Resonant Periodic Gain Research Articles

Monolithic VECSEL for stable kHz linewidth.

Vertical-external-cavity surface-emitting semiconductor lasers (VECSELs) are of increasing interest for applications requiring ultra-coherence and/or low noise at novel wavelengths; performance that is currently achieved via high-Q, air-spaced resonators to achieve long intra-cavity photon lifetimes (for the so-called class-A low noise regime), power scaling and high beam quality. Here, we report on the development of a compact, electronically tunable, monolithic-cavity, class-A VECSEL (monolithic VECSEL) for ultra-narrow free-running linewidths. A multi-quantum-well, resonant periodic gain structure with integrated distributed Bragg reflector (DBR) was optically-bonded to an air-gap-free laser resonator created inside a right-angle fused-silica prism to suppress the influence of environmental noise on the external laser oscillation, thus achieving high stability. Mode-hop-free wavelength tuning is performed via the stabilized temperature; or electronically, and with low latency, via a shear piezo-electric transducer mounted on the top of the prism. The free-running linewidth, estimated via the frequency power spectral density (PSD), is sub-kHz over ms timescales and <1.9 kHz for time sampling as long as 1s, demonstrating at least two orders-of-magnitude improvement in noise performance compared to previously reported single frequency VECSELs. The stable, total internal reflection resonator concept is akin to the prevalent monolithic non-planar ring oscillator (NPRO), however the monolithic VECSEL has several important advantages: tailored emission wavelength (via semiconductor bandgap engineering), no relaxation oscillations, no applied magnetic field, and low requirements on the pump beam quality. This approach is power-scalable in principle and could be applied to VECSELs at any of the wavelengths from the visible to the mid-infrared at which they are already available, to create a range of robust, ultra-coherent laser systems with reduced bulkiness and complexity. This is of particular interest for remote metrology and the translation of quantum technologies, such as optical clocks, from research laboratories into real world applications.

Optimization of the gain chip for optically pumped semiconductor disk lasers

Strain-compensated quantum wells (QWs) could greatly improve the performance of semiconductor disk lasers, such as improving the epitaxy quality of the semiconductor gain chip and obtaining a higher gain. To optimize the output characteristics of the semiconductor gain chip, the strain-compensated thickness, well depth, band energy, emission wavelength, and peak gain versus In and P compositions are investigated in detail. The results show that increasing the P composition of the strain-compensated layer will slightly reduce the emission wavelength, deepen the well depth, and increase the peak gain. But the P composition is not the higher the better. When designing a gain chip, it should have a pre-offset between the emission wavelength of the QWs at room temperature and the target wavelength of the laser at an intense pump, and the pre-offset values of the wavelength should be designed deliberately to guarantee the resonant periodic gain structure working normally when the temperature in the active region approaches the desired value. This theoretical investigation may provide guidance for optimizing the laser performance of semiconductor disk lasers and other QW lasers.

Thermal Characteristics of GHz Passively Mode-Locked Semiconductor Disk Laser

A 1.3 GHz repetition rate optically pumped passively mode-locked semiconductor disk laser with 5.5 ps pulse duration and 204 mW average output power at 978 nm wavelength was demonstrated. Main factors preventing the mode-locked laser from higher output power, such as the epitaxy structure of wafer and thermal characteristics of gain chip were investigated, and corresponding methods to improve the thermal performance of the laser, e.g., growing the wafer in a reversed sequence, removing substrate entirely and using a diamond as heat spreader, were proposed. The experimental results showed that both the microcavity mode of laser and the emission wavelength of QWs redshifted when the pump power increased, but their redshift rates were different. This suggested us to consider a pre-offset between the emission wavelength of QWs at room temperature and the target wavelength of laser at an intense pump, and the pre-offset values of the wavelength should be designed deliberately to guarantee the resonant periodic gain structure working properly when the temperature in the active region approaches the desired value.

Widely tunable external-cavity surface-emitting laser using various methods.

