Optimisation of distributed feedback laser biosensors

A new integrated optical sensor chip is proposed, based on a modified distributed- feedback (DFB) semiconductor laser. The semiconductor layers of different refractive indices that comprise a laser form the basis of a waveguide sensor, where changes in the refractive index of material at the surface are sensed via changes in the evanescent field of the lasing mode. In DFB lasers, laser oscillation occurs at the Bragg wavelength. Since this is sensitive to the effective refractive index of the optical mode, the emission wavelength is sensitive to the index of a sample on the waveguide surface. Hence, lasers are modelled as planar waveguides and the effective index of the fundamental transverse electric mode is calculated as a function of index and thickness of a thin surface layer using the beam propagation method. We find that an optimised structure has a thin upper cladding layer of ~0.15 mum, which according to this model gives detection limits on test layer index and thickness resolution of 0.1 and 1.57 nm, respectively, a figure which may be further improved using two lasers in an interferometer-type configuration.     

Characterization for imperfect polarizers under imperfect conditions

The principles for measuring the extinction ratio and transmittance of a polarizer are formulated by use of the principal Mueller matrix, which includes both polarization and depolarization. The extinction ratio is about half of the depolarization, and the contrast is the inverse of the extinction ratio. Errors in the extinction ratio caused by partially polarized incident light and the misalignment of polarizers can be corrected by the devised zone average method and the null method. Used with a laser source, the null method can measure contrasts for very good polarizers. Correct algorithms are established to deduce the depolarization for three comparable polarizers calibrated mutually. These methods are tested with wire-grid polarizers used in the 3-5-mum wavelength region with a laser source and also a lamp source. The contrasts obtained from both methods agree.     

Optical response of laser-doped silicon carbide for an uncooled midwave infrared detector

An uncooled mid-wave infrared (MWIR) detector is developed by doping an n-type 4H-SiC with Ga using a laser doping technique. 4H-SiC is one of the polytypes of crystalline silicon carbide and a wide bandgap semiconductor. The dopant creates an energy level of 0.30 eV, which was confirmed by optical spectroscopy of the doped sample. This energy level corresponds to the MWIR wavelength of 4.21 μm. The detection mechanism is based on the photoexcitation of electrons by the photons of this wavelength absorbed in the semiconductor. This process modifies the electron density, which changes the refractive index, and, therefore, the reflectance of the semiconductor is also changed. The change in the reflectance, which is the optical response of the detector, can be measured remotely with a laser beam, such as a He-Ne laser. This capability of measuring the detector response remotely makes it a wireless detector. The variation of refractive index was calculated as a function of absorbed irradiance based on the reflectance data for the as-received and doped samples. A distinct change was observed for the refractive index of the doped sample, indicating that the detector is suitable for applications at the 4.21 μm wavelength.     

Measurement of air refractive index fluctuation based on interferometry with two different reference cavity lengths

A measurement method based on interferometry with two different reference cavity lengths is presented and applied in air refractive index measurement in which the two cavity lengths and a laser wavelength are combined to generate two wavelength equivalents of cavity. Corresponding calculation equations are derived, and the optical path configuration is designed, which is inspired by the traditional synthetic wavelength method. Theoretical analyses indicate that the measurement uncertainty of the determined index of refraction is about 2.3×10^-8, which is mainly affected by the length precision of the long vacuum cavity and the ellipticity of polarization components of the dual-frequency laser, and the range of nonambiguity is 3.0×10^-5, which is decided by the length difference of the two cavities. Experiment results show that the accuracy of air refractive index measurement is better than 5.0×10^-8 when the laboratory conditions changes slowly. The merit of the presented method is that the classical refractometry can be also used without evacuation of the gas cavity during the experiment. Furthermore, the application of the traditional synthetic wavelength method may be extended by using the wavelength equivalents of cavity, any value of which can be easily acquired by changing cavity length rather than using actual wavelengths whose number is limited.     

Scanning flow cytometer modified to distinguish phytoplankton cells from their effective size, effective refractive index, depolarization, and fluorescence

A laser flow cytometer based on scanning flow cytometry has been assembled. The unpolarized and linearly polarized light-scattering profiles, as well as the side emitted light in different spectral bands, were measured, allowing the simultaneous and real-time determination of the effective size and the effective refractive index of each spherelike particle. Additionally, each particle could be identified from depolarization and fluorescence measured simultaneously. The tests with aqueous samples of polystyrene spheres, fluorescent or nonfluorescent, and phytoplankton cells demonstrate that the system is able to retrieve size and refractive index with an accuracy of 1% and that the depolarization and fluorescence measurements allow the classification of particles otherwise indistinguishable.     

