Towards realisation of an efficient continuous wave terahertz source using quantum dot devices

We demonstrate a diode laser system which is suitable for high-resolution spectroscopy in the 1.2 μm and yellow spectral ranges. It is based on a two-facet quantum dot chip in a Littrow-type external cavity configuration. The laser is tunable in the range 1125–1280 nm, with an output power of more than 200 mW, and exhibits a free-running line width of 200 kHz. Amplitude and frequency noise were characterized, including the dependence of the frequency noise on the cavity length. Frequency stabilization to a high-finesse reference cavity is demonstrated, whereby the line width was reduced to approx. 30 kHz. Using a femtosecond frequency comb, the residual frequency instability was determined and found to be below 300 Hz on the time scales 1–300 s. Yellow light (>3 mW) at 578 nm was generated by frequency doubling in an enhancement cavity containing a PPLN crystal. The source has potential application for precision spectroscopy of ultra-cold Yb atoms and cold molecular hydrogen ions.

A tunable external cavity laser (ECL) near 3.24 μm was developed using semiconductor laser gain chips based on GaSb. The type-1 interband laser diodes were grown by molecular beam epitaxy using 17 nm InGaAsSb compressively-strained quantum wells with 30 nm AlInGaAsSb quinary barriers for improved hole confinement. In continuous-wave operation with a diode temperature of 10°C, the ECLs produce up to 1.8 mW of singlemode output power with a tuning range of 60 nm. The devices are tunable through the fundamental ν 3 vibrational absorption features of methane gas, providing a platform for highly-sensitive detection of trace hydrocarbons.

We report on the unique highly-configurable wavelength tuning and switching properties of a tunable external cavity laser based on multiplexed volume holographic gratings (VHGs) and a micromirror device. The ultra-compact laser has a 3 THz bandwidth and exhibits single mode operation in either single or multiple wavelengths with narrow linewidth (<7.5 MHz), and a switching rate of 0.66 kHz per wavelength. A prototype laser exhibited 40 mW of output power for wavelengths from 776 - 783 nm. The unique discrete-wavelength-switching features and low power consumption of this laser make it well suited as a source for continuous-wave (cw) terahertz signal generation in portable photomixing systems.

A stable tunable external cavity diode laser (ECDL) which uses an intracavity electro- optic deflector incorporated into a Littrow-type external cavity as a wavelength tunable element is proposed. The deflector comprises an antireflection-coated rectangle Lead Lanthanum Zirconate Titanate (PLZT) electro-optic ceramic with a pair of parallel trapezoidal electrodes on the up and down surfaces, which can simultaneously change the cavity length and the beam-grating angle to achieve mode-hop-free tuning. The theory for mode-hop-free tuning will be presented in this paper, and the model calculations predict that the laser can be tuned continuously by as much as 110GHz. This new approach is simple in structure with no mechanical moving parts and has potential configuration of compact grating-stabilized ECDL. Keywords: Laser tuning, Mode-hop-free, Electro-optic defl ectors, External cavity diode laser, Ceramics 1. INTRODUCTION Due to the merits of single-mode, narrow-linewidth, fine tuna bility and large tunable range, external cavity diode lasers (ECDLs) have found more and more applications in high resolution spectroscopy, synthetic aperture laser radar, optical communication systems, and so on [1~3]. Usually, the wavelength of ECDLs is usually tuned by rotating a diffraction grating in the Littrow configuration [4] or a mirror in the Littman–Metcalf configuration [5] via a piezo-electrical actuator. Continuous single-mode scanning without mode hopping can be achieved by synchronous tuning of external cavity modes and grating selectivity [6]. However, the modulation frequency and reliability are restricted by the movement mechanical construction of the laser system. To avoid the drawbacks referred above, using a deflector as the tuning element without moving component in the extended cavity appears to be more attractive, among which the electro-optic crystal (EOC), acousto-optic modulator (AOM), and nematic liquid-crystal (NLC) deflectors have had a large development [7~10]. However, the mode-hop-free tuning range is limited to 10 GHz because of limitation of the high driving voltage, as reported in reference [7]. Although using two AOMs or NLC can achieve wider tuning rage (including mode hops), up to 225GHz for AOMs [9] and 12nm for NLC [10], these kinds of tuning have a drawback in application for high repetition rates.

