Jasman Zainal

Hydrocarbon level detection with nanosecond laser ablation

Nanosecond laser induced breakdown in liquid is used as a technique to detect hydrocarbon levels in water. A Q-switched Nd:YAG laser was focused to generate optical breakdown associated with shock wave generation. The shock wave was propagated at the speed of sound in the medium after travelling 1 μs outward from the center of optical breakdown. Different amplitudes of sound were traced with the aid of an ultrasonic probe. The optical properties of the hydrocarbon solution were quantified via fundamental refractive index measurement (the Snell law). A continuous mode diode pumped solid state laser with second harmonic generation was used as the illumination light source. A CCD video camera with Matrox version 4.2 software was utilized to analyze the recording image. Option line analysis was performed to analyze the intensity of optical breakdown at different input energies. Gray level analysis was also conducted on the scattering light after passing through the hydrocarbon solution at different concentrations. The hydrocarbon solution comprised impurities or particles that varied according to the concentration. The average of the gray level is assumed to present the size of the particle. Inherently, as the acoustic wave propagates outward, it transports the mass (particles or impurities) and impacts on the ultrasonic probe. As a result a higher concentration of hydrocarbons reveals a larger amplitude of sound waves. This phenomenon is identified as a finger print for hydrocarbon levels between 100 and 1000 ppm. The transient detection, without complicated sampling preparation and no hazardous chemical involvement, makes laser ablation a promising technique to detect in situ hydrocarbon levels in water.

Mode matching for efficient laser diode to single mode fiber coupling

This paper presents some analysis for the matching between the elliptical mode field of 1550nm high power laser diode with the circular mode field of the single mode fiber in order to obtain high coupling efficiency with relaxed misalignment tolerances. Three coupling schemes namely Butt, single ball lens and double ball lenses coupling schemes have been employed in pigtailing the butterfly laser diode module using laser welding technique with dual beams from Nd:YAG laser welding system for the attachment of coupling components. The process of fiber attachment to laser diode and welding of various coupling components, such as lens holders, fiber ferrule and welding clips have been performed in what is so called active alignment process, where the system continues measuring the coupled power during the process of coupling and welding of coupling components in their holder to each other and to the main substrate. It has been found that double ball lenses coupling scheme is efficient and more effective for mode matching of highly elliptical (large divergence ratio) laser diode mode field with the circular mode field of a single mode fiber, whereas for small divergence ratios the single ball lens is adequate.

Estimation of coupling power parameter of fused coupled fibers

Fiber couplers are widely used in telecommunication and industry as a passive splitting power device. The effective power coupling and transmitting from one fiber to another is mainly determined by both coupling length and coupling coefficient. Coupling length can be calculated directly but for coupling coefficient it depends upon the refractive index and separation fiber axis. After fusion processes of two SMF-28e(R) couplers, the refractive index is unknown due to change in the radius of fiber (rcladd<40μm and rcore<1.5μm). Coupling coefficient range is obtained from a distribution of coupling ratio and compared with the empirical formula which also enables to calculate refractive index. In this experiment, coupling coefficient in the range of 0.6-0.9/mm is calculated as a function of separation fiber axis and refractive index of core and cladding. The result shows a good correlation between experimental results and theoretical calculation.

The comparative study of optical fiber coupling scheme in pigtailing of semiconductor optical amplifiers

In this paper we present analysis on three different coupling systems, i.e., butt, single ball lens, and two ball lenses between the tips of two coupled single mode fiber in semiconductor optical amplifier (SOA) module. The coupling components inside the module can be aligned in an active alignment process and attached by means of dual beam from an Nd: YAG laser welding system. The tips of the coupled fiber are ferruled inside metallic tubes to enable the attachment to the substrates through saddle-shaped welding clips. Investigations of the variations of coupling efficiency with the with working distance for the three schemes showed that two ball lenses is more efficient with coupling efficiency of 75% followed by single ball lens at 55% and butt coupling mode at 20% maximum. In addition dual ball lens configuration have shown to have better longitudinal tolerant even with an elliptical beam profile from the source fiber. This is however not the case in single ball lens and butt coupling scheme. We also observe however the optimum separation between the two lenses at in the range between 0.35 mm - 0.45 mm. This is to ensure the coupling efficiency is the highest possible within the acceptable tolerant misalignments.

