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Applications of the lock-in amplifier


Lock-in amplifier is widely used in temperature detection, photoelectric detection, biological signal detection, geological exploration and so on.

Lock-in amplifier is widely used in temperature detection, photoelectric detection, biological signal detection, geological exploration and so on. The following is its basic application:

High-precision spectral measurement

高精度光谱测量

Diagram of spectral measurement  (F: SSI OE3001optical chopper   G: grating spectrometer  B: photoelectric probe 

D: praseodymium rubidium glass samples  C: sample box  E: bromine tungsten light source  H: computer)


The light from the bromine tungsten lamp modulated by optical chopper OE3001 becomes a light with certain frequency information. Then it becomes monochromatic light after passing through the grating spectrometer.  Depending on the structure of the sample, it will absorbs the light of a particular wavelength when the monochromatic light hits the sample. Then the light converts to an electric siganl when shines on probe B. At last, the lock-in amplifier will detect this weak electric signal. 

TDLAS technology

Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology is a detection technology to measure gas component concentrations and temperature fields, or even velocity fields in industrial applications. TDLAS is based on Beer-Lambert's law.  When the laser passes through the gaseous molecular medium,  a portion of the  laser will be absorbed as long as its wavelength is equal to the level transition of any one of the molecules. The relationship between the outgoing light intensity and incident light intensity is as follows:

光强与入射光强

Where, α(cm-1 is the absorption coefficient of unit length medium;  L (cm) is the effective absorption length; N (cm-3) is the number of molecules per unit volume (that is concentration); σ(cm-2) is the molecular absorption cross-section. After frequency modulation, and va / v0 << 1, the above equation is written as:

TDLAS

From the above equation, it can be seen that the molecule absorbs high-order components of the laser.  In the signal detection path, we use phase-locked amplification technology to achieve the second harmonic detection of the modulated signal and then get  the first, second or third derivative of Gaussian function respectively at 1f, 2f or 3f frequency.

应用1.jpg

In this laser phase-locked system, the most crucial part is the realization of dual-channel detection of test gas and reference gas or even three-channel detection of test gas, reference gas and laser at the same time by lock-in amplifier. OE1022 can successfully solve this problem due to its multiple harmonic measurement function.

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Solar cell quantum efficiency testing
There are two kinds of solar cell quantum efficiency.
External quantum efficiency (EQE):  the ratio of the number of charge carriers in a solar cell to the number of  photons with a certain energy incident on the surface of the solar cell.  

Internal Quantum Efficiency (IQE): the ratio of the number of charge carriers in a solar cell to the number of photons with a certain energy that incident on the surface of the solar cell but not reflected  by the solar cel or not penetrate it.

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The system of solar cell quantum efficiency detection includes a dual-channel lock-in