Vertical External Cavity Surface Emitting Laser Research Articles

Analytical study of vertical external-cavity surface-emitting organic lasers

In this paper we report a detailed study of emission dynamics of an organic solid-state laser structure so-called VECSOL standing for Vertical External-Cavity Surface-emitting Organic Laser recently developed in our group. An optical-optical efficiency of 43% and 6.3% was reported for a 4-mm-long cavity incorporating 18- μ m-thick film of Poly(methyl methacrylate) (PMMA) doped with 1 wt.% of Rhodamine 640 when pumped with 7-ns-long and 0.5-ns-long pulses respectively. In order to understand the difference seen in lasing efficiency as a function of different parameters such as cavity length or pump pulse duration, Tang-Statz-deMars cavity rate equations are used to model the emission behavior in a pulsed regime. Based on this model, conversion efficiency could be optimized practically to values as high as 57%. Furthermore, some characteristics of this laser architecture such as lasing lifetime (up to 140 000 pulses at two times above lasing threshold), wavelength tuning (over 40 nm) and the system power scalability with potential operation up to mJ level are investigated.

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.

Femtosecond VECSEL with tunable multi-gigahertz repetition rate

We present a femtosecond vertical external cavity surface emitting laser (VECSEL) that is continuously tunable in repetition rate from 6.5 GHz up to 11.3 GHz. The use of a low-saturation fluence semiconductor saturable absorber mirror (SESAM) enables stable cw modelocking with a simple cavity design, for which the laser mode area on SESAM and VECSEL are similar and do not significantly change for a variation in cavity length. Without any realignment of the cavity for the full tuning range, the pulse duration remained nearly constant around 625 fs with less than 3.5% standard deviation. The center wavelength only changed ±0.2 nm around 963.8 nm, while the output power was 169 mW with less than 6% standard deviation. Such a tunable repetition rate is interesting for various metrology applications such as optical sampling by laser cavity tuning (OSCAT).

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.

1220–1280-nm Optically Pumped InAs Quantum Dot-Based Vertical External-Cavity Surface-Emitting Laser

We report an InAs quantum dot-based optically pumped vertical external-cavity surface-emitting laser (VECSEL), with a continuously variable emission wavelength from 1220 to 1280 nm through the use of epitaxial gradient across the wafer. We demonstrate the performance of two designs of this VECSEL. The first design, 4 × 3, makes use of a resonant periodic gain structure where three dot-in-a-well (DWELL) layers are located at the antinode of the E-field standing wave, with a total of four sets being used. The second design, 12 × 1, makes use of a single DWELL layer per antinode which is repeated 12 times for the same total of 12 DWELL layers. We demonstrate the lasing performance of the two structures as a function of distance from the center of the wafer and use known radial composition gradients in the molecular beam epitaxy reactor to achieve 50 nm continuous wavelength variation in the 4 × 3 structure and 60 nm in the 12 × 1 structure. We also demonstrate that the performance of the 12 × 1 structure is significantly improved compared to the 4 × 3 structure for all measured laser parameters, possibly due to increased pump absorption in the former structure's thicker barriers.

Heat Management in High-Power Vertical-External-Cavity Surface-Emitting Lasers

The thermal properties of a high-power vertical-external-cavity surface-emitting laser (VECSEL) are studied experimentally, focusing on the generation, distribution, and removal of excess heat under extreme pumping conditions. Different heat-spreading and heat-transfer approaches are analyzed. The performance of the device is optimized yielding a maximum emitted power beyond 70 W from a single spot. Finally, the potential for power-scaling in VECSELs and its restrictions are examined.

VECSEL Optimization Using Microscopic Many-Body Physics

Vertical external cavity surface-emitting lasers (VECSELs) are designed and analyzed using an approach based on fully microscopically computed material properties like gain and carrier recombination rates. Very good agreement between theoretical predictions and measured characteristics of the realized devices is demonstrated. The high accuracy of the theoretical models allows one to determine even small deviations between the nominal designs and actual realizations. The models are used to find optimization strategies. It is shown how the external efficiency can be strongly improved using surface coatings that reduce the pump reflection while retaining the gain-enhancing cavity effects at the lasing wavelength. It is shown how incomplete pump absorption can be detrimental to the device performance and how this problem can be reduced using optimized distributed Bragg reflectors and metallization layers. A combination of improved metallization and use of such a coating more than doubles the external efficiency and maximum power for a realized VECSEL operating at 1010 nm and the theory indicates that further significant improvements are possible.

