Miniature self-aligned external cavity tunable single frequency laser for THz generation

A continuous wave (CW) terahertz source emitting in a broad frequency range (1-5THz) is promising towards achieving a compact, high power, finely tunable, room temperature terahertz generation system which will be of immense significance towards the realisation of terahertz applications in spectroscopy, communication, sensing, and imaging among others. We have demonstrated a tunable continuous-wave Quantum Dot external cavity laser emitting at two frequencies for continuous wave terahertz emission in a Quantum dot Photoconductive Antenna (PCA). The external cavity QD Laser has been characterised with tunability of 152nm and a tuning range from 1143nm -1295.8nm that lies within the THz difference frequency for the generation of THz radiation from QD based PCAs.

The wavelength tuning properties of a tunable external cavity laser based on multiplexed volume holographic gratings and a commercial micromirror device are reported. The 3x3x3 cm(3) laser exhibits single mode operation in single or multi colors between 776 nm and 783 nm with less than 7.5 MHz linewidth, 37 mW output power, 50 μs rise/fall time constant and a maximum switching rate of 0.66 KHz per wavelength. The unique discrete-wavelength-switching features of this laser are also well suited as a source for continuous wave Terahertz generation and three-dimensional metrology.

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.

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.

Diffraction waveguides are widely used in augmented reality devices as information display systems for the introduction of data into the human visual field in order to supplement information about the world around us. This paper formulates the principles of radiation conversion in diffraction waveguides made of photo-thermo-refractive glass on the basis of multiplexed volume holographic gratings, and the advantages and disadvantages of high spectral-angular selectivity are analyzed. In the optical scheme, each of the superimposed volume holographic gratings in the diffraction waveguide forms a corresponding part of the composite angular field of view of the augmented reality device. A proposed mathematical model based on angular multiplexing made it possible to synthesize the diffraction optical element for a new type of diffraction waveguide made from photo-thermo-refractive glass and to create a prototype with an angular resolution of at least 3.0 ± 0.5′, with a brightness change in the image of less than 20% and with a composite angular field of view of 32°.

High coherence telecom systems and more recently data center applications require tunable external cavity lasers with very narrow linewidth and reliability over whole device lifetime. Tunability by using Liquid Crystal on Silicon component is a desirable approach with no moving part, able to grant linewidth lower than 100 KHz and Telcordia qualifiable devices. The use of subassemblies built up in parallel, together with a cheap glue-based fixing technique is attractive due to its orientation towards high volume industrial production. The issue of incomplete glue cross-linking when using curing temperatures lower than liquid crystal clearing temperature is demonstrated through a low-cost experimental approach, and a solution is given, sustained by Avrami-Erofe’ev kinetic model.

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.

A noncontact 3D imaging technique based on tunable lasers is investigated to assess its performance compared to commercially available methods. In this technique, an object is flood illuminated by an external cavity tunable diode laser. As the laser frequency is scanned, the time-varying speckle-intensity pattern provides information about the depth of the scattering object. The patterns are recorded with a CCD camera, and the object's height profile is then extracted from the 3D fast Fourier transform. This paper presents the first quantitative comparison of results using this technique with those from a well-known standard instrument--the coordinate measuring machine. The object used for the comparison is a pressed sheet-metal part with dimensions of approximately 1-- by 100 by 20 mm. We found the standard deviation (sigma) of the difference between the two profiles to be less than 0.2 (Delta) z, where (Delta) z is the raw range resolution of the speckle-pattern-sampling technique.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Tunable high-power blue external cavity semiconductor laser

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.

The limited availability of rugged and economical infra-red laser sources that can be tuned over a certain wavelength range is still a bottleneck for widespread application of spectroscopic methods to environmental, industrial, and medical trace gas monitoring. Conventional semiconductor lasers in distributed feedback (DFB) configuration offer only limited wavelength tuning around the center wavelength. External cavity lasers (ECLs) can provide the large tuning ranges needed for laser spectroscopy. Micro-electro-mechanical systems (MEMS) technology is a viable option for integrating and manipulating optical elements and thus fabricating a hybrid integrated ECL at chip level dimensions. With this thesis, I present a strong foundation for building a widely tunable MEMS based ECL. The major contributions are: 1) design and fabrication of different rotary comb actuators to build micro-opto-electromechanical systems (MOEMS) devices for various applications, including ECLs; 2) development of two silicon sidewall smoothing techniques which can provide optical quality reflecting surfaces; and 3) design of an extremely long arm MEMS actuator with zipped comb fingers and structural re-enforcement, and development of an efficient method of mounting external micro mirrors onto MEMS devices. The first rotary comb actuator presented in this thesis provides a large deflection angle of 3° at a, for MEMS technology, reasonably low voltage of 100 V. This actuator has a high resonance frequency (>1 kHz) and can be used for optical applications where kHz level frequency operation is required. The virtual pivot point actuator has a pivot point located distant from the physical actuator and offers more options for in-plane optical path alignment. The detailed design and the implementation of the virtual pivot point actuator are presented. The full rotation angle for this actuator is 3° (± 1.5°) for an applied voltage of 190 V. Both of these designs provide an accessible sidewall which can be used as a reflecting surface for on-chip integration. The sidewalls of the actuators were smoothed using either an over-passivated or a two-step oxidation + BOE (buffered oxide etch) technique. The over-passivated recipe uses controlled over-passivation during reactive ion etching with the Bosch process to obtain a smooth sidewall. An rms roughess of as low as 3.93 nm was achieved for the etched sidewall of the rotary comb actuator using this process. The second process uses a thermal oxidation step followed by buffered oxide etching. Repeating this process and using a carefully chosen temperature and oxidation time resulted in the desired optical quality sidewall. This method has no theoretical limitations in terms of smoothing thick structures and the rms roughness achieved for a 100 µm thick silicon sidewall was 2.56 nm. A third actuator was designed with a long movable arm that can support relatively large externally mounted optical components. This actuator will be especially useful for optical applications that require larger optical beams and therefore also large reflectors. For an ECL, a large, high quality external mirror can maximize the reflected light fed back into the gain medium and increase the wavelength tuning range.

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.

We demonstrate a 10-Gb/s tunable external cavity laser with a polymer Bragg reflector and a highly integrated semiconductor gain chip including an asymmetric ring resonator reflector. The output power of the tunable laser is increased by adjusting the coupling strengths of two multimode interferences (MMIs) of the ring resonator reflector. The output power of the tunable laser is 6 mW at gain and semiconductor optical amplifier currents of 100 mA, and at a temperature of 25°C when the cross-coupling strengths ratios of two MMI couplers are 0.36 and 0.5. The tunable laser successfully provides 10-Gb/s transmissions through 20-km standard single mode fiber (SMF). The power penalty of the tunable laser is less than 2 dB after a 20-km SMF transmission for a 16-nm tuning range. The power penalty variation is less than 2 dB during the blue-shifted wavelength tuning.

Broad continuous frequency tuning of a diode laser with an external cavity

A robust biomedical sensor for ultrasensitive detection of biomarkers in breath based on a tunable external cavity laser (ECL) and an off-axis cavity-enhanced absorption spectroscopy (OA-CEAS) using an amplitude stabilizer is developed. A single-mode, narrow-linewidth, tunable ECL is demonstrated. A broadly coarse wavelength tuning range of 720 cm⁻¹ for the spectral range between 6890 and 6170 cm⁻¹ is achieved by rotating the diffraction grating forming a Littrow-type external-cavity configuration. A mode-hop-free tuning range of 1.85 cm⁻¹ is obtained. The linewidths below 140 kHz are recorded. The ECL is combined with an OA-CEAS to perform laser chemical sensing. Our system is able to detect any molecule in breath at concentrations to the ppbv range that have absorption lines in the spectral range between 1450 and 1620 nm. Ammonia is selected as target molecule to evaluate the performance of the sensor. Using the absorption line of ammonia at 6528.76 cm⁻¹, a minimum detectable absorption coefficient of approximately 1×10⁻⁸ cm⁻¹ is demonstrated for 256 averages. This is achieved for a 1.4-km absorption path length and a 2-s data-acquisition time. These results yield a detection sensitivity of approximately 8.6×10⁻¹⁰ cm⁻¹ Hz(-1/2). Ammonia in exhaled breath is analyzed and found in a concentration of 870 ppb for our example.