We demonstrate the application of a commercially available widely tunable continuous-wave external cavity quantum cascade laser as a spectroscopic source for the simultaneous detection of multiple gases. We measured broad absorption features of benzene and toluene between 1012 and 1063 cm(-1) (9.88 and 9.41 microm) at atmospheric pressure using an astigmatic Herriott multipass cell. Our results show experimental detection limits of 0.26 and 0.41 ppm for benzene and toluene, respectively, with a 100 m path length for these two gases.

Compact, inexpensive laser sources in the 2-4 μm wavelength region are of growing interest for applications in environmental detection of trace gases as well as medical diagnostics, process control and combustion analysis. Many organic compounds, including hazardous pollutants, have characteristic absorption spectra in this region. Although external cavity diode lasers (ECDLs) are commercially available from 0.6 μm to 1.6 μm 1, 2, they have not yet made it into the 2 μm wavelength range. Others have recently demonstrated tunability in an external cavity at 3.3 μm3 which was not continuous or single mode. We present room temperature operation of single longitudinal and single spatial mode, external cavity tunable diode lasers operating around 2 μm. We also present high-resolution absorption measurements of CO2 over the 250 cm−1 laser tuning range.

We demonstrate Terahertz (THz) signal sources based on photoconductive (PC) antenna devices comprising active layers of InAs semiconductor quantum dots (QDs) on GaAs. Antenna structures comprised of multiple active layers of InAs:GaAs PC materials are optically pumped using ultrashort pulses generated by a Ti:Sapphire laser and CW dualwavelength laser diodes. We also characterised THz output signals using a two-antenna coherent detection system. We discuss preliminary performance data from such InAs:GaAs THz devices which exhibit efficient emission of both pulsed and continuous wave (CW) THz signals and significant optical-to-THz conversion at both absorption wavelength ranges, ≤850 nm and ≤1300 nm.

Quantum cascade lasers are unipolar semiconductor lasers based on intersubband transitions in heterostructures. These lasers, which have demonstrated continuous wave operation at room temperature in the mid-infrared spectral range, are well suited for the realization of compact, ultra-sensitive, trace-gas sensors based on absorption spectroscopy. Up to now, only distributed feedback (DFB) single-mode devices have been used for such applications. DFB quantum cascade lasers have proven to be effective for gas sensing, but their relatively narrow tuning range, smaller or equal to about 1% of the wavelength, makes them not very versatile and limits their usefulness for spectroscopic investigations. In this thesis we developed broadly tunable external cavity quantum cascade lasers. The main advantage of these sources compared with DFBs is their broader tuning range, which is limited only by the spectral bandwidth of the gain element. We particularly studied broad gain bandwidth active regions based on bound-to-continuum designs. With that kind of active region, we have demonstrated a tuning range equal to 15% of the center wavelength at ~ 10 m, which was three times broader than the best values reported in the literature at that time, as well as good performance in pulsed mode at room temperature. Using a strain-compensated bound-to-continuum design emitting near 5.2 m, we have demonstrated for the first time continuous-wave operation of an external cavity quantum cascade laser on a thermoelectric cooler. The tuning range was comparable to that of pulsed devices, but with a much better side-mode suppression ratio and a much narrower linewidth. This continuous-wave device has successfully been applied to the spectroscopy of nitric oxide in collaboration with Prof. Tittel's Laser Science Group at Rice University. High resolution absorption spectra of that gas could be acquired over a large wavelength range. We also studied heterogeneous cascade structures, that is quantum cascade structures in which the various stages emit at several different wavelengths, as a way to increase further the tuning range of external cavity quantum cascade lasers. A pulsed laser tunable from 8.2 to 10.4 m, that is over 24% of the center wavelength has been realized using an active region composed of two substacks of bound-to-continuum designs. We conclude on the feasibility of room temperature, continuous-wave, broadly-tunable external cavity quantum cascade lasers in the mid-infrared spectral range. These devices will be useful for the analysis of multi-component gases, for high resolution spectroscopic investigations in fundamental science, and for the detection of heavy molecules with broad absorption features which DFB quantum cascade lasers cannot scan entirely.

