Flexible lock-in detection system based on synchronized computer plug-in boards applied in sensitive diode-laser gas spectroscopy

摘要

A flexible and compact digital lock-in detection system, with software-based balanced detection for tuneable diode-laser spectroscopy measurements is described. The system is based on two data acquisition cards for a standard computer. One data acquisition card contains a software-controlled arbitrary waveform generator (CH-3150, Exacq Technologies) that is used for laser modulation. The scanning frequency is typically 130 Hz and a sinusoidal modulation of about 132 kHz is applied. The second card contains four externally clocked A/D converters (NI-6132, National Instruments) that are used for acquisition of sensor signals synchronously. Detected signals are averaged before software based lock-in detection is carried out. By using synchronous acquisition and averaging, a comb filter is formed that filters the signals [1]. Thus, it is possible to detect the 1f, 2f, 3f, and 4f lock-in components out of the same data set. Averaged or raw data may be stored on disc for later analysis. No external lock-in amplifier, oscilloscope or function generator is needed. A pigtailed single mode VCSEL laser (L2K-P760-LD-SM, Laser2000) is used to scan across oxygen lines at about 760 nm. After the laser, the light is split by a single-mode fiber splitter. 10% of the laser light is guided into a reference detector (UDT-455LN, OSI Optoelectronics) while 90% is guided, via the sample, to a sample detector (SD394-70-72-661, Advanced Photonix); see Fig. 1. By analysing the signal from the reference beam, optical noise contributions in the sample beam, such as optical interference fringes, are suppressed [2]. Finally, detector signals are high-pass filtered and amplified before the A/D converters. Thus, 14 bit A/D converters are adequate to resolve a weak lock-in signal that corresponds to absorptions in the order of 10{sup}(-5).