Room Temperature Continuous Wave Research Articles

Diode-pumped continuous wave Tm:Lu2SiO5 laser with narrow linewidth output

A room-temperature narrow linewidth continuous wave (CW) Tm:Lu2SiO5 laser is investigated by using a Volume Bragg grating and a Fabry-Perot etalon. The output wavelength of 2054.24nm and a linewidth of 0.12nm were obtained experimentally, and 1.64W of output power was achieved under an absorbed pump power of 9.2W, corresponding to an optical-to-optical conversion efficiency of 17.8% and a slope efficiency of 23.6%. The laser beam quality was optimized and measured to be M2∼1.2 in the x-axis direction and 1.3 in the y-axis direction, respectively.

Progress in high-power continuous-wave quantum cascade lasers [Invited

Multi-watt continuous-wave room temperature operation with efficiency exceeding 10% has been demonstrated for quantum cascade lasers essentially in the entire mid-wave and long-wave infrared spectral regions. Along with interband cascade lasers, these devices are the only room-temperature lasers that directly convert electrical power into mid- and long-infrared optical power. In this paper, we review the progress in high-power quantum cascade lasers made over the last 10 years. Specifically, an overview of the most important active region, waveguide, and thermal design techniques is presented, and various aspects of die packaging for high-power applications are discussed. Prospects of power scaling with lateral device dimensions for reaching optical power level in the range from 10W to 20W are also analyzed. Finally, coherent and spectral beam-combining techniques for very high-power infrared platforms are discussed.

Efficient continuous wave and quasi-continuous wave operation of a 2.8 μm Er:Lu2O3 ceramic laser.

We have demonstrated a highly efficient 2.8 μm Er-doped Lu2O3 ceramic laser and investigated the lasing dynamics by time-resolved spectroscopy. During room-temperature continuous wave operation, a slope efficiency of 22% was achieved with a high-quality transparent ceramic. To our knowledge, this is the highest slope efficiency obtained by an Er:Lu2O3 ceramic laser. In addition, an output peak power of 1.2 W was obtained during quasi-continuous wave operation. Time-resolved spectroscopy showed that the emission wavelengths exhibited a red shift from 2715 to 2845 nm, which indicated that continuous wave operation may be possible at 2740 and 2845 nm.

Room temperature continuous wave quantum dot cascade laser emitting at 7.2 μm.

We demonstrate a quantum cascade laser with active regions consisting of InAs quantum dots deposited on GaAs buffer layers that are embedded in InGaAs wells confined by InAlAs barriers. Continuous wave room temperature lasing at the wavelength of 7.2 μm has been demonstrated with the threshold current density as low as 1.89 kA/cm2, while in pulsed operational mode lasing at temperatures as high as 110 °C had been observed. A phenomenological theory explaining the improved performance due to weak localization of states had been formulated.

Improvement of Buried Grating DFB Quantum Cascade Lasers by Small-Angle Tapered Structure

Small-angle tapered waveguide structure combining with the sampled grating was adopted to improve the output power and achieve room temperature continuous-wave (CW) operation of buried grating distributed feedback quantum cascade lasers (DFB QCLs). By utilizing the first-order channel of sampled grating, single-mode DFB QCLs at four different wavelengths were fabricated simultaneously. At 298 K, all four devices can emit more than 130 mW optical power in CW operation. Fundamental transverse mode operation with excellent beam quality was preserved.

High power, low divergent, substrate emitting quantum cascade ring laser in continuous wave operation

We demonstrate a surface grating coupled substrate emitting quantum cascade ring laser with high power room temperature continuous wave operation at 4.64 μm. A second order surface metal/semiconductor distributed-feedback grating is used for in-plane feedback and vertical out-coupling. A device with 400 μm radius ring cavity exhibits an output power of 202 mW in room temperature continuous wave operation. Single mode operation with a side mode suppression ratio of 25 dB is obtained along with a good linear tuning with temperature. The far field measurement exhibits a low divergent concentric ring beam pattern with a lobe separation of ∼0.34°, which indicates that the device operates in fundamental mode (n = 1).

