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Application Note - LTC-1141 in Laser Spectroscopy

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Application Note - LTC-1141 in Laser Spectroscopy Application Note - LTC-1141 in Laser Spectroscopy Application note written in highly appreciated collaboration with IPM – Fraunhofer Institut Freiburg, Germany Meerstetter Engineering GmbH Schulhausgasse 12 CH-3113 Rubigen Switzerland Phone: +41 31 712 01 01 Email: [email protected] Meerstetter Engineering GmbH (ME) reserves the right to make changes without further notice to the product described herein. Information furnished by ME is believed to be accurate and reliable. However typical parameters can vary depending on the application and actual performance may vary over time. All operating parameters must be validated by the customer under actual application conditions. Release date: 28 August 2020 Developed, assembled and tested in Switzerland 5240C Meerstetter Engineering GmbH 1 5240C Meerstetter Engineering GmbH 2 Index 1 Abstract ...................................................................................................................... 4 2 Device Overview ........................................................................................................ 5 3 Application .................................................................................................................. 6 3.1 Application Theory ...................................................................................................... 6 3.2 Application Description ............................................................................................... 8 4 Results and Benefits ................................................................................................... 9 A Change History ..........................................................................................................10 5240C Meerstetter Engineering GmbH 3 1 Abstract This application notes describes how Meerstetter’s LTC-1141 Laser Diode driver with inte- grated TEC controller is used in a laser spectroscopic set up for detection and concentration measurement of gases. Laser spectrometers for gas analysis consist of the laser, the sample volume and a detector. For operation a laser driver, signal acquisition and processing is re- quired. These components are combined on the LTC-1141. The LTC-1141 provides a univer- sal and powerful platform for laser spectrometer demonstrator setups. The laser driver is applicable to laser diodes, interband and quantum cascade lasers. The TEC driver stabilizes the heat sink temperature exact and constant. The programmable integrated waveform generator is used to drive current ramps of the laser current. Through the laser current modulation, the laser emission wavelength is swept and the gas absorption line is scanned. The high speed 16-bit ADC is used to sample the voltage output of the preamplifier of the receiving detector. The onboard FPGA is used to process the incoming data stream with an application specific, customized algorithm in real time. The processed measurement values are output via the digital interface. 5240C Meerstetter Engineering GmbH 4 2 Device Overview The LTC family controllers are laser diode drivers with an integrated TEC controller (based on the TEC-1091). The core of the LTC controllers consists of a system-on-chip featuring high performance pro- cessing capabilities in combination with fast ADC, DAC and memory. This allows fast modula- tion, sampling as well as onboard data processing. Object (laser diode, sensor, etc.) cooling is managed by the onboard TEC controller featuring high temperature stability and high measurement precision. Product Highlights: Applications: Low noise laser diode current Spectroscopy High bandwidth (up to 0.5 MHz) Radar High efficiency TEC controller (DC output) Medical diagnostics Very high temperature stability (0.005 °C) Chemical analysis Auto tuning for PID values of TEC controller General measurement systems Fast 16-bit A/D and D/A conversion Integrated signal processing Application data processing: - 11 configurable digital or 5 analog IOs (X3) Main Features: - 1 fast analog input (differential) reserved for Laser Diode Driver (LDD): sampling and measurements (X2) - 0.5 MHz modulation bandwidth - 1 fast analog output (X4) - Integrated signal generator - Custom current waveforms TEC/Peltier controller (TEC): - Synchronous sampling and measuring - Fast and high precision temperature control - Capacity for data processing, sampling, LDD and TEC integrated on one board measurement sequences and oscilloscope LDD and TEC fully digitally controlled functionality Please refer to the User Manual and the Data Sheet for details. 