Frequency-modulation spectroscopy with blue diode lasers

Abstract

Frequency-modulation spectroscopy provides ultrasensitive absorption measurements. The technique is especially adaptable to diode lasers, which can be modulated easily, and has been used extensively in the near-infrared and infrared spectral regions. The availability of blue diode lasers now means that the accessible wavelength region can be increased. We successfully demonstrate wavelength-modulation spectroscopy and two-tone frequency-modulation spectroscopy for the weak second resonance line of potassium at 404.8 nm and for the transition at 405.8 nm in lead, starting from the thermally populated 6p 2 3 P 2 metastable level. Information on the modulation parameters is obtained with a fitting procedure. Experimental signal-to-noise ratios at different absorption levels are compared with theoretical signal-to-noise ratios and show good agreement. Detection sensitivities of 2 ϫ 10 Ϫ6 and 5 ϫ 10 Ϫ6 for wavelength and two-tone frequency-modulation spectroscopy, respectively, for a 120-Hz bandwidth are demonstrated.

Wavelength Modulation Spectroscopy: Combined Frequency and Intensity Laser Modulation

“…[1][2][3][4] It was then applied to optical communication lasers in the near infrared 8 -13 and more recently to the novel quantum-cascade lasers in the mid-infrared [5][6][7] and using visible laser diodes. 14 WMS is based on the modulation of the light emitted by a laser that is slowly tuned through an absorption feature of the species to be detected. The interaction of the modulated light with the absorption line leads to the generation of signals at different harmonics of the modulation frequency.…”

Experimental method based on wavelength-modulation spectroscopy for the characterization of semiconductor lasers under direct modulation

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] This sensitive technique is based on sinusoidal modulation of the light emitted by a laser diode that is slowly tuned through an absorption feature of the species to be detected. The interaction of the modulated light with the absorption feature leads to the generation of signals at the different harmonics of the modulation frequency, which can be detected with a lock-in amplifier.…”

Environmental monitoring using optical techniques

“…With the fast development of the semiconductor laser technology, new wavelength regions are becoming available. We recently demonstrated first single-mode diode laser spectroscopy in the violet spectral range, frequency-modulation spectroscopy with blue diodes, and also their usefulness in sum-frequency generation to the UV region, also accessing the mercury line at 254 nm [28][29][30]. We are also, like many other groups, performing diode laser spectroscopy in the IR range, accessible by differencefrequency generation or directly by quantum cascade lasers.…”