Depolarization of diffusely reflecting man-made objects

The polarization properties of light scattered or diffusely reflected from seven different man-made samples are studied. For each diffusely reflecting sample an in-plane Mueller matrix bidirectional reflectance distribution function is measured at a fixed bistatic angle using a Mueller matrix imaging polarimeter. The measured profile of depolarization index with changing scattering geometry for most samples is well approximated by an inverted Gaussian function. Depolarization is minimum for specular reflection and increases asymptotically in a Gaussian fashion as the angles of incidence and scatter increase. Parameters of the Gaussian profiles fitted to the depolarization data are used to compare samples. The dependence of depolarization on the incident polarization state is compared for each Stokes basis vector: horizontal, vertical, 45 degrees, 135 degrees, and right- and left-circular polarized light. Linear states exhibit similar depolarization profiles that typically differ in value by less than 0.06 (where 1.0 indicates complete depolarization). Circular polarization states are depolarized more than linear states for all samples tested, with the output degree of polarization reduced from that of linear states by as much as 0.15. The depolarization difference between linear and circular states varies significantly between samples.     

Influence of Er:YAG and Nd:YAG wavelengths on laser-induced breakdown spectroscopy measurements under air or helium atmosphere

Laser-induced breakdown spectroscopy (LIBS) is widely dependent on the conditions of its implementation in terms of laser characteristics (wavelength, energy, and pulse duration), focusing conditions, and surrounding gas. In this study two wavelengths, 1.06 and 2.94 microm, obtained with Nd:YAG and Er:YAG lasers, respectively, were used for LIBS analysis of aluminum alloy samples in two conditions of surrounding gas. The influence of the laser wavelength on the laser-produced plasma was studied for the same irradiance by use of air or helium as a buffer gas at atmospheric pressure. We used measurements of light emission to determine the temporally resolved space-averaged electron density and plasma temperature in the laser-induced plasma. We also examined the effect of laser wavelength in two different ambient conditions in terms of spectrochemical analysis by LIBS. The results indicate that the effect of the surrounding gas depends on the laser wavelength and the use of an Er:YAG laser could increase linearity by limiting the leveling in the calibration curve for some elements in aluminum alloys. There is also a significant difference between the plasma induced by the two lasers in terms of electron density and plasma temperature.     

Differences in the evolution of surface-microstructured silicon fabricated by femtosecond laser pulses with different wavelength

We experimentally investigate the differences in the evolution of surface-microstructured silicon fabricated by femtosecond laser pulses with different wavelength as a function of irradiated laser energy. The results show that when laser energy absorbed by the silicon material is the same, laser pulses with a shorter wavelength can form the surface-microstructured silicon with less laser energy, while the corresponding spike height is much lower than that of laser pulses with a longer wavelength. This is because the penetration depth of the laser pulses increases exponentially at the increase of the laser wavelength. Additionally, for two laser pulses with the certain wavelength and the certain absorption efficiency of silicon, the proportional relations between their formed spike height and irradiated laser energy should be determined. In particular, the average spike height is 3 times with 8 times corresponding energy for 800 nm laser pulses than that of 400 nm. These results are a benefit for the fast and optimum-morphology preparation of microstructured silicon.     

Emission polarization of roughened glass and aluminum surfaces

Ellipsometer measurements of the effective complex refractive index at a wavelength of 10.6 μm are made on a series of glass and aluminum surfaces of increasing surface roughness. The measured values are then used to calculate the degree of emission polarization and are shown to be in agreement with the experimentally determined values when depolarization is small. Comparisons are also made with calculations based on the Kirchhoff scattering theory. Both the theory and the experimental results indicate that it is the local surface slope and not the roughness magnitude that is the prime factor in determining the degree of emission polarization from the samples studied.     