Laterally coupled distributed feedback lasers emitting at 2 μm with quantum dash active region and high-duty-cycle etched semiconductor gratings

Single-mode diode lasers on an InP(001) substrate have been developed using InAs/In0.53Ga0.47As quantum dash (Qdash) active regions and etched lateral Bragg gratings. The lasers have been designed to operate at wavelengths near 2 μm and exhibit a threshold current of 65 mA for a 600 μm long cavity, and a room temperature continuous wave output power per facet >5 mW. Using our novel growth approach based on the low ternary In0.53Ga0.47As barriers, we also demonstrate ridge-waveguide lasers emitting up to 2.1 μm and underline the possibilities for further pushing the emission wavelength out towards longer wavelengths with this material system. By introducing experimentally the concept of high-duty-cycle lateral Bragg gratings, a side mode suppression ratio of >37 dB has been achieved, owing to an appreciably increased grating coupling coefficient of κ ∼ 40 cm−1. These laterally coupled distributed feedback (LC-DFB) lasers combine the advantage of high and well-controlled coupling coefficients achieved in conventional DFB lasers, with the regrowth-free fabrication process of lateral gratings, and exhibit substantially lower optical losses compared to the conventional metal-based LC-DFB lasers.

High-power dual-wavelength lasing in bimodal-sized InGaAs/GaAs quantum dots.

In this paper, we demonstrate high power, dual-wavelength (dual-λ) lasing stemming from bimodal-sized InGaAs/GaAs quantum dots (QDs). The device exhibits simultaneous dual-λ lasing at 1015.2 nm and 1023.0 nm with total power of 165.6 mW at 700 mA under room temperature continuous wave (CW) mode. Gaussian fitting analyses of the electroluminescence (EL) spectrum attribute the excellent performance to independent carrier transitions from the first excited states of large dot ensemble (LD ES1) and small dot ensemble (SD ES1), respectively. This formation provides a new possibility to achieve high power dual-λ operation only using Fabry-Pérot (FP) cavity, which is significant for compact size and low fabrication cost.

Above room temperature continuous wave operation of a broad-area quantum-cascade laser

We describe the design and implementation of a broad-area (w ≈ 30 μm) quantum-cascade laser operating in a continuous wave mode up to heat-sink temperatures beyond +100 °C. The room-temperature emission wavelength is 4.6 μm. The temperature gradient in the active region of such a wide laser stripe is essentially perpendicular to the epitaxial layers and the resulting steady-state active region temperature offset scales approximately with the square of the number of cascades. With only 10 cascades in the active region, the threshold electrical power density in the current quantum-cascade laser in the continuous-wave mode is as low as Vth × Ith = 3.8 V × 0.9 kA/cm2 = 3.4 kW/cm2 at room temperature for 2 mm-long two-side high-reflectivity coated laser stripe. A 4 mm-long one-side high-reflectivity coated laser stripe delivers in continuous-wave mode above 0.6 W at +20 °C and above 1.3 W at −27 °C (cooled with a single-stage Peltier element). A 2 mm-long two-side high-reflectivity coated laser stripe demonstrates continuous-wave lasing up to at least +102 °C (375 K). The thermal conductance, Gth, ranges between 235 W/K cm2 and 140 W/K cm2 for temperatures between −33 °C and +102 °C. This demonstration opens the route for continuous-wave power scaling of quantum-cascade lasers via broad-area laser ridges.

