Highly Sensitive Sensor Based On Intracavity Laser Absorption Spectroscopy By Means of Relative Intensity Noise

This thesis concerns with the main aspects of medical trace molecules detection by means of intracavity laser absorption spectroscopy (ICLAS), namely with the equirements for highly sensitive, highly selective, low price, and compact size sensor. A novel two modes semiconductor laser sensor is demonstrated. Its operation principle is based on the competition between these two modes. The sensor sensitivity is improved when the sample is placed inside the two modes laser cavity, and the competition between the two modes exists. The effects of the mode competition in ICLAS are discussed theoretically and experimentally. The sensor selectivity is enhanced using external cavity diode laser (ECDL) configuration, where the tuning range only depends on the external cavity configuration. In order to considerably reduce the sensor cost, relative intensity noise (RIN) is chosen for monitoring the intensity ratio of the two modes. RIN is found to be an excellent indicator for the two modes intensity ratio variations which strongly supports the sensor methodology. On the other hand, it has been found that, wavelength tuning has no effect on the RIN spectrum which is very beneficial for the proposed detection principle. In order to use the sensor for medical applications, the absorption line of an anesthetic sample, propofol, is measured. Propofol has been dissolved in various solvents. RIN has been chosen to monitor the sensor response. From the measured spectra, the sensor sensitivity enhancement factor is found to be of the order of 10^(3) times of the conventional laser spectroscopy.

Adhesion to Er : YAG laser-prepared dentin after long-term water storage and thermocycling

This in vitro study evaluated the microtensile bond strength of a resin composite to Er:YAG-prepared dentin after long-term storage and thermocycling. Eighty bovine incisors were selected and their roots removed. The crowns were ground to expose superficial dentin. The samples were randomly divided according to cavity preparation method (I-Er:YAG laser and II-carbide bur). Subsequently, an etch & rinse adhesive system was applied and the samples were restored with a resin composite. The samples were subdivided according to time of water storage (WS)/number of thermocycles (TC) performed: A) 24 hours WS/no TC; B) 7 days WS/500 TC; C) 1 month WS/2,000 TC; D) 6 months WS/12,000 TC. The teeth were sectioned in sticks with a cross-sectional area of 1.0-mm(2), which were loaded in tension in a universal testing machine. The data were subjected to two-way ANOVA, Scheffe and Fisher`s tests at a 5% level. In general, the bur-prepared group displayed higher microtensile bond strength values than the laser-treated group. Based on one-month water storage and 2,000 thermocycles, the performance of the tested adhesive system to Er:YAG-laser irradiated dentin was negatively affected (Group IC), while adhesion of the bur-prepared group decreased only within six months of water storage combined with 12,000 thermocycles (Group IID). It may be concluded that adhesion to the Er:YAG laser cavity preparation was more affected by the methods used for simulating degradation of the adhesive interface.

Laser emission of a Nd-doped mixed tellurite and zinc oxide glass

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Temperature rise during Er:YAG cavity preparation of primary enamel

This study aimed to assess in vitro thermal alterations taking place during the Er:YAG laser cavity preparation of primary tooth enamel at different energies and pulse repetition rates. Forty healthy human primary molars were bisected in a mesio-distal direction, thus providing 80 fragments. Two small orifices were made on the dentin surface to which type K thermocouples were attached. The fragments were individually fixed with wax in a cylindrical PlexiglassA (R) abutment and randomly assigned to eight groups, according to the laser parameters (n = 10): G1 -aEuro parts per thousand 250 mJ/ 3 Hz, G2 -aEuro parts per thousand 250 mJ/ 4 Hz, G3 -aEuro parts per thousand 250 mJ/ 6 Hz, G4 -aEuro parts per thousand 250 mJ/10 Hz, G5 -aEuro parts per thousand 250 mJ/ 15 Hz, G6 -aEuro parts per thousand 300 mJ/ 3 Hz, G7 -aEuro parts per thousand 300 mJ/ 4 Hz and G8 -aEuro parts per thousand 300 mJ/ 6 Hz. An area of 4 mm(2) was delimited. Cavities were done (2 mm long x 2 mm wide x 1 mm thick) using non-contact (12 mm) and focused mode. Temperature values were registered from the start of laser irradiation until the end of cavity preparation. Data were analyzed by one-way ANOVA and Tukey test (p a parts per thousand currency signaEuro parts per thousand 0.05). Groups G1, G2, G6, and G7 were statistically similar and furnished the lowest mean values of temperature rise. The set 250 mJ/10 and 15 Hz yielded the highest temperature values. The sets 250 and 300 mJ and 6 Hz provided temperatures with mean values below the acceptable critical value, suggesting that these parameters ablate the primary tooth enamel. Moreover, the temperature elevation was directly related to the increase in the employed pulse repetition rates. In addition, there was no direct correlation between temperature rise and energy density. Therefore, it is important to use a lower pulse frequency, such as 300 mJ and 6 Hz, during cavity preparation in pediatric patients.

