Modulated Laser Source Research Articles

Laser generation of narrow-band and directed ultrasound

An aluminium hemicylindrical sample has been irradiated with an array of laser lines, with each line acting as a source of acoustic waves. Detection of the generated ultrasonic waves was performed using both a wide-band stabilized Michelson interferometer and a 20 MHz piezoelectric transducer. Experimental and theoretical results are presented which reveal that the use of a spatially modulated laser source produces significant narrow-banding of the detected ultrasound, compared with a single point or single line source case. Additionally, for a given line spacing, ultrasound of a particular frequency can be directed. Owing to the nature of the acoustic signals generated by each individual array element, superposition of several signals does not result in any energy directivity similar to that encountered in phase electromagnetic array antennas. While time or frequency feature enhancement may be obtained in a desired direction, in most cases the far field energy directivity pattern is simply the incoherent sum of the energy directivity of each array element.

Mechanism of compensation in heavily silicon-doped gallium arsenide grown by molecular beam epitaxy

Silicon-doped GaAs grown by molecular beam epitaxy has been characterized by Hall measurements, infrared local vibrational mode (LVM) absorption, secondary ion and laser source mass spectroscopy. Highly doped samples with [Si]∼3×1019 cm−3 show only a low carrier concentration of 8×1017 cm−3. LVM spectroscopy shows that SiGa donors are compensated predominantly by [Si-X] complexes, where X has been assigned previously to a gallium vacancy (VGa). Other compensating impurities are not present in the layers at significant concentrations.

Multiplexing of interferometric sensors using phase carrier techniques

This paper demonstrates the multiplexing of fiber-optic interferometric sensors using a CW phase generated carrier technique. The technique employs modulated diode laser sources at different carrier frequencies, nearly balanced interferometers (∼4-cm path difference), and phase generated carrier demultiplexing demodulation. This approach leads to a simple all-passive sensor array which has intrinsically low crosstalk. The system is analyzed in terms of shot noise performance and crosstalk. An experimental all optical implementation of a four sensor array was demonstrated; both the single sensor and multisensor arrays were limited by the laser phase noise to a sensitivity of \sim 18 \mu rad/ \sqrt{Hz} . Crosstalk between individual channels was better than -60 dB and crosstalk between three sensors and the test sensor was better than -55 dB. In the absence of laser phase noise the demodulator/demultiplexer demonstrated \sim 2-\mu rad performance with both single sensor and four element array operation.

Wide-range directional LDV employing modulated semiconductor laser source

A laser Doppler velocimeter (LDV) based on a semiconductor laser is described. Information on the direction of flow is obtained by applying a single-sideband technique using direct modulation of the laser diode. A measuring range of 8.4 MHz is reached with this technique, where the nonideal FM properties of the semiconductor laser are dealt with by an equalisation of the modulation current.

Single and multimode fiber bandwidth measurements with single and multilongitudinal mode lasers operating at 0.8-, 1.3-, and 1.5-µm wavelength

The bandwidth characteristics of single and multimode optical fibers have been investigated with single and multilongitudinal mode laser sources operating at 0.8, 1.3, and 1.5 μm. It is shown that single-mode fiber with a cutoff wavelength of 1.3 μm can support 1 Gb/s transmission over at least 7.5 km with a 0.8-μm laser source.

Coherent optical-fibre sensors with modulated laser sources

Coherent optical-fibre sensing based on environmentally induced phase modulation can be extremely sensitive, but interferometric detection limits dynamic range. This letter examines a separate class of applications where the very high sensitivity is not required, but where remote optical sensing of relatively large physical variations is desirable. We describe a system whereby source modulation may be used to determine the state of a remote interferometer accurately and unambiguously over phase modulations from a fraction of an optical wavelength to tens or even hundreds of wavelengths. The concept may also be utilised as the basis of a multiplexed sensor highway, and preliminary results are reported.

Real-time observation of laser-induced ir fluorescence of organic pollutants in the ambient atmosphere: suggested mechanism of laser-induced ir fluorescence

A modified optical system has been developed for the continuous monitoring of hydrocarbons in the atmosphere by laser-induced i.r. fluorescence. The system includes a large gathering mirror, a modulated laser source to eliminate interferences from thermal emission, and phase-shifting equipment to eliminate interferences from back-scatter, particularly at the resonance wavelengths. The mechanism of excitation is discussed and a proposal put forward to explain how several photons may be simultaneously absorbed, causing fluorescence at wavelengths shorter than that of the excitation beam. Some data on the variation of the fluorescence intensity from ambient pollutants with time of day are presented for a TGS detector and a cryogenic detector.