We report a widely tunable optically pumped vertical-external-cavity surface-emitting laser. The multiple quantum wells in the active region of the gain chip are generally designed to form the resonant periodic gain structure, and three different methods are used to tune the oscillating wavelength. The maximum wavelength coverage of 45 nm is obtained when a 2 mm thickness birefringent filter is introduced in a straight-line cavity, while the tuning range of 8 nm is performed by employing a 0.15 mm thickness uncoated Fabry-Perot etalon. For the first time, to the best of our knowledge, we present an 11 nm tuning range by the use of an inserted blade as the tuning element, and the related wavelength tuning mechanism is analyzed.

45 nm broadband continuously tunable semiconductor disk laser

A broadband continuously tunable semiconductor disk laser is reported in this paper. The active region of gain chip is composed of InGaAs multiple quantum wells with resonant periodic gain structure, and its fluorescence peak wavelength is around 965 nm. Using the wideband characteristics of the quantum wells in gain chip, along with the simple linear cavity that is formed by a high reflectivity external mirror, the laser has a low cavity loss and a wide tuning range. The continuously tunable laser wavelength can be obtained by inserting birefringent filters with different thickness into the cavity. When the thickness of the birefringent filter is 2 mm, the wavelength tuning range of the laser is 45 nm, the maximum output power is 122 mW, and the beam quality &lt;i&gt;M&lt;/i&gt;&lt;sup&gt;2&lt;/sup&gt; factors in the &lt;i&gt;X-&lt;/i&gt; and the &lt;i&gt;Y-&lt;/i&gt;directions are 1.00 and 1.02, respectively. The temperature characteristics of the surface-emitting spectra of gain chip and the narrowing effect of birefringent filter on laser linewidth h are also discussed.

DBR‐free semiconductor disc laser on SiC heatspreader emitting 10.1 W at 1007 nm

We report a distributed Bragg reflector-free semiconductor disc laser which emits 10 W continuous wave output power at a wavelength of 1007 nm when pumped with 40 W at 808 nm, focused into a 230 μm diameter spot on the gain chip. By introducing a birefringent filter plate in the laser cavity the wavelength could be tuned from 995 to 1020 nm. The laser consisted of a gain chip located at the beam waist of a linear concentric resonator with an output coupling of 2.15%. The gain chip consists of a 1.574-μm-thick resonant periodic gain structure, with ten In 0.13 Ga 0.87 As quantum wells embedded in strain-compensating GaAs 0.94 P 0.06 barrier layers, van der Waals bonded to a silicon carbide intra-cavity heat spreader.

Type-II vertical-external-cavity surface-emitting laser with Watt level output powers at 1.2 μm

Semiconductor laser characteristics based on type-II band-aligned quantum well heterostructures for the emission at 1.2 μm are presented. Ten “W”-quantum wells consisting of GaAs/(GaIn)As/Ga(AsSb)/(GaIn)As/GaAs are arranged as resonant periodic gain in a vertical-external-cavity surface-emitting laser. Its structure is analyzed by X-ray diffraction, photoluminescence, and reflectance measurements. The laser's power curves and spectra are investigated. Output powers at Watt level are achieved, with a maximum output power of 4 W. It is confirmed that laser operation only involves the type-II transition. A blue shift of the material gain is observed while the modal gain exhibits a red shift.

Fully microscopic modeling of mode locking in microcavity lasers

Microscopic many-body theory is employed to analyze the mode-locking dynamics of a vertical external-cavity surface-emitting laser with a saturable absorber. The quantum wells are treated microscopically through the semiconductor Bloch equations and the light field using Maxwell’s equations. Higher-order correlation effects such as polarization dephasing and carrier relaxation are approximated using effective rates fitted to second Born–Markov evaluations. The theory is evaluated numerically for vertical external cavity surface-emitting lasers with resonant periodic gain media. For a given gain, the influence of the loss conditions on the very-short pulse generation in the range above 100 fs is analyzed. Optimized operational parameters are identified.