5240C Meerstetter Engineering GmbH 5 3 Application 3.1 Application Theory Figure 1. Scheme of a laser spectrometer and its main components. The LTC-1141 merges all components indicated by the blue boxes. The heart of a laser spectrometer consists of the optical unit: laser, gas cell and detection. The aim is to determine the gas concentration by measuring the absorption by the gas. The sensi- tivity is mainly determined by the absorption coefficient of the gas line, the optical path length through the gas and the potential optical interferences. However, the non-gas-specific perfor- mance is determined by the electronics and digital periphery. In addition to the optics (laser, gas cell and detection), a laser spectrometer consists in principle (Figure 1) of the driver for the thermoelectric cooler (TEC), the current end stage, the modulation unit for the laser current, data acquisition, signal and data processing. These components must be centrally controlled and synchronized. Usually separate bench-top devices controlled by a computer are used for laboratory set-ups of laser spectrometers. Such space-consuming setups are complex and error-prone due to the many degrees of freedom and adjustment possibilities. The LTC-1141 offers the ability to combine all of this in a compact package on a platform that can operate both stand-alone and computer-controlled. Figure 2. The LTC-1141 combines TEC controller, la- ser current driver, current modulation, data acquisition and system controller on one board. Figure 2 shows the technical specifications of the LTC-1141. Laser drivers for TEC and laser current are integrated on the board. The laser current can be modulated by a waveform gen- erator with a sawtooth signal (DAC, Analog out), so that a gas absorption line is scanned with the laser current, for example the tunable laser spectroscopy (Figure 3). If the sawtooth is repeated at 1 kHz, absorption spectra are generated at 1 kHz. In order to average a series of spectra, it is extremely important that the measurement is jitter-free and reproducible. For a reproducible gas measurement, it is very important to keep the laser temperature very stable. 5240C Meerstetter Engineering GmbH 6 The temperature is measured by a thermistor on the cold side of the TEC and kept stable at 0.005 K by a PID loop. In addition, the temperature of the heat sink can also be included in the control loop. The fact that there is a common clock for DAC, ADC and FPGA ensures optimal synchronization of the processes (jitter-free). The current driver is suitable for the use of differ- ent lasers. Laser diodes and interband cascade lasers with a compliance voltage of less than 5 V as well as QCLs up to 14 V can be operated. The intelligent protective circuits prevent damage of the laser chip in case of incorrect laser temperature or laser current values. On the detection side the detector signals are digitized by a fast 16-bit ADC. By means of a corresponding customer-specific firmware, the signal is e.g. averaged or filtered and then passed on to the CPU for an analysis algorithm. The digital interfaces enable the transfer of the measurement results to the user side. Figure 3. The typical tuneable laser spectroscopy method works with a sawtooth laser current modulation, which causes a wavelength scan of the gas spectrum. The detector signals are sampled by an ADC and averaged by the FPGA and used for calculation of the gas concentration on the CPU. A customized firmware can be imple- mented and determines the data analysis method. 5240C Meerstetter Engineering GmbH 7 3.2 Application Description For safety and maintenance, potential gas leaks of a gas pipeline need to be found and ana- lysed. An apparatus was built consisting of an emitting laser with frequency sweep. Stray light was received with a photo diode. The received intensity is aligned with the transmitted fre- quency via a trigger signal. Frequency specific absorption leads to a local decrease in received light intensity. The amount of decrease correlates with the gas concentration of the gas under investigation (Figure 4). Figure 4. The LTC-1141 is used for laser spectrometry for remote gas detection. The LTC-1141 is the central control unit for laser driving, data acquisition and data analysis. 5240C Meerstetter Engineering GmbH 8 4 Results and Benefits The LTC-1141 is the central part of a compact laser spectrometer development designed for remote gas detection. It was important that all components were integrated on a single board for operation and portability. Due to the good laser driver and synchronization, high quality gas spectra are generated. On the FPGA, the signals are averaged, resulting in a low-noise spec- trum. Absorption features smaller than 0.0001 can be resolved. The FPGA allows fast pre- processing of the data so that measured values can be calculated, e.g. at 60 Hz. The firmware for the data analysis was designed by the user. This process was quickly and skillfully sup- ported by
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