Yellow laser light generation by frequency doubling of the output from a master oscillator fiber power amplifier system

We present a power-scalable approach for yellow laser-light generation based on standard Ytterbium (Yb) doped fibers. To force the cavity to lase at 1154 nm, far above the gain-maximum, measures must be taken to fulfill lasing condition and to suppress competing amplified spontaneous emission (ASE) in the high-gain region. To prove the principle we built a fiber-laser cavity and a fiber-amplifier both at 1154 nm. In between cavity and amplifier we suppressed the ASE by 70 dB using a fiber Bragg grating (FBG) based filter. Finally we demonstrated efficient single pass frequency doubling to 577 nm with a periodically poled lithium niobate crystal (PPLN). With our linearly polarized 1154 nm master oscillator power fiber amplifier (MOFA) system we achieved slope efficiencies of more than 15 % inside the cavity and 24 % with the fiber-amplifier. The frequency doubling followed the predicted optimal efficiency achievable with a PPLN crystal. So far we generated 1.5 W at 1154nm and 90 mW at 577 nm. Our MOFA approach for generation of 1154 nm laser radiation is power-scalable by using multi-stage amplifiers and large mode-area fibers and is therefore very promising for building a high power yellow laser-light source of several tens of Watt.

Direct real-time cavity mode assessment and optimization technique for fiber lasers

A simple technique for direct real-time assessment of a fiber laser cavity-mode condition during operation is demonstrated. Mode stabilization and optimization with this cavity-mode monitoring and conditioning feedback scheme shows significant improvements to the output performance.

Theoretical study of Talbot-like bands observed using a laser diode below threshold

We report on a theoretical study of an interferometric system in which half of a collimated beam from a broadband optical source is intercepted by a glass slide, the whole beam subsequently being incident on a diffraction grating and the resulting spectrum being viewed using a linear CCD array. Using Fourier theory, we derive the expression of the intensity distribution across the CCD array. This expression is then examined for non-cavity and cavity sources for different cases determined by the direction from which the slide is inserted into the beam and the source bandwidth. The theoretical model shows that the narrower the source linewidth, the higher the deviation of the Talbot bands' visibility (as it is dependent on the path imbalance) from the previously known triangular shape. When the source is a laser diode below threshold, the structure of the CCD signal spectrum is very complex. The number of components present simultaneously increases with the number of grating lines and decreases with the laser cavity length. The model also predicts the appearance of bands in situations not usually associated with Talbot bands.

270-km ultralong raman fiber laser

We analyze the physical mechanisms limiting optical fiber resonator length and report on the longest ever laser cavity, reaching 270 km, which shows a clearly resolvable mode structure with a width of ~120??Hz and peak separation of ~380Hz in the radio-frequency spectrum.

All-fiber passively mode-locked femtosecond laser using a 45º-tilted fiber grating polarization element

We report on the demonstration of an all-fiber femtosecond erbium doped fiber laser passively mode-locked using a 45º tilted fiber grating as an in-fiber polarizer in the laser cavity. The laser generates 600 fs pulses with output pulse energies ~1 nJ. Since the 45° tilted grating has a broad polarization response, the laser output has shown a tunabilty in wavelength from 1548 nm to 1562 nm by simply adjusting the polarization controllers in the cavity.

Fiber laser incorporating an intracavity micro-channel for refractive index and temperature sensing

In this letter, we present a standard linear cavity fiber laser incorporating a microchannel for refractive index (RI) and temperature sensing. The microchannel of ~6µm width created by femtosecond laser aided chemical etching provides an access to the external liquid; therefore, the laser cavity loss changes with the liquids of different RIs. Thus, at a fixed pump power, the output laser power will vary with the change of RI in the microchannel. The results show that the proposed sensing system has a linear response to both the surrounding medium RI and temperature. The RI sensitivity of the laser system is on the order of 10-3, while the temperature sensitivity is about 0.02 C. Both sensitivities could be further enhanced by employing a more sensitive photodetector and using higher pump power.

