Two-section Laser Research Articles

Sideband generation of coupled-cavity terahertz semiconductor lasers under active radio frequency modulation.

The radio frequency (RF) modulation is a powerful tool, which is used for generating sidebands in semiconductor lasers for active mode-locking. The two-section coupled-cavity laser geometry shows advantages over traditional Fabry-Pérot cavities in the RF modulation efficiency, because of its reduced device capacitance of short section cavity. Further, it has been widely used for active/passive mode-locking of semiconductor diode lasers. For semiconductor-based quantum cascade lasers (QCLs) emitting in the far-infrared or terahertz frequency bands, the two-section coupled-cavity configuration can strongly prevent the laser from multimode emissions. This is because of its strong mode selection (loss modulation), which the cavity geometry introduces. Here, we experimentally demonstrate that the coupled-cavity terahertz QCL can be actively modulated to generate sidebands. The RF modulation is efficient at the frequency that equals the difference frequency between the fundamental and higher order transverse modes of the laser, and its harmonics. We show for the first time that, when the laser is modulated at the second harmonic of the difference frequency, the sideband generation in coupled-cavity terahertz QCLs and the generated sidebands are equally spaced by the injected microwave frequency. Our results, which are presented here, provide a novel approach for modulating terahertz coupled-cavity lasers for active mode-locking. The coupled-cavity geometry shows advantages in generating dense modes with short cavities for potential high-resolution spectroscopy. Furthermore, the short coupled-cavity laser consumes less electrical power than Fabry-Pérot lasers that generate a similar mode spacing.

Stabilization of self-mode-locked QDash lasers subject to simultaneous continuous-wave optical injection and optical feedback.

In this paper, we report stabilization of self-mode-locked two-section quantum-dash lasers on the widest range of delay using simultaneous optical injection and optical feedback. With continuous-wave optical injection, various wavelengths spanning a range from 1568 to 1578nm were investigated and optimum wavelengths (1571.210 to 1572.710) yielding the narrowest RF linewidth and reduced timing jitter of slave laser were identified. In addition, the dependence of RF linewidth and pulse repetition rate on injected wavelength was further explored. Our results indicate that simultaneous optical feedback and optical injection significantly improves the RF linewidth across the widest delay range compared to optical feedback alone. Under fully resonant feedback and optimum injection parameters, a minimum RF linewidth of 1kHz (instrument limited) was achieved with simultaneous optical injection plus optical feedback, which was >2× lower than optical feedback alone and more than 100× lower than free-running. This stabilization technique is implemented in an all-optical arrangement without optical/electrical conversion, which is ideal for high-repetition-rate devices and photonic integration.

Mode locking in a tapered two-section quantum dot laser: design and experiment.

In this Letter, the pulse generation and pulse train stability of a tapered two-section InAs/InGaAs quantum dot laser emitting at 1250nm are numerically predicted and experimentally verified. Simulations based on a multi-section delayed differential equation model are used to properly design a laser source able to generate stable mode-locked pulses at a 15GHz repetition rate with picosecond width and output power larger than 1W, and to identify the device stability regions depending on the bias conditions. Possible instabilities are associated with the existence of a leading or trailing edge net gain window outside the optical pulse. Experimentally, we confirm the existence of different stability regions where instabilities manifest in broadband or multi-periodic pulse train amplitude modulations. Our results confirm the correctness to the design and may be helpful in achieving high-power pulses while avoiding detrimental pulse train instabilities, both being important for time-critical applications.

Stabilization of self-mode-locked quantum dash lasers by symmetric dual-loop optical feedback.

We report experimental studies of the influence of symmetric dual-loop optical feedback on the RF linewidth and timing jitter of self-mode-locked two-section quantum dash lasers emitting at 1550 nm. Various feedback schemes were investigated and optimum levels determined for narrowest RF linewidth and low timing jitter, for single-loop and symmetric dual-loop feedback. Two symmetric dual-loop configurations, with balanced and unbalanced feedback ratios, were studied. We demonstrate that unbalanced symmetric dual loop feedback, with the inner cavity resonant and fine delay tuning of the outer loop, gives the narrowest RF linewidth and reduced timing jitter over a wide range of delay, unlike single and balanced symmetric dual-loop configurations. This configuration with feedback lengths of 80 and 140 m narrows the RF linewidth by ∼ 4-67x and ∼ 10-100x, respectively, across the widest delay range, compared to free-running. For symmetric dual-loop feedback, the influence of different power split ratios through the feedback loops was determined. Our results show that symmetric dual-loop feedback is markedly more effective than single-loop feedback in reducing RF linewidth and timing jitter, and is much less sensitive to delay phase, making this technique ideal for applications where robustness and alignment tolerance are essential.