Processing and characterisation of II–VI ZnCdMgSe thin film gain structures

Lattice-matched II–VI selenide quantum well (QW) structures grown on InP substrates can be designed for emission throughout the visible spectrum. InP has, however, strong visible-light absorption, so that a method for epitaxial lift-off and transfer to transparent substrates is desirable for vertically-integrated devices. We have designed and grown, via molecular beam epitaxy, ZnCdSe/ZnCdMgSe multi-QW gain regions for vertical emission, with the QWs positioned for resonant periodic gain. The release of the 2.7μm-thick ZnCdSe/ZnCdMgSe multi-QW film is achieved via selective wet etching of the substrate and buffer layers leaving only the epitaxial layers, which are subsequently transferred to transparent substrates, including glass and thermally-conductive diamond. Post-transfer properties are investigated, with power and temperature-dependent surface- and edge-emitting photoluminescence measurements demonstrating no observable strain relaxation effects or significant shift in comparison to unprocessed samples. The temperature dependent QW emission shift is found experimentally to be 0.13nm/K. Samples capillary-bonded epitaxial-side to glass exhibited a 6nm redshift under optical pumping of up to 35mW at 405nm, corresponding to a 46K temperature increase in the pumped region; whereas those bonded to diamond exhibited no shift in QW emission, and thus efficient transfer of the heat from the pumped region. Atomic force microscopy analysis of the etched surface reveals a root-mean-square roughness of 3.6nm. High quality optical interfaces are required to establish a good thermal and optical contact for high power optically pumped laser applications.

Growth and testing of vertical external cavity surface emitting lasers (VECSELs) for intracavity cooling of Yb:YLF

Optically-pumped vertical external cavity surface emitting lasers (VECSELs) have unique characteristics that make them attractive for use in intracavity optical cooling of rare earth doped crystals. We present the development of high power VECSELs at 1020nm for cooling ytterbium-doped yttrium lithium fluoride (Yb:YLF). The VECSEL structures use AlAs/GaAs distributed Bragg reflectors and InGaAs/GaAsP resonant periodic gain epitaxially grown by metal-organic vapor phase epitaxy. To achieve the necessary output power, we investigated thinning the substrate to improve the thermal characteristics. We demonstrated a VECSEL structure that was grown inverted, bonded to the heat sink, and the substrate removed by chemical etching. The inverted structure allows us to demonstrate 15W output with 27% slope efficiency. Wavelength tuning of 30nm around 1020nm was achieved by inserting a birefringent quartz window into the cavity. The window also narrows the VECSEL emission, going from a FWHM of 5nm to below 0.5nm at a pump power of 40W.

In-Well Pumped Blue GaN-Based Vertical-External-Cavity Surface-Emitting Lasers

We describe the properties of in-well pumped blue InGaN/GaN vertical-external-cavity surface-emitting lasers. The laser hetero-structures were deposited on bulk GaN substrates by using metal–organic vapor phase epitaxy near atmospheric pressure. The active zones are comprised of up to 20 InGaN quantum wells distributed in a resonant periodic gain configuration. High-reflectivity dielectric distributed Bragg-reflectors were used as mirrors. Lasing was achieved at a wavelength of about 440–445 nm by exclusively exciting the quantum wells with the 384 nm emission line of a dye/N2 laser. The laser threshold was about 240 kW/cm2. The small pump spot diameter of about 20 µm and the usage of dielectric mirrors result in a rather high thermal resistance, which was experimentally determined by using an all optical measurement technique based on the temperature-dependent change of the refractive index of the device.

In-well pumping of InGaN/GaN vertical-external-cavity surface-emitting lasers

In-well-pumped blue InGaN/GaN vertical-external-cavity surface-emitting lasers are demonstrated. The laser structures were grown on bulk GaN substrates by using metal-organic vapor phase epitaxy near atmospheric pressure. The active zone consisted of up to 20 InGaN quantum wells distributed in a resonant periodic gain configuration. High-reflectivity dielectric distributed Bragg-reflectors were used as mirrors. Laser emission with a single longitudinal mode at 440 nm was achieved by exclusively pumping the quantum wells with the 384 nm emission line of a dye-/N2-laser.

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.

Comprehensive modeling of vertical external cavity surface emitting laser with resonant periodic gain

One of the promising laser constructions having much attention in the past years is thin-disc semiconductor lasers with the resonant periodic gain in a form of multiple quantum wells. The 3D numerical model is developed based on combining bi-directional beam propagation method with the round-trip operator technique. The standing wave effect in resonant gain structure and diffraction in the external laser cavity are taken into account explicitly. Characteristics of optical modes are found numerically for cylindrical vertical extended cavity surface emitting laser with above-threshold pumping. Variation of distance to the external mirror is found to result in notable changes in power and optical quality of the output beam. The decisive role of gain and index non-linearities in these changes is identified.

