Mode-locked Vertical External Cavity Surface Emitting Lasers Research Articles

Mode-locked VECSEL emitting 5 ps pulses at 675 nm

A picosecond GaInP/AlGaInP/GaAs vertical external-cavity surface-emitting laser (VECSEL) at 675 nm is reported. The laser is mode-locked with a GaInP/AlGaInP/GaAs saturable absorber mirror and emitted ~5.1 ps pulses at a 973 MHz repetition rate and an average power of 45 mW. To our knowledge, this is the first demonstration of a passively mode-locked VECSEL emitting fundamental laser radiation at the visible part of the spectrum.

Sub-60-fs Timing Jitter of a SESAM Modelocked VECSEL

We present noise measurements of a pulse train emitted from an actively stabilized semiconductor-saturable-absorber-mirror (SESAM) modelocked vertical external cavity surface emitting laser (VECSEL). The laser generated 6-ps pulses with 2-GHz pulse-repetition rate and 40-mW average output power. The repetition rate was phase locked to a reference source using a piezo actuator. The timing phase noise power spectral density of the laser output was detected with a highly linear photodiode and measured with a signal source analyzer. The resulting RMS timing jitter integrated over an offset frequency range from 1 Hz to 100 MHz gives a value of below 60 fs, lower than previous modelocked VECSELs and comparable with the noise performance of ion-doped solid-state lasers. The RMS amplitude noise was below 0.4% (1 Hz to 40 MHz) and not influenced by the timing phase stabilization.

Subpicosecond pulse generation from a 156 μm mode-locked VECSEL

Near-transform-limited subpicosecond pulses at 1.56 μm were generated from an optically pumped InP-based vertical-external-cavity surface-emitting laser (VECSEL) passively mode-locked at 2 GHz repetition rate with a fast InGaAsNSb/GaAs semiconductor saturable absorber mirror (SESAM). The SESAM microcavity resonance was adjusted via a selective etching of phase layers specifically designed to control the magnitude of both the modulation depth and the intracavity group delay dispersion of the SESAM. Using the same VECSEL chip, we observed that the mode-locked pulse duration could be reduced from several picoseconds to less than 1 ps with a detuned resonant SESAM.

Electrically Pumped Vertical External Cavity Surface Emitting Lasers Suitable for Passive Modelocking

Modelocked optically pumped vertical external cavity surface emitting lasers (VECSELs) have generated up to 6.4-W average power, which is higher than for any other semiconductor lasers. Electrical pumping of modelocked VECSELs is the next step toward a higher level of integration. With continuous wave (cw) electrically pumped (EP) VECSELs, an average output power of 900 mW has been demonstrated from the undisclosed proprietary novalux extended cavity surface emitting laser (NECSEL) design. In contrast, modelocked NECSELs have only been demonstrated at 40 mW. Recently, we presented a numerical study of EP-VECSELs suitable for modelocked operation; here, we demonstrate the first realization of this design. Power scaling is achieved with a lateral mode size increase. The competing electrical and optical requirements are, on the electrical side, low ohmic resistance, and on the optical side, low optical losses and low dispersion. Additionally, the device needs to operate in a fundamental mode for stable modelocking. We have fabricated and characterized 60 EP-VECSELs with varying dimensions and compared their lasing performance with our numerical simulations. The tradeoff between good beam quality and output power is discussed with an outlook to the modelocking of these EP-VECSELs. Initial EP-VECSEL devices have generated >;100 mW of cw output power.

Timing Jitter Characterization of a Free-Running SESAM Mode-locked VECSEL

We present timing jitter measurements of an InGaAs quantum well vertical external cavity surface emitting laser (VECSEL) passively mode locked with a quantum dot semiconductor saturable absorber mirror (SESAM) at 2-GHz repetition rate. It generates 53-mW average output power in 4.6-ps pulses at 953 nm. The laser housing was optimized for high mechanical stability to reduce acoustic noise. We use a fiber-coupled multimode 808-nm pump diode, which is mounted inside the laser housing. No active cavity length stabilization is employed. The phase noise of the free-running laser integrated over a bandwidth from 100 Hz to 1 MHz corresponds to an RMS timing jitter of ≈212 fs, which is lower than previously obtained for mode-locked VECSELs. This clearly confirms the superior noise performance expected from a high-Q-cavity semiconductor laser. In contrast to edge-emitting semiconductor diode lasers, the cavity mode is perpendicular to the quantum well gain layers, which minimizes complex dispersion and nonlinear dynamics.