Mode-Locked Lasers with “Thin” Quantum Wells in 1.55 μm Spectral Range

We present the results of an experimental study of two-section semiconductor lasers with active region consisting of three 3.1 nm InGaAs/InGaAlAs quantum wells. Passive mode-locking regime with a repetition rate of 10 GHz is realized. Different approaches have been compared to determine the stability of pulse repetition.

Синхронизация мод в лазерах спектрального диапазона 1.55 mum на основе "тонких" квантовых ям

AbstractWe present the results of an experimental study of two-section semiconductor lasers with active region consisting of three 3.1 nm InGaAs/InGaAlAs quantum wells. Passive mode-locking regime with a repetition rate of 10 GHz is realized. Different approaches have been compared to determine the stability of pulse repetition.

InGaAsP多量子阱双稳态激光器的实验及理论研究

Influences of the bias states of saturable absorbers on bistable InGaAsP multi-quantum-well(MQW) common cavity tandem section(CCTS) semiconductor lasers were investigated experimentally and theoretically. The experiment demonstrated that the bistability characteristic of P-I curves can be more significant with increasing reversed bias voltage in the saturable absorber(SA), and a negative differential resistance phenomenon was found in V-I curves. When the voltage was -3 V, the hysteresis width was broadened to 13.5 mA, with the on-off ratio up to 21:1. The theoretical analysis proves that higher passive voltage in SA and shorter carrier escape time can result in better bistability. The maximum on-off ratio as high as 107:1 promises that a common cavity two-section laser can be switching between the two-steady state.

Spectral dynamical behavior in two-section, quantum well, mode-locked laser at 1.064 μm**Project supported by the National Basic Research Program of China (Grant Nos.~2013CB933304 and 2012CB932701), the National Natural Science Foundation of China (Grant Nos. 61274125 and 61435012), and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (Grant No. XDB01010200).

In this study, two-section mode-locked semiconductor lasers with different numbers of quantum wells and different types of waveguide structures are made. Their ultrashort pulse features are presented. The spectral dynamical behaviors in these lasers are studied in detail. In the simulation part, a two-band compressive-strained quantum well (QW) model is used to study thermally induced band-edge detuning in the amplifier and saturable absorber (SA). A sudden blue shift in laser spectrum is expected by calculating the peak of the net gain. In the experiment part, the sudden blue shift in the emission spectrum is observed in triple QW samples under certain operating conditions but remains absent in single QW samples. Experimental results reveal that blue shift phenomenon is connected with the difference between currents in two sections. Additionally, a threshold current ratio for blue-shift is also demonstrated.

High-speed directly modulated widely tunable two-section InGaAlAs DBR lasers.

We report widely tunable two-section distributed Bragg reflector (DBR) lasers, which have InGaAlAs multiple quantum wells (MQWs) as the gain material. By butt-jointing InGaAsP, which has a photoluminescence wavelength of 1.4 μm as the material of the DBR section, a wavelength tuning range of 12 nm can be obtained by current injection into the DBR section. The direct modulation bandwidth of the lasers is greater than 10 GHz over the entire wavelength tuning range up to 40°C. Compared with InGaAsP DBR lasers having the same structure, the InGaAlAs lasers have smaller variations in both the threshold current and slope efficiency with the temperature because of the better electron confinement in the InGaAlAs MQWs. Moreover, the DBR-current-induced decreases in the modulation bandwidth and side mode suppression ratio (SMSR) of the optical spectra are notably smaller for the InGaAlAs lasers than for the InGaAsP lasers.