Interferometric investigation and simulation of refractive index in glass matrixes containing nanoparticles of varying sizes

The relationship between refractive index and nanoparticle radii of cadmium selenide (CdSe) nanoparticles embedded within glass matrixes was investigated experimentally and by simulations. A homemade automated Michelson interferometer arrangement employing a rotating table and a He-Ne laser source at a wavelength of 632.8 nm determined the refractive index versus nanoparticle radii of embedded cadmium selenide (CdSe) nanoparticles. The refractive index was found to decrease linearly with nanoparticle radius increase. However, one sample showed a step increase in refractive index; on spectroscopic analysis, it was found that its resonant wavelength matched that of the He-Ne source wavelength. The simulations showed that two conditions caused the step increase in refractive index: low plasma frequency and matched sample and source resonances. This simple interferometer setup defines a new method of determining the radii of nanoparticles embedded in substrates and enables refractive index tailoring by modification of exact annealing conditions.     

Fiber-Bragg-grating force sensor based on a wavelength-switching actively mode-locked erbium-doped fiber laser

A fiber-Bragg-grating (FBG) transverse-force sensor based on a wavelength-switching actively mode-locked erbium-doped fiber laser is proposed, in which a FBG is used as both the sensing element and the wavelength-selection element of the laser. When a force is applied to the FBG, the induced birefringence in the FBG causes the laser to emit pulses at two close wavelengths, whose separation is proportional to the applied force. To suppress the interference between the two wavelengths, the laser is made to emit at the two wavelengths alternately by use of a polarization-switching technique. The wavelength separation is converted into a time difference by transmission of the laser pulses through a dispersive single-mode fiber, so the wavelength measurement is replaced by the less-expensive time measurement. The output of the sensor is insensitive to temperature and axial strain changes along the FBG. To interrogate similar FBG sensing elements connected in series it is necessary only to change the modulating frequency of an electro-optic modulator to select the corresponding laser cavity. The practicability of this approach was demonstrated experimentally with two multiplexed sensing elements.     

Integrated all-optical infrared switchable plasmonic quantum cascade laser

We report a type of infrared switchable plasmonic quantum cascade laser, in which far field light in the midwave infrared (MWIR, 6.1 μm) is modulated by a near field interaction of light in the telecommunications wavelength (1.55 μm). To achieve this all-optical switch, we used cross-polarized bowtie antennas and a centrally located germanium nanoslab. The bowtie antenna squeezes the short wavelength light into the gap region, where the germanium is placed. The perturbation of refractive index of the germanium due to the free carrier absorption produced by short wavelength light changes the optical response of the antenna and the entire laser intensity at 6.1 μm significantly. This device shows a viable method to modulate the far field of a laser through a near field interaction.     

Optical properties of titanium dioxide nanoparticles/3-(2-benzothiazolyl)-7-N,N-diethylaminocoumarin/polymethyl methacrylate composite films

3-(2-benzothiazolyl)-7-N,N-diethylaminocoumarin organic laser dye-polymethyl methacrylate (PMMA) composite films doped with inorganic titanium dioxide (TiO₂) particles are fabricated by spin-coating technique. TiO₂ nanoparticles exhibit a strong influence on optical properties of the organic laser dye/PMMA composite films. The refractive index and absorbance (absorption intensity) of organic laser dye/PMMA composite film with micro- and nanoparticles of TiO₂ are reduced, compared to those without TiO₂ particles. The organic laser dye/PMMA composite film with TiO₂ nanoparticles has the lowest refractive index and absorbance values. Photoluminescence intensities of all systems exhibit a maximum peak around the excitation wavelength, close to that of the organic laser dye, at 450 nm and the minimum around the excitation wavelength of 350 nm. Photoluminescence intensity of the organic laser dye/PMMA composite film with TiO₂ microparticles is always the lowest at all excitation wavelengths. However, the photoluminescence intensity of the organic laser dye/PMMA composite film with TiO₂ nanoparticles has the highest value at excitation wavelengths of 330 and 380 nm, while the photoluminescence intensity of composite film without TiO₂ particles is more than that with nanoparticles at other excitation wavelengths.     

Laser action in polymeric gain media containing scattering particles

Optical pumping of polymer sheets containing laser dyes and TiO(2) nanoparticle scatterers results in emissions that exhibit laser behavior with linewidths as low as 4 nm. The input-output characteristics, effects of index matching on the emission, damage thresholds, wavelength stability, and possible applications of these new materials are discussed.     