A tolerant coupling scheme for pigtailing laser diode module in active alignment process

A laser diode transmitter packaged in a butterfly module is coupled into a single mode fiber using double small ball lenses. The process of alignment and fixing of all the components inside the module is performed in an active alignment procedure, where the laser diode is powered and the output power is continuously measured during the alignment process of all coupling components to determine the optimum positions for maximum coupling efficiency and then fixed in their holders and to the main substrate by laser welding technique using dual beam Nd:YAG laser welding. The double ball lenses coupling scheme found to be very effective in mode matching between laser diode and single mode fiber. The axial, lateral and angular 1dB misalignment tolerances are enhanced for the transformed laser mode field radii in both X and Y directions. The experimentally measured coupling efficiency of the proposed coupling system was around 75% with a relaxed working distance (separation of the coupling system from the fiber tip) in the range of (2-4mm) by optimizing the separation between the two lenses as well as the separation between the first lens and the facet of the laser diode. The experimental results match very well with those obtained theoretically by employing ABCD ray tracing matrix.

Investigation of noise effects of CMS laser lineshape and frequency fluctuations

Lineshape and frequency fluctuations actually cause the optical field of current modulation semiconductor laser (CMS) laser beam to deviate from a pure sine wave [1]. This paper investigates the lineshape of a CMS laser for two distinct types of frequency noise. In the first case the investigation analyzes on the low-pass filter, the frequency noise level is a constant δo below a cutoff frequency but zero above this threshold. In the second case on high-pass filter the frequency noise level is a constant δo above a cutoff frequency but zero below this threshold. Investigations observed inspire us to focus for separation of the frequency noise spectrum into two regions wherein due to the effect of noise on the lineshape is radically different in the slow modulation area in a left side of the diagonal line where the noise contributes to the laser linewidth, with a Gaussian shape, and the other one the fast modulation area in a right side of the diagonal line, where the noise contributes only to the wings of the line (sidebands) and not to the linewidth, thus transforming the lineshape from Gaussian to Lorentzian.

Active alignment and reliable pigtailing of laser diode transmitter

In this paper we present theoretical and experimental analysis on Nd:YAG laser microwelding for pigtailing laser diode transmitter through two ball lenses that are employed for effectively matching the elliptical mode field of the laser diode with the circular on of the single mode fiber. The fiber attachment and the fixing of various coupling components have been performed in what is so called active alignment process. The system continues measuring the coupled power during the processes of alignment and attachment of various coupling components as well as the working distance and misalignment tolerances optimizations. Results of theoretical modeling of laser weld penetration depth agree with the experimentally measured results in the low laser pulse energy range. Moreover the laser pulse parameters such as, duration, energy, number of pulse shoots as well as the focusing position over the workpiece and angle of laser pulse incidence are found to have very significant effects on the weld yields and greatly affect the laser weld depth to width ratio. Optimization of all the mentioned parameters found to be necessary for achieving strong laser microwelds with more penetration and less width in the attachments of the sensitive optical components inside the packaged photonic devices modules.

ESTIMATION OF THE COUPLING POWER PARAMETER OF FUSED COUPLED FIBERS

Fiber couplers are widely used in telecommunications and industry as passive splitting power devices. The effective power coupling and transmission from one fiber to another is mainly determined by both the coupling length and the coupling coefficient. The coupling length can be calculated directly but the coupling coefficient depends upon the refractive index and the separation fiber axis. After the fusion processes of two SMF-28e® couplers, the refractive index is unknown due to a change in the radius of the fiber (rcladd < 40 μm and rcore < 1.5 μm). The coupling coefficient range is obtained from a distribution of the coupling ratio and compared with the empirical formula, which also enables one to calculate the refractive index. In this experiment, the coupling coefficient in the range of 0.6–0.9/mm is calculated as a function of the separation fiber axis and the refractive index of the core and cladding. The result shows a good correlation between experimental results and theoretical calculation.

A partial coupling power of single mode fiber fusion

Coupled fibers are successfully fabricated by injecting hydrogen flow at 1bar and fused slightly by unstable torch flame in the range of 800-1350°C. Optical parameters may vary significantly over wide range physical properties. Coupling coefficient and refractive index are estimated from the experimental result of the coupling ratio distribution from 1% to 75%. The change of structural and geometrical fiber affects the normalized frequency (V) even for single mode fibers. Coupling ratio as a function of coupling coefficient and separation of fiber axis changes with respect to V at coupling region. V is derived from radius, wavelength and refractive index parameters. Parametric variations are performed on the left and right hand side of the coupling region. At the center of the coupling region V is assumed constant. A partial power is modeled and derived using V, normalized lateral phase constant (u), and normalized lateral attenuation constant, (w) through the second kind of modified Bessel function of the l order, which obeys the normal mode, LP01 and normalized propagation constant (b). Total power is maintained constant in order to comply with the energy conservation law. The power is integrated through V, u and w over the pulling length range of 7500-9500 μm for 1-D where radial and angle directions are ignored. The core radius of fiber significantly affects V and power partially at coupling region rather than wavelength and refractive index of core and cladding. This model has power phenomena in transmission and reflection for industrial application of coupled fibers.