Switching between the mode-locking and Q-switching modes in two-section QW lasers upon a change in the absorber properties due to the Stark effect

The passive Q-switching and mode-locking modes are implemented in two-section lasers with three quantum wells. It is demonstrated that raising the reverse bias on the absorbing section changes its spectral and dynamic properties and, accordingly, leads to a change from the Q-switching mode to mode-locking. The pulse-repetition frequency in the mode-locking mode is 75 GHz, with the product of the pulse duration by the spectrum bandwidth being 0.49, which is close to the theoretical limit. It is shown that, in structures with three quantum wells, strong absorption at the lasing wavelength gives rise to a photocurrent across a section of the saturable absorber, which is sufficient for compensation of the applied bias.

Design and simulation of two-section DFB lasers with short active-section lengths.

Distributed feedback lasers comprised of a reflection section and an active section have been proposed for high direct-modulation bandwidth. The reflection section has the same core layer as the active section so butt-joint re-growth is avoided. Without current injection the reflection section will be pumped to near transparency by the emission from the laser itself so high reflection (> 0.75) can still be achieved as confirmed by the simulation. Therefore a short (150 µm) active section can be used, which enables a low threshold current (~5 mA) and a high direct modulation bandwidth (>30 GHz) as demonstrated by the simulation.

Self-induced transparency mode locking, and area theorem.

Self-induced transparency mode locking (or coherent mode locking, CML), which is based on intracavity self-induced transparency soliton dynamics, potentially allows achievement of nearly single-cycle intracavity pulse durations, much below the phase relaxation time T2 in a laser which, despite having great promise, has not yet been realized experimentally. We develop a diagram technique which allows us to predict the main features of CML regimes in a generic two-section laser far from the single-cycle limit. We show that CML can arise directly at the first laser threshold if the phase relaxation time is large enough. Furthermore, we discuss the stability of the corresponding mapping. We also predict the existence of "super-CML regimes," with a pulse coupled to several Rabi oscillations in the nonlinear medium.

Self-starting stable coherent mode-locking in a two-section laser

Coherent mode-locking (CML) uses self-induced transparency (SIT) soliton formation to achieve, in contrast to conventional schemes based on absorption saturation, the pulse durations below the limit allowed by the gain line width. Despite the great promise it is difficult to realize it experimentally because a complicated setup is required. In all previous theoretical considerations CML is believed to be non-self-starting. In this paper we show that if the cavity length is selected properly, a very stable (CML) regime can be realized in an elementary two-section ring-cavity geometry, and this regime is self-developing from the non-lasing state. The stability of the pulsed regime is the result of a dynamical stabilization mechanism arising due to finite-cavity-size effects.

Passive mode locking of a GaSb-based quantum well diode laser emitting at 2.1 μm

We demonstrate passive mode locking of a GaSb-based diode laser emitting at 2.1 μm. The active region of the studied device consists in two 10-nm-thick GaInSbAs/GaAlSbAs quantum wells. Passive mode locking has been achieved in a two-section laser with one of the sections used as a saturable absorber. A microwave signal at 20.6 GHz, measured in the electrical circuit of the absorber, corresponds to the fundamental photon round-trip frequency in the laser resonator. The linewidth of this signal as low as ∼10 kHz has been observed at certain operating conditions, indicating low phase noise mode-locked operation.

On coherent mode-locking in a two-section laser

Currently existing lasers with passive mode-locking are two-section systems including an amplifier section and a saturable absorber section. The mechanism of generation of short pulses in them is based on the effects of saturation of the gain in the amplifier and absorption in the absorber, which prevent obtaining pulses with durations shorter than the relaxation time of the polarization in the amplifying and absorbing media. In this work, the new possibility of the generation of ultrashort pulses in a laser with passive mode-locking owing to the coherent character of the interaction of light with matter in the amplifying and absorbing media in ring and linear cavities has been theoretically analyzed. A practically interesting case where the amplifying and absorbing media are separated in the space of the cavity, rather than being homogeneously mixed in its volume, as was previously considered for such type of lasers, has been studied. It has been shown that the width of the spectrum of generated pulses can be much larger than the width of the amplification line and can vary depending on the parameters of the amplifying and absorbing media of the laser. The existence of nonsoliton scenarios of the regime of coherent passive mode synchronization, as well as the possibility of generation without injection of an auxiliary pulse, has been demonstrated.