Direct measurement of the spectral reflectance of OP-SDL gain elements under optical pumping

We report on a direct measurement method for acquiring highly precise reflectance spectra of gain elements for semiconductor disk lasers under optical pumping. The gain element acts as an active mirror, and the active mirror reflectance (AMR) was measured with a weak and tunable probe beam coincident on the gain element with a high-power pump beam. In particular, we measured the spectral AMR of a gain element designed to have a broad and flat AMR spectrum by being anti-resonant at the center wavelength and employing a parametrically optimized anti-reflection structure. We were able to confirm that this sophisticated gain element performs according to design, with an almost constant AMR of ∼103% over a wavelength range of nearly 35 nm, very well matching the simulated behavior. Such gain characteristics are useful for optically pumped semiconductor disk lasers (OP-SDLs) designed for broadband tuning and short-pulse generation through mode-locking. The measurement technique was also applied to a conventional resonant periodic gain element designed for fixed wavelength OP-SDL operation; its AMR spectrum is markedly different with a narrow peak, again in good agreement with the simulations.

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.

Simultaneous dual-wavelength generation from vertical external-cavity surface-emitting semiconductor lasers

We demonstrate dual-wavelength emission from optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) with two different gain mirror designs. The first gain mirror design (S1) generated an optical output power of 0.39 W at λS = 984 nm and 0.30 W at λL = 1042 nm. A VECSEL using the second gain mirror design (S2) generated a maximum optical output power of 0.75 W at λS = 966 nm and 1.38 W at λL = 1047 nm. VESCELs using either gain mirror design exhibited self-pulsating behaviour in the high output power regime. In the VECSEL using the S1 gain mirror design, selfpulsation started beyond the λS output power of 140 mW and λL output power of 115 mW. The corresponding output power limits in the VECSEL using the S2 gain mirror were 300 mW for both wavelengths. Keywords: vertical external-cavity surface-emitting lasers, resonant periodic gain structure, semiconductor lasers

Multi-watt 1.25 [micro sign]m quantum dot VECSEL

An InAs quantum dot (QD) vertical external cavity surface-emitting laser (VECSEL) operating at a wavelength of 1.25 µm is reported. The gain region of the laser consists of a resonant periodic gain structure using single QD layers at 12 adjacent electrical field antinodes – a design optimised to reduce strain accumulation and improve QD quality. A fibre-coupled 808 nm laser diode is used as a pump source and a spherical external mirror completes the cavity. Thermal management is accomplished by bonding the VECSEL sample to a chemical vapour deposition (CVD) diamond heat spreader. Continuous wave output powers in excess of 3 W have been achieved at room temperature. The broad emission spectrum makes this device an ideal candidate for wavelength tunable or modelocked applications.

Record pulsed power demonstration of a 2 μm GaSb-based optically pumped semiconductor laser grown lattice-mismatched on an AlAs/GaAs Bragg mirror and substrate

An optically pumped semiconductor laser resonant periodic gain structure, grown lattice-mismatched on an AlAs/GaAs Bragg mirror, exhibits a peak pulsed power of 70 W when pumped with a pulsed 1064 nm neodymium doped yttrium aluminum garnet laser.

High-power semiconductor disk laser based on InAs∕GaAs submonolayer quantum dots

An optically pumped semiconductor disk laser using submonolayer quantum dots (SML QDs) as gain medium is demonstrated. High-power operation is achieved with stacked InAs∕GaAs SML QDs grown by metal-organic vapor-phase epitaxy. Each SML-QD layer is formed from tenfold alternate depositions of nominally 0.5 ML InAs and 2.3 ML GaAs. Resonant periodic gain from a 13-fold nonuniform stack design of SML QDs allows to produce 1.4W cw at 1034nm. The disk laser demonstrates the promising potential of SML-QD structures combining properties of QD and quantum-well gain media for high-power applications.