Simultaneously Photoacoustic Measurement of Carbon Dioxide and Nitrous Oxide Using a Quantum Cascade Laser

In this paper a photoacoustic senor for carbon dioxide and nitrous oxide detection is described which uses a quantum cascade laser. The sensor relies on a 4.43 μm continuous-wave room temperature quantum-cascade laser source and a homemade photoacoustic cell based on a cylindrical acoustic resonator. Primary laboratory tests have been performed for estimation of the achievable detection limits and possible applications for in situ and real time atmosphere measurements. It is demonstrated that the minimum detectable concentration of 13CO2 and N2O under laboratory conditions is 8 ppbv and 0.45 ppbv, respectively.

Room temperature continuous wave operation of InAs-based quantum cascade lasers at 15 µm.

We report low threshold InAs/AlSb quantum cascade lasers emitting near 15 µm. The devices are based on a vertical design similar to those employed previously in far infrared InAs-based QCLs, whereas the doping level of the active core is considerably decreased. The lasers exhibit a threshold current density as low as 730 A/cm2 in pulsed mode at room temperature and can operate in this regime up to 410K. The continuous wave regime of operation has been achieved in these devices at temperatures up to 20°C. The cw regime is demonstrated for InAs-based QCLs for the first time at room temperature.

Low-threshold external-cavity quantum cascade laser around 7.2 μ m

Room-temperature continuous wave (CW) operation of a tunable external-cavity quantum cascade laser (EC-QCL) at center wavelength around 7.2 μm is presented. The EC-QCL was implemented in a Littrow configuration. The gain chip is based on a diagonal bound-to-continuum design with a high-reflection coating on the back facet. A two-layer antireflection (AR) coating consisting of Al2O3 and ZnSe was designed and deposited on the front facet of the chip to suppress the Fabry–Perot modes. With this AR coating, single-mode tuning range of 128 cm−1 was achieved, from 1346.7 to 1475.3 cm−1 (6.78 to 7.43 μm). High side-mode suppression ratio over 30 dB was achieved near the center gain region. A very low-threshold current density of 0.89 kA/cm2 and a high output power of 50 mW were obtained when the EC-QCL was operated in CW mode at 20°C.

Room-temperature CW operation of a nitride-based vertical-cavity surface-emitting laser using thick GaInN quantum wells

We demonstrated a room-temperature (RT) continuous-wave (CW) operation of a GaN-based vertical-cavity surface-emitting laser (VCSEL) using a thick GaInN quantum well (QW) active region and an AlInN/GaN distributed Bragg reflector. We first investigated the following two characteristics of a 6 nm GaInN 5 QWs active region in light-emitting diode (LED) structures. The light output power at a high current density (∼10 kA/cm2) from the 6 nm GaInN 5 QWs was the same or even higher than that from standard 3 nm 5 QWs. In addition, we found that hole injection into the farthest QW from a p-layer was sufficient. We then demonstrated a GaN-based VCSEL with the 6 nm 5 QWs, resulting in the optical confinement factor of 3.5%. The threshold current density under CW operation at RT was 7.5 kA/cm2 with a narrow (0.4 nm) emission spectrum of 413.5 nm peak wavelength.

Compact TDLAS based optical sensor for ppb-level ethane detection by use of a 3.34 μm room-temperature CW interband cascade laser

A mid-infrared ethane (C2H6) sensor based on a wavelength modulation spectroscopy (WMS) technique was developed using a thermoelectrically cooled (TEC), continuous-wave (CW) interband cascade laser (ICL) emitting at 3.34μm and a dense multi-pass gas cell (MPGC, 17×6.5×5.5cm3) with a 54.6m optical path length. A compact optical sensor system with a physical size of 35.5×18×12.5cm3 was designed and constructed. An ICL was employed for targeting a strong C2H6 line at 2996.88cm−1 at <100Torr gas pressure in the fundamental absorption band of C2H6. The sensor performance, including the minimum detection limit (MDL) and the stability were improved by reducing the effect of laser power drift by means of the 2f/1f-WMS technique. A MDL of ∼1.2 parts per billion (ppbv) for 2f-WMS and ∼1.0ppbv for 2f/1f-WMS were achieved, respectively, with a measurement time of 4s. The MDL was further improved from 299pptv (@108s for 2f-WMS) to 239pptv (@208s for 2f/1f-WMS), based on an Allan deviation analysis. The rise time (@0→100ppbv) and fall time (@100→0ppbv) were determined to be ∼64s and ∼48s, respectively, at a gas pressure of <100Torr for the C2H6 sensor operation.