Strain compensated 1120 nm GaInAs/GaAs vertical external-cavity surface-emitting laser grown by molecular beam epitaxy

We report on the development of high quality, strain compensated gain mirror for 1120 nm vertical external-cavity surface-emitting lasers (VECSELs). The gain mirror was grown by molecular beam epitaxy and comprised a total of six Ga 0.69In 0.31As quantum wells. The effect of the strain compensation has been assessed by measuring the curvature of the wafer and by mapping the photoluminescence to identify the non-emissive dark areas. We demonstrate that ∼91% strain compensation with GaAs 0.85P 0.15 is sufficient to remove the dark lines corresponding to areas with structural defects in the gain mirror structure. Rapid thermal annealing studies revealed that the strain compensation is efficient in preventing the appearance of dark lines even for samples that were annealed at temperatures as high as 700 °C for a considerable time. The strain compensated gain mirror was used to demonstrate a VECSEL emitting at 1120 nm.

A VECSEL worth its salt

Vertical external cavity surface emitting laser (VECSEL) that has a continuous tuning over 3.22 to 3.3 μm at temperatures up to 5°C has been demonstrated. This PbSe quantum well VECSEL, developed for hydrocarbon absorption spectroscopy, is one of a range of lead-salt-based VECSELs from a team at ETH Zurich, Switzerland, for mid-IR applications in the 2.5-10 μm wavelength range.

Continuously tunable singlemode VECSEL at 3.3 µm wavelength for spectroscopy

A singlemode tunable vertical external cavity surface emitting laser (VECSEL) has been realised. The VECSEL can be tuned continuously over more than 80 nm from ∼3.22 to 3.3 µm wavelength. Tuning is performed by changing the cavity length with a piezoelectric driver. Covering this wavelength range allows various potential applications, especially for gas spectroscopy of organic compounds. The VECSEL is optically pumped, and emits a maximum output power of 20 mWp when cooled with a Peltier element to −20°C. The active medium consist of PbSe quantum wells embedded in Pb1−xSrxSe (x=0.08) barrier layers and is grown on a Si-substrate.

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.

Modular PbSrS/PbS mid-infrared vertical external cavity surface emitting laser on Si

A mid-infrared vertical external cavity surface emitting laser (VECSEL) based on undoped PbS is described herein. A 200 nm-thick PbS active layer embedded between PbSrS cladding layers forms a double heterostructure. The layers are grown on a lattice and thermal expansion mismatched Si-substrate. The substrate is placed onto a flat bottom Bragg mirror again grown on a Si substrate, and the VECSEL is completed with a curved top mirror. Pumping is done optically with a 1.55 μm laser diode. This leads to an extremely simple modular fabrication process. Lasing wavelengths range from 3–3.8 μm at 100–260 K heat sink temperature. The lowest threshold power is ∼210 mWp and highest output power is ∼250 mWp. The influence of the different recombination mechanism as well as free carrier absorption on the threshold power is modeled.

Femtosecond high-power quantum dot vertical external cavity surface emitting laser

We report on the first femtosecond vertical external cavity surface emitting laser (VECSEL) exceeding 1 W of average output power. The VECSEL is optically pumped, based on self-assembled InAs quantum dot (QD) gain layers, cooled efficiently using a thin disk geometry and passively modelocked with a fast quantum dot semiconductor saturable absorber mirror (SESAM). We developed a novel gain structure with a flat group delay dispersion (GDD) of ± 10 fs2 over a range of 30 nm around the designed operation wavelength of 960 nm. This amount of GDD is several orders of magnitude lower compared to standard designs. Furthermore, we used an optimized positioning scheme of 63 QD gain layers to broaden and flatten the spectral gain. For stable and self-starting pulse formation, we have employed a QD-SESAM with a fast absorption recovery time of around 500 fs. We have achieved 1 W of average output power with 784-fs pulse duration at a repetition rate of 5.4 GHz. The QD-SESAM and the QD-VECSEL are operated with similar cavity mode areas, which is beneficial for higher repetition rates and the integration of both elements into a modelocked integrated external-cavity surface emitting laser (MIXSEL).

Tunable ultraviolet vertically-emitting organic laser

A solid-state organic thin-film laser with intracavity frequency doubling is reported. Tunable ultraviolet emission from 309 to 322 nm is achieved from a vertical external cavity surface-emitting organic laser, with 2% efficiency (1 μJ at 315 nm). The laser comprises a poly(methyl methacrylate) layer doped with Rhodamine 640, spun-cast onto a plane mirror, a remote concave mirror, a nonlinear crystal, and a dichroic separator. The output is spectrally narrow (<0.5 nm full width at half maximum) and tunable through phase-matching selection of the fundamental radiation lasing modes. These results highlight a low-cost and portable alternative to tunable UV laser sources, useful for spectroscopic applications.