A new optical wavelength ratio measurement apparatus: the fringecounting sigmameter

A new, compact and achromatic Michelson-type interferometer with a variable path difference is presented. This “fringe-counting” sigmameter allows measurement of optical wavelength ratios between a laser of unknown wavelength and a reference laser of known wavelength. This apparatus, maintained in a vacuum, measures interference order variations in two stages: integer counting of around 400000 and fractional counting (also called “excess fraction”) with an uncertainty of 10^-3. From these measurements, this “sigmameter” can determine laser wavelength from 0.36 μm to 1.5 μm with an accuracy of 1.10^-8 using a reference stabilized He-Ne laser     

Tunable single longitudinal mode S-band fiber laser using a 3 m length of erbium-doped fiber

In this paper a tunable single-longitudinal mode (SLM), short-wavelength band (S-band) fiber laser using a conventional erbium-doped fiber (EDF) with a length of 3?m and a step index erbium dopant profile as opposed to the commonly used depressed cladding erbium-doped fiber (DC-EDF) is proposed and demonstrated. The proposed SLM fiber laser has a tuning range of 1496 to 1507?nm in a ring configuration using two 0.15?m of EDF which acts as saturable absorbers (SAs). The highest peak power measured is about ?0.6?dBm at a wavelength range of 1502 to 1507?nm. The measured signal-to-noise ratio (SNR) is approximately 74?dB for the same wavelength range. The line-width of the SLM output is measured to be 140?kHz.     

Light intensification modeling of coating inclusions irradiated at 351 and 1053 nm

Electric-field modeling provides insight into the laser damage resistance potential of nodular defects. The laser-induced damage threshold for high-reflector coatings is 13x lower at the third harmonic (351 nm) than at the first harmonic (1053 nm) wavelength. Linear and multiphoton absorption increases with decreasing wavelength, leading to a lower-third harmonic laser resistance. Electric-field effects can also be a contributing mechanism to the lower laser resistance with decreasing wavelength. For suitably large inclusions, the nodule behaves as a microlens. The diffraction-limited spot size decreases with wavelength, resulting in an increase in intensity. Comparison of electric-field finite-element simulations illustrates a 3x to 16x greater light intensification at the shorter wavelength.     

Bismuth activated alumosilicate optical fibers fabricated by surface-plasma chemical vapor deposition technology

Plasma chemical technology is experimentally applied to the fabrication of a Bi-activated alumosilicate-core pure-silica-cladding fiber preform. To the best of our knowledge, this is the first time this technology has been applied in this way. We measure gain efficiency at pumping by a 1058 nm wavelength Yb fiber laser in a piece of a newly obtained fiber 20 m in length within 100-1200 nm wavelengths band. The gain efficiency reaches as high as 0.2 dB/mW. Bi-activated alumosilicate-core pure-silica-cladding fiber that is not more than 12 m in length serves a basis for a 1 W output power fiber laser emitting at the wavelength of 1160 nm with 8% slope efficiency. We also measure the photoluminescence spectrum and kinetics of Bi centers responsible for laser emission under the excitation of 193 nm wavelength ArF laser pulses.     

Measurement of the physical properties of cyclohexane using a laser interferometric technique

A laser Mach-Zehnder interferometric modified technique providing the necessary high precision measurements of the physical properties of the most frequently used solvents for laser dyes is applied. This technique offers several advantages in the determination of the absolute value of the refractive index of cyclohexane (C₆H₁₂) at 20°C for a wavelength 632.8 nm and also its thermal coefficient. About fourteen various macroscopic and microscopic physical constants that can be derived from the refractive index are presented. In comparison with the other methods described in the literature, the accuracy and sensitivity of this technique are discussed.     

Studies on the two-photon pumped upconverted fluorescence and superradiance of a new organic dye material in solutions

The linear and nonlinear optical properties of a new organic dye, trans-4-[p-(N-ethyl-N-ethylamino)-styryl]-N-methyl-pyridinium tris(thiocyanato) cadmates (II), are reported in this paper. When pumped with a picosecond laser at the wavelength range of 850-1200 nm, intense upconversion fluorescence can be obtained. The upconversion efficiencies at different pump energies were measured when pumped with a 1064-nm laser beam from a mode-locked Nd:YAG laser. The highest upconversion efficiencies were measured to be 5.8% and 7.6% in dimethyl formamide (DMF) and methanol. The lifetime of the dye in DMF was measured to be 75 ps. The strongest nonlinear absorption was at the wavelength of 940 nm, and the highest upconversion efficiency was at the wavelength of 1030 nm. The difference of the two wavelengths was caused by excited state absorption in the dye at wavelengths shorter than 1000 nm. The dye solution in DMF and methanol show a clear optical power limiting effect.