In this paper we discuss how different feedback gratings affect the tuning range and the output power of external feedback diode laser systems. A tunable high-power narrow-spectrum external-cavity diode laser system around 455 nm is investigated. The laser system is based on a high-power GaN diode laser in a Littrow external-cavity. Both a holographic diffraction grating and a ruled diffraction grating are used as feedback elements in the external cavity. The output power, spectral bandwidth, and tunable range of the external cavity diode laser system are measured and compared with the two gratings at different injected currents. When the holographic grating is used, the laser system can be tuned over a range of 1.4 nm with an output power around 530 mW. When the ruled grating is used, the laser system can be tuned over a range of 6.0 nm with an output power around 80 mW. The results can be used as a guide for selecting gratings for external-cavity diode lasers for different requirements.

We report a diffraction-limited photonic terahertz (THz) source with linewidth <10 MHz that can be used for nonlinear THz studies in the continuous wave (CW) regime with uninterrupted tunability in a broad range of THz frequencies. THz output is produced in orientation-patterned (OP) gallium arsenide (GaAs) via intracavity frequency mixing between the two closely spaced resonating signal and idler waves of an optical parametric oscillator (OPO) operating near λ = 2 μm. The doubly resonant type II OPO is based on a periodically poled lithium niobate (PPLN) pumped by a single-frequency Yb:YAG disc laser at 1030 nm. We take advantage of the enhancement of both optical fields inside a high-finesse OPO cavity: with 10 W of 1030 nm pump, 100 W of intracavity power near 2 μm was attained with GaAs inside cavity. This allows dramatic improvement in terms of generated THz power, as compared to the state-of-the art CW methods. We achieved >25 μW of single-frequency tunable CW THz output power scalable to >1 mW with proper choice of pump laser wavelength.

Terahertz (THz) imaging is increasingly used in the cultural heritage field. In particular, continuous wave (CW) and low frequency THz is attracting more attention. The first application of the THz technique inherent to the cultural heritage field dates back 10 years ago. Since 2006, tangible improvements have been conducted in the refinement of the technique, with the aim to produce clear maps useful for any art restorer. In this paper, a CW THz (0.1 THz) imaging system was used to inspect paintings on canvas both in reflection and in transmission modes. In particular, two paintings were analyzed: in the first one, similar materials and painting execution of the original artwork were used, while in the second one, the canvas layer is slightly different. Flash thermography was used herein together with the THz method in order to observe the differences in results for the textile support materials. A possible application of this method for the detection of artwork forgery requires some parameterization and analysis of various materials or thickness influence which will be addressed in a future study. In this work, advanced image processing techniques including principal component thermography (PCT) and partial least squares thermography (PLST) were used to process the infrared data. Finally, a comparison of CW THz and thermographic results was conducted.

Influence of Antireflection Coating on the Performance of Broadly Tunable Quantum Dot External Cavity Laser

The terahertz quantum-cascade (QC) VECSEL is a recently demonstrated approach to designing single-mode terahertz lasers based on the coupling of an amplifying reflect-array metasurface with an external optical cavity. The QC-VECSEL has demonstrated single-mode terahertz lasing with high output power and near-diffraction limited beam quality. The QC-VECSEL is also a natural candidate for demonstrating broadband, continuous, single-mode frequency tuning as the VECSEL's lasing frequency is determined by the length of the its external cavity, which can be mechanically tuned. In this work, we use a piezoelectric translational stage to actively adjust the length of the QC-VECSEL's external cavity and demonstrate >500 GHz of single-mode tuning around a center frequency of 3.5 THz (>20% fractional tuning). High-quality, circular beam patterns are observed with a divergence angle of ~15° throughout the tuning range, and tens of milliwatts of peak terahertz output power are observed. In order to maintain single mode behavior, the external cavity is made to be extremely short, increasing the spacing between the external cavity's neighboring longitudinal resonances. Cavity lengths as short as 250 µm have been studied, but the free-spectral range of the external cavity could not be made larger than the gain bandwidth of the metasurface, providing testament to the bandwidth of both the metasurface and the QC-gain material.

Narrow linewidth tunable diode lasers are an important tool for spectroscopic instrumentation. Conventional external cavity diode lasers are designed as laboratory instrument and do not allow hand-held operation for portable instruments. A new miniaturized type of tunable external cavity tunable diode laser will be presented. The presentation will focus on requirements on the assembly technology of micro-optic components as well as on the physical properties of such devices. Examples for the realization of this new technology will be given in the NIR for Alkaline Spectroscopy as well as in the MIR at 1908nm.

Tunable high-power blue external cavity semiconductor laser