High power 1.25 μm InAs quantum dot vertical external-cavity surface-emitting laser

The authors demonstrate InAs quantum dot (QD)-based optically pumped vertical external-cavity surface-emitting lasers grown by molecular beam epitaxy. Active region designs utilizing two different resonant periodic gain (RPG) structures are compared. The first RPG structure is a more traditional design consisting of three QD layers per antinode of the E-field standing wave, repeated four times, for a total of 12 QD layers. The second RPG has a single-QD layer per antinode, repeated 12 times. The single-QD layer per antinode structure allows for both superior strain relief as well as more complete pump absorption and thus results in significantly improved device performance over the traditional multi-QD layer per antinode design. The authors were able to demonstrate over 3 W of output power at room temperature using a thermal grade polycrystalline chemical-vapor deposition diamond heat spreader mounted on the backside of a sample thinned by mechanical polishing.

High-power MIXSEL: an integrated ultrafast semiconductor laser with 64 W average power

High-power ultrafast lasers are important for numerous industrial and scientific applications. Current multi-watt systems, however, are based on relatively complex laser concepts, for example using additional intracavity elements for pulse formation. Moving towards a higher level of integration would reduce complexity, packaging, and manufacturing cost, which are important requirements for mass production. Semiconductor lasers are well established for such applications, and optically-pumped vertical external cavity surface emitting lasers (VECSELs) are most promising for higher power applications, generating the highest power in fundamental transverse mode (>20 W) to date. Ultrashort pulses have been demonstrated using passive modelocking with a semiconductor saturable absorber mirror (SESAM), achieving for example 2.1-W average power, sub-100-fs pulse duration, and 50-GHz pulse repetition rate. Previously the integration of both the gain and absorber elements into a single wafer was demonstrated with the MIXSEL (modelocked integrated external-cavity surface emitting laser) but with limited average output power (<200 mW). We have demonstrated the power scaling concept of the MIXSEL using optimized quantum dot saturable absorbers in an antiresonant structure design combined with an improved thermal management by wafer removal and mounting of the 8-µm thick MIXSEL structure directly onto a CVD-diamond heat spreader. The simple straight cavity with only two components has generated 28-ps pulses at 2.5-GHz repetition rate and an average output power of 6.4 W, which is higher than for any other modelocked semiconductor laser.

Observation of Slow Light in the Noise Spectrum of a Vertical External Cavity Surface-Emitting Laser

The role of coherent population oscillations is evidenced in the noise spectrum of an ultralow noise laser. This effect is isolated in the intensity noise spectrum of an optimized single-frequency vertical external cavity surface-emitting laser. The coherent population oscillations induced by the lasing mode manifest themselves through their associated dispersion that leads to slow light effects probed by the spontaneous emission present in the nonlasing side modes.

Dynamic regimes of the dual-wavelength vertical external cavity surface-emitting laser

The stability of the stationary state and periodic oscillations of the radiation intensity of the dual-wavelength vertical external cavity surface-emitting laser is analyzed. It is demonstrated that the stationary state is always stable for the laser with independent active areas that contain quantum dots of different depths. A map of dynamic regimes with the values of parameters that correspond to the cw lasing, periodic oscillations of intensity, and complicated dynamics (quasi-periodic or chaotic regimes) is constructed for the laser with coupled active areas. The dynamics of the laser radiation in the quasi-periodic regime is numerically simulated. It is demonstrated that relatively slow variations in the amplitude with characteristic times of several or several tens of microseconds are typical of the short laser pulses that are formed at time intervals of tens of picoseconds. The period of the pulse train is approximately equal to the round-trip time of the external laser cavity.

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.

Short wavelength red-emitting AlGaInP-VECSEL exceeds 1.2 W continuous-wave output power

We present a vertical external cavity surface-emitting laser system based on a multi-quantum-well structure with 20 compressively strained GaInP quantum wells for an operation wavelength of around 665 nm with a monolithic integrated distributed Bragg reflector. With the help of an intra-cavity diamond heatspreader the laser operates in continuous-wave mode. Operation with a TEM00 Gaussian beam profile and a beam propagation factor of M2≤1.05 is shown as well as a high resolution spectrum of the laser line, which shows the etalon effect of the diamond. The laser can be operated at a maximum output power exceeding 1.2 W with a slope efficiency of ηdiff=18%. At maximum output power the wavelength of the laser resonance is at 670 nm, which is shortest reported until now at powers exceeding 1 W. By rotating a birefringent filter in an extended folded cavity arrangement a wavelength tuning of 21 nm was attained.