1.55-μm mode-locked quantum-dot lasers with 300 MHz frequency tuning range

Passive mode-locking of two-section quantum-dot mode-locked lasers grown by metalorganic vapor phase epitaxy on InP is reported. 1250-μm long lasers exhibit a wide tuning range of 300 MHz around the fundamental mode-locking frequency of 33.48 GHz. The frequency tuning is achieved by varying the reverse bias of the saturable absorber from 0 to −2.2 V and the gain section current from 90 to 280 mA. 3 dB optical spectra width of 6–7 nm leads to ex-facet optical pulses with full-width half-maximum down to 3.7 ps. Single-section quantum-dot mode-locked lasers show 0.8 ps broad optical pulses after external fiber-based compression. Injection current tuning from 70 to 300 mA leads to 30 MHz frequency tuning.

Design Optimization of Passively Mode-Locked Semiconductor Lasers With Intracavity Grating Spectral Filters

We consider design optimization of passively mode-locked two-section semiconductor lasers that incorporate intracavity grating spectral filters. Our goal is to develop a method for finding the optimal wavelength location for the filter in order to maximize the region of stable mode-locking as a function of drive current and reverse bias in the absorber section. In order to account for material dispersion in the two sections of the laser, we use analytic approximations for the gain and absorption as a function of carrier density and frequency. Fits to measured gain and absorption curves then provide inputs for numerical simulations based on a large signal accurate delay-differential model of the mode-locked laser. We show how a unique set of model parameters for each value of the drive current and reverse bias voltage can be selected based on the variation of the net gain along branches of steady-state solutions of the model. We demonstrate the validity of this approach by demonstrating qualitative agreement between numerical simulations and the measured current-voltage phase-space of a two-section Fabry-Perot laser. We then show how to adapt this method to determine an optimum location for the spectral filter in a notional device with the same material composition, based on the targeted locking range, and accounting for the modal selectivity of the filter.

A Widely Tunable Directly Modulated DBR Laser With High Linearity

We demonstrate a widely tunable and high-modulation-bandwidth distributed Bragg reflector (DBR) laser. With the selected butt-joint material (λg = 1.44 μm) and butt-joint angle (45°) for the grating section, a widely tuning range of 13.88 nm with 19 consecutive channels is obtained for the two-section DBR laser. Furthermore, a small-signal modulation bandwidth of 16 GHz with in-band flatness of ±3 dB is realized at 10 °C. A uniform modulation bandwidth of more than 13 GHz is obtained for ten consecutive channels at 25 °C. Meanwhile, the DBR laser shows a high linearity, with an input 1-dB compression point of 17 dBm and an input third-order intercept point of 30 dBm at 13 GHz.

Influence of GaAs spacer-layer thickness on quantum coupling and optical polarization in a ten-layer system of vertically correlated InAs/GaAs quantum dots

The polarization anisotropy of electroluminescence and absorption in a ten-layer system of vertically correlated InAs quantum dots separated by 8.6-nm-thick GaAs spacer layers is investigated experimentally. The quantum-dot system is built into a two-section laser structure with sections of equal length. It is found that the polarization anisotropy in this system is smaller than the anisotropy in similar systems with a single layer of quantum dots or quantum-dot molecules, but larger than that in a quantum-dot superlattice. The spectra of differential absorption in the structure under study for different strengths of the applied electric field are also investigated. The rate of variation in the Stark shift as a function of the electric field is determined, the results giving evidence of controlled quantum coupling between adjacent quantum dots in tenlayer vertically correlated InAs/GaAs quantum-dot systems with 8.6- and 30-nm-thick GaAs spacer layers. The measured polarization dependences are explained by the participation of heavy-hole ground states in optical transitions. This effect is defined by the two dimensional nature of the system under study.

Dual-wavelength InP quantum dot lasers

We have demonstrated a two-section dual-wavelength diode laser incorporating distributed Bragg reflectors, with a peak-wavelength separation of 62.5 nm at 300 K. Each lasing wavelength has a different temperature dependence, providing a difference-tuning of 0.11 nm/K. We discuss the mechanisms governing the light output of the two competing modes and explain how the short wavelength can be relatively insensitive to output changes at the longer wavelength. Starting from an initial condition when the output at both wavelengths are equal, a 500% increase in the long wavelength output causes the short wavelength output to fall by only 6%.