Fibre laser using a microchannel based loss tuning element for refractive index sensing

We have proposed and demonstrated a fibre laser system using a microchannel as a cavity loss tuning element for surrounding medium refractive index (SRI) sensing. A ~6µm width microchannel was created by femtosecond (fs) laser inscription assisted chemical etching in the cavity fibre, which offers a direct access to the external liquids. When the SRI changes, the microchannel behaves as a loss tuning element, hence modulating the laser cavity loss and output power. The results indicate that the presented laser sensing system has a linear response to the SRI with a sensitivity in the order of 10-5. Using higher pump power and more sensitive photodetector, the SRI sensitivity could be further enhanced.

All-fiber polarization locked vector soliton laser using carbon nanotubes

We report an all-fiber mode-locked erbium-doped fiber laser (EDFL) employing carbon nanotube (CNT) polymer composite film. By using only standard telecom grade components, without any complex polarization control elements in the laser cavity, we have demonstrated polarization locked vector solitons generation with duration of ~583fs , average power of ~3 mW (pulse energy of 118pJ ) at the repetition rate of ~25.7 MHz.

All-fiber-integrated soliton-similariton laser with in-line fiber filter

We demonstrate an all-fiber-integrated Er-doped fiber laser operating in the soliton-similariton mode-locking regime. In the similariton part of the cavity, a self-similarly evolving parabolic pulse with highly linear chirp propagates in the presence of normal dispersion. Following an in-line fiber-based birefringent filter, the pulse evolves into a soliton in the part of the cavity with anomalous dispersion. The similariton and the soliton pulses are dechirped to 75.5 and 167.2 fs, respectively, outside of the cavity. Mode-locked operation is very robust, owing to the influence of the two similariton and soliton attractors that predominate each half of the laser cavity. The experimental results are supported with numerical simulations, which provide good agreement.

Multiwavelength fiber laser based on nonlinear polarization rotation

Multiwavelength fiber laser is a perfect light source for future wavelength-division-multiplexing optical communication systems. A multiwavelength fiber laser based on nonlinear polarization rotation with up to 18 wavelengths has been proposed and demonstrated. The intensity- and wavelength-dependent loss induced by nonlinear polarization rotation effect is used to alleviate the mode competition in the homogeneous broadening gain medium of erbium-doped fiber. Instead of traditional filters, a polarization-maintaining fiber is inserted into the laser cavity, with which the polarization-dependent isolator composes an equivalent Lyot birefringent fiber filter. The in-line birefringence fiber filter is used to simplify the laser configuration, which benefits systematic integration. The effect of the 980 nm pump power on the multiwavelength generation is investigated. It is shown that the pump power contributes a lot to the evenness of the multiwavelength spectra due to the intensity dependence of nonlinear polarization rotation effect.

Wideband-tuneable, nanotube mode-locked, fibre laser

Ultrashort-pulse lasers with spectral tuning capability have widespread applications in fields such as spectroscopy, biomedical research and telecommunications1–3. Mode-locked fibre lasers are convenient and powerful sources of ultrashort pulses4, and the inclusion of a broadband saturable absorber as a passive optical switch inside the laser cavity may offer tuneability over a range of wavelengths5. Semiconductor saturable absorber mirrors are widely used in fibre lasers4–6, but their operating range is typically limited to a few tens of nanometres7,8, and their fabrication can be challenging in the 1.3–1.5 mm wavelength region used for optical communications9,10. Single-walled carbon nanotubes are excellent saturable absorbers because of their subpicosecond recovery time, low saturation intensity, polarization insensitivity, and mechanical and environmental robustness11–16. Here, we engineer a nanotube–polycarbonate film with a wide bandwidth (>300 nm) around 1.55 mm, and then use it to demonstrate a 2.4 ps Er31-doped fibre laser that is tuneable from 1,518 to 1,558 nm. In principle, different diameters and chiralities of nanotubes could be combined to enable compact, mode-locked fibre lasers that are tuneable over a much broader range of wavelengths than other systems.