Room temperature continuous wave, monolithic tunable THz sources based on highly efficient mid-infrared quantum cascade lasers.

A compact, high power, room temperature continuous wave terahertz source emitting in a wide frequency range (ν ~ 1–5 THz) is of great importance to terahertz system development for applications in spectroscopy, communication, sensing, and imaging. Here, we present a strong-coupled strain-balanced quantum cascade laser design for efficient THz generation based on intracavity difference frequency generation. Room temperature continuous wave emission at 3.41 THz with a side-mode suppression ratio of 30 dB and output power up to 14 μW is achieved with a wall-plug efficiency about one order of magnitude higher than previous demonstrations. With this highly efficient design, continuous wave, single mode THz emissions with a wide frequency tuning range of 2.06–4.35 THz and an output power up to 4.2 μW are demonstrated at room temperature from two monolithic three-section sampled grating distributed feedback-distributed Bragg reflector lasers.

Room temperature continuous wave operation of injection quantum dot microdisk lasers

We present results of characterization of injection microdisk lasers with InAs/InGaAs self-organized quantum dot active region grown on a GaAs substrate. The minimal microdisk diameter is 15 μm. The microlasers operate in continuous wave regime at room temperature without external cooling. Lasing wavelength is around 1.26... 1.27 μm, minimal threshold current is 1.6 mA (threshold current density 900 A/cm2). Specific thermal impedance is estimated to be 5·0-3 °C·cm2/W.

Ppb-level formaldehyde detection using a CW room-temperature interband cascade laser and a miniature dense pattern multipass gas cell.

A ppb-level formaldehyde (H2CO) sensor was developed using a thermoelectrically cooled (TEC), continuous-wave (CW) room temperature interband cascade laser (ICL) emitting at 3.59 μm and a miniature dense pattern multipass gas cell with >50 m optical path length. Performance of the sensor was investigated with two measurement schemes: direct absorption (DAS) and wavelength modulation spectroscopy (WMS). With an integration time of less than 1.5 second, a detection limit of ~3 ppbv for H2CO measurement with precision of 1.25 ppbv for DAS and 0.58 ppbv for WMS, respectively, was achieved without zero air based background subtraction. An Allan-Werle variance analysis indicated that the precisions can be further improved to 0.26 ppbv @ 300s for DAS and 69 pptv @ 90 s for WMS, respectively. A side-by-side comparison between two measurement schemes is also discussed in detail.

Low-threshold-current operation of membrane distributed-feedback laser with surface grating bonded on Si substrate

A lateral-current-injection (LCI) membrane distributed-feedback (DFB) laser with a surface grating structure (incorporating a slight step) emitting in the 1550 nm wavelength range was fabricated on a Si substrate by the adhesive bonding of benzocyclobutene. For a semiconductor membrane thickness of 158 nm and a surface grating depth of 30 nm, a threshold current of as low as 390 µA, with a corresponding threshold current density of 540 A/cm2, was obtained for a cavity length of 360 µm and a stripe width of 0.2 µm under a room-temperature continuous-wave (RT-CW) condition.

Interview

Dr Fedor Zubov of St Petersburg Academic University in Russia talks about the work behind the paper ‘Suppression of sublinearity of light–current curve in 850 nm quantum well laser with asymmetric barrier layers’ page 1106. Dr Fedor Zubov My research relates to the development of modern injection lasers and my current interests include diode lasers of novel design – lasers with asymmetric barriers (LABs). LAB-concept was proposed in 2003 by a theorist, a member of our research team and co-author of our Letter - Levon Asryan, a professor at Virginia Tech. In such lasers, special, thin (several nanometers) asymmetric barrier layers (ABLs) on both sides of the active region block undesired transport of charge carriers, suppressing parasitic recombination of charge carriers in the waveguide. After almost 10 years our research team has realised this concept for the first time. It was a part of my Ph.D. research that I carried out in the Nanophotonics Lab under supervision of Prof. A.E. Zhukov. Recently, in 2013, the scientific community celebrated the 50th anniversary of semiconductor laser invention and now there is no sphere where they are not used. Among the most significant applications are data transmission, material processing and various medical applications. Today, diode lasers continue to be intensively investigated, new promising spectral ranges are being developed, and lasers of nondiode type are being displaced by them. In respect to application of our short-wave 0.85 µm LABs, the most interesting application seems to be high-power laser diodes used for optical pumping of nondiode lasers. Here, the problem of parasitic recombination is especially challenging. Our previous calculations have shown that the implementation of ABLs should improve device power characteristics owing to a decrease of current component related to parasitic recombination. Actually, within this work we are confirming this thesis experimentally. Our LABs, in comparison to a reference laser of traditional design, have demonstrated a significant improvement of the LCC linearity, so that the maximal output power was reached with 60% less pumping. The concept of asymmetric barrier lasers is universal, in the sense that it can be applied to diode lasers of different material systems and active region types. We have shown that suppression of parasitic recombination due to ABLs allows enhancement of main device characteristics: threshold current, efficiency, output optical power and temperature stability. This approach does not complicate the technology. In fact, temperature stability improvement would significantly simplify diode laser temperature-stabilisation systems. Such devices will provide a basis for new applications of injection lasers under elevated temperature conditions. A previous approach for suppression of parasitic recombination used a thinner waveguide in combination with a larger bandgap, resulting in the increase of vertical beam divergence and the decrease of COMD threshold. The attributes that should be improved to meet toughening application requirements are the lasing power, efficiency, reliability, brightness and cost. If we consider a 100 µm aperture single emitter for continuous wave room temperature (RT) operation, the present demand is 20 W. In the last 10 years the annual growth was 15% per year, and an achievement of this magnitude in the next 10 years seems to be realistic. Much more challenging are future diode laser pump systems for fusion energy installations. To achieve the required ∼100 GW pump power, the system should consist of ∼200 Mbars with 500W/bar at $0.01/W price. This could be achieved by using uncooled diodes, while maintaining the high efficiency and output power. I believe, in this regard, our lasers of ABL-design with improved temperature stability and power characteristics look very attractive. One part of our research is the development of whispering gallery mode microcavity quantum-dot lasers. These tiny (∼1-10 µm) semiconductor lasers are promising light sources for application in photonic ICs, biological sensing and imaging. Recently, we demonstrated RT lasing in the 1.3 µm wavelength range for microlasers as small as 1 µm, achieved under optical pumping. Our current research is the study of lasing under electrical pumping in such lasers. Another research area is the development of novel waveguide designs. We are targeting the realisation of symmetrical far-field radiation profiles with low divergence, which is necessary for effective coupling of laser emission to optical fibres. The current structure uses additional tunnel-coupled passive waveguide, suppressing high-order optical modes.

Continuous wave room temperature external ring cavity quantum cascade laser

An external ring cavity quantum cascade laser operating at ∼5.2 μm wavelength in a continuous-wave regime at the temperature of 15 °C is demonstrated. Out-coupled continuous-wave optical powers of up to 23 mW are observed for light of one propagation direction with an estimated total intra-cavity optical power flux in excess of 340 mW. The uni-directional regime characterized by the intensity ratio of more than 60 for the light propagating in the opposite directions was achieved. A single emission peak wavelength tuning range of 90 cm−1 is realized by the incorporation of a diffraction grating into the cavity.