Timing Jitter Reduction of a Mode-Locked VECSEL Using an Optically Triggered SESAM

A technique for actively stabilizing the repetition rate of a mode-locked vertical external-cavity surface-emitting laser is presented using an optically triggered semiconductor saturable absorber mirror. This method enables a significant reduction of the timing jitter measured in the frequency range [100 Hz, 10 MHz] and makes this kind of externally locked laser suitable for high-speed sampling of microwave signals. The experimental demonstration shows jitter reduction by a factor of 16 from 8 ps down to 423 fs for a 1.68-GHz repetition rate pulse train. The hold-in locking bandwidth and stabilization efficiency are shown to be a function of the average power and modulation index of the optical control signal.

Ultrashort pulse generation from 156 µm mode-locked VECSEL at room temperature

We report on a picosecond pulse source delivering near transform-limited pulses in the 1.55 µm wavelength region, based on an optically pumped InP-based mode locked Vertical External Cavity Surface Emitting Laser (VECSEL). The cavity combines two semiconductor elements, a gain structure which includes six strained InGaAlAs quantum wells and a hybrid metal-metamorphic Bragg bottom mirror bonded onto a CVD diamond substrate, and a single quantum well GaInNAs SEmiconductor Saturable Absorber Mirror (SESAM). The laser operates at a repetition frequency of 2 GHz and emits near-transform-limited 1.7 ps pulses with an average output power of 15 mW at room temperature, using 1.7 W pump power at 980 nm. The RF line width of the free running laser has been measured to be less than 1 kHz.

Experimental verification of soliton-like pulse-shaping mechanisms in passively mode-locked VECSELs

We present a detailed experimental study on the influence of dispersion on the pulse formation in passively modelocked vertical-external-cavity surface-emitting lasers (VECSELs). We have demonstrated that the shortest pulse duration requires slightly positive dispersion to balance the nonlinear phase shift induced by strong semiconductor gain and absorber saturation. This is in contrast to soliton modelocking in ion-doped solid-state lasers - but similarities remain and will be discussed. Our results are in good qualitative agreement with numerical simulations and confirm the quasi-soliton modelocking mechanism of ultrafast VECSELs.

Optimization of Ultrashort Pulse Generation in Passively Mode-Locked Vertical External-Cavity Semiconductor Lasers

We present a genetic algorithm based method to optimize the gain medium design of a passively mode-locked vertical external-cavity surface-emitting laser (VECSEL) for ultrashort pulse generation. Beginning with a given standard VECSEL design, we show that a simple modification to the DBR within the active mirror can significantly reduce both the duration and the chirp of the generated pulse. We demonstrate, using realistic numerical simulations, that the full-width at half-maximum of the pulse can be reduced by more than a factor of two. The required design changes come at the cost of increased sensitivity to perturbations in the active chip growth. Methods of counteracting this trend are presented.

All-GaInNAs ultrafast lasers: Material development for emitters and absorbers

Defect engineering is a key feature in material development for active and passive laser devices. Active devices such as surface emitting lasers require excellent material quality with low defect concentration and good strain management. In contrast, passive devices such as saturable absorbers benefit from nonradiative recombination via defect states. Different molecular beam epitaxy (MBE) growth conditions and annealing parameters were developed to optimize GaInNAs for both active and passive devices. We have demonstrated for the first time an all-GaInNAs modelocked vertical external-cavity surface-emitting laser (VECSEL) at 1.3 μm. We combined a GaInNAs VECSEL with a GaInNAs semiconductor saturable absorber mirror (SESAM) in a laser cavity. The VECSEL was optically pumped by an 808 nm semiconductor diode laser. The intracavity GaInNAs SESAM self-starts stable modelocking and generates a pulse duration of 18.7 ps with a pulse repetition rate of 6.1 GHz at 57 mW of average output power at a center wavelength of 1308 nm. In this paper, we briefly review the modelocking result and then focus on the MBE growth and fabrication of both active and passive GaInNAs devices.

Femtosecond synchronously mode-locked vertical-external cavity surface-emitting laser

The behavior of a room temperature synchronously mode-locked vertical-external cavity surface-emitting laser (VECSEL) operating at 980 nm is reported. The laser performance was found to be qualitatively the same for different pump pulse duration (3.6 ps and 70 fs). The pulse duration of the laser is limited by strong self-phase modulation to around 10-40 ps. By compressing the strongly chirped pulses generated directly from the laser, ultrashort pulses with duration of around 200 fs with maximum peak powers of 1.3 kW at 80 MHz were obtained. Multiple pulsing of the laser was observed and the effects of cavity length detuning on pulse width and spectral bandwidth have been investigated.