Jia Chen

Wavelength modulation spectroscopy with a widely tunable InP-based 2.3 μm vertical-cavity surface-emitting laser

We report on the successful application of recently developed 2.3 microm InP-based vertical-cavity surface-emitting lasers with a buried tunnel junction in a wavelength modulation spectroscopy measurement for carbon monoxide (CO) detection. The electrically pumped devices operate at room temperature under cw operation with stable single-mode emission and should allow for parts in 10(6) (ppm) level resolution measurement of CO with a standard optical setup.

Stress and thermomechanical properties of amorphous hydrogenated germanium thin films deposited by glow discharge

Stress measurements of a-Ge:H thin films deposited by rf glow discharge using a large variety of deposition conditions are reported. It was observed that the stress of the films is strongly related to their structure. Tensile films are usually porous or have many defects, while compressive films are usually homogeneous. High quality films are always compressive. A strong correlation of the stress in the films with the unbonded hydrogen concentration was observed, which may explain the origin of the compressive stress. There was no systematic or consistent link between the stress and the bonded hydrogen content or the deposition rate. The thermal expansion coefficient and the elastic constant were determined for high quality films.

Experimental characterization of the frequency modulation behavior of vertical cavity surface emitting lasers

The frequency response of the current-to-wavelength tuning rate (FM response) was measured for two different vertical cavity surface emitting lasers (VCSELs) up to frequencies of 1MHz: GaAs-based VCSEL (763nm) and InP-based VCSEL (1853nm). Both of them show the same qualitative FM response behavior, which can be described by a square root law and therefore cannot be modeled by a first order low pass. The square root law behavior is a significant advantage for laser spectroscopy applications with VCSELs because the decrease of the current-to-wavelength tuning coefficient is less severe as in the case of the first order low pass.

Room‐temperature 1.5 μm luminescence of co‐deposited erbium and germanium

Nanocrystalline germanium films containing erbium were deposited by thermal evaporation under 0.6 Torr of argon onto crystalline silicon wafer substrates. Weak broad photoluminescence (PL) around 1.5 μm was observed at room temperature. Annealing under 10−7 Torr of vacuum for 3 h at 500 °C produced no change in the PL spectrum. After 1 h oxidation in air at 500 °C the PL intensity increased by an order of magnitude with reduction of the spectral linewidth and appearance of distinct structures, a portion of which is similar to that observed for Er‐implanted Si:O. Subsequent increase in oxidation time reduced the PL intensity slightly with no change in the spectral shape. The PL intensity exhibits a sublinear increase with pump power and approaches saturation at 200 mW. Raman spectra before and after anneal are also presented. Annealing increased the average grain size from 5 to 10 nm. The PL spectrum of erbium metal after oxidation in air at 500 °C is quite different from that of these oxidized Ge:Er films.

Low-level and ultralow-volume hollow waveguide based carbon monoxide sensor

We demonstrate an ultralow sample volume optical carbon monoxide sensor with detection sensitivity of 180 parts in 10(9) (1σ at 1 Hz). The utilization of a 2.3 μm surface-emitting laser directly coupled to a 3 m hollow capillary fiber as the gas cell is proven to be a compact, sensitive, and cost-efficient gas sensing concept. By mechanical vibration of the fiber, an absorbance resolution of 10(-5) is achieved, which is comparable to single-reflective (double-pass) cells. An improvement of sensitivity over the conventional single-reflective cell is thus approximately linearly scaled with the enhancement of the optical path length, which is usually more than 1 order of magnitude.

Phase rainbow refractometry for accurate droplet variation characterization

We developed a one-dimensional phase rainbow refractometer for the accurate trans-dimensional measurements of droplet size on the micrometer scale as well as the tiny droplet diameter variations at the nanoscale. The dependence of the phase shift of the rainbow ripple structures on the droplet variations is revealed. The phase-shifting rainbow image is recorded by a telecentric one-dimensional rainbow imaging system. Experiments on the evaporating monodispersed droplet stream show that the phase rainbow refractometer can measure the tiny droplet diameter changes down to tens of nanometers. This one-dimensional phase rainbow refractometer is capable of measuring the droplet refractive index and diameter, as well as variations.

GaSb and InP-based VCSELs at 2.3 μm emission wavelength for tuneable diode laser spectroscopy of carbon monoxide

We present long-wavelength buried tunnel junction (BTJ) VCSELs for emission wavelengths around 2.3 μm. Two different device concepts have been realized utilizing either InP- or GaSb-based materials. The InP-VCSELs are based on a BTJ-design which has been well-proven for wavelengths up to 2 μm in recent years. To extend this range up to emission wavelengths around 2.3 μm, the main focus is set on an optimization of the active region. In this context, we use a graded and heavily strained quantum well design in conjunction with optimized growth conditions. The photoluminescence and x-ray characterization shows a very good material quality. Room-temperature operated VCSELs exhibit around 0.5 mW of output power with singlemode-emission at 2.36 μm representing the longest wavelength that has been achieved with InP-based interband lasers so far. GaSb-based devices comprise an epitaxial back mirror and a dielectric output mirror while the basic BTJ-principle is maintained. Using GaInAsSb quantum wells, the active region reveals excellent gain characteristics at 2.3 μm. Singlemode VCSELs show room temperature threshold currents around 1 mA and output powers of 0.7 mW, respectively. Both laser types have been implemented in a tuneable diode laser spectroscopy (TDLS) setup to evaluate their capability for sensing of carbon monoxide. Using an absorption path length of only 10 cm, concentration measurements down to a few ppm have been successfully demonstrated.

Resolution limits of laser spectroscopic absorption measurements with hollow glass waveguides.

In this paper, resolution limits of laser spectroscopy absorption measurements with hollow capillary fibers are investigated. Furthermore, a concept of sensitive near-infrared sensing utilizing hollow fiber directly coupled with vertical-cavity surface-emitting lasers is developed. By performing wavelength modulation spectroscopy, the smallest absorbance that can be detected by the fiber sensor was determined to be 10(-4), limited by a random modulation of the fiber transmission function (modal noise). By mechanically vibrating the fiber, a sensor resolution of 10(-5) in absorbance is achieved. Because the random modulation on the fiber transmission function limits the detection sensitivity, its physical reasons are analyzed. One contribution is found to be the partial integration of the far field, and the amplitude of the spectral features is inversely proportional to the square root of the integrated speckle points number. Therefore, careful design of the fiber-detector outcoupling is necessary. It turned out that incoupling alignment is not of much influence with respect to the spectral background. The residual spectral background is caused by mode-dependent effects and can be lowered by vibrating the fiber mechanically.

The effects of sulphur incorporation on the optoelectronic properties of hydrogenated amorphous germanium

Abstract Hydrogenated amorphous germanium-sulphur (a-G1 −xSx : H) alloys have been prepared by the glow-discharge technique from mixtures of hydrogen, germane and sulphur hexafluoride gases. The sulphur content of the films prepared at the powered and the unpowered electrodes varied between 2 and 30 at.%. Addition of sulphur decreases the activation energy of the conductivity from a typical value of 0.5 eV for hydrogenated amorphous germanium (a-Ge: H) to minima of 0.14 and 0.28 eV for the alloys deposited on the unpowered electrode and powered electrode respectively. Concurrent with this, for the cathode-deposited films, the ημτ product of electrons is increased by one order of magnitude to a maximum of 2 × 10−6 cm2 V −1, and the ambipolar diffusion length decreases from 470 to 350 A. From these trends, and with a consideration of various additional experimental findings, it is inferred that n-type doping occurs when sulphur is added to a-Ge: H. Sulphur incorporation improves the quality of films prepare...

Simplified Model of the Dynamic Thermal Tuning Behavior of VCSELs

In this letter, a simplified thermal model is used to derive a closed-form expression for the frequency dependency of the current-to-wavelength tuning coefficient (FM response) of vertical-cavity surface-emitting lasers (VCSELs). It explains the experimentally observed square root law and shows very good agreement with measured FM responses of two different VCSEL types. It yields significantly improved accuracy compared to the usually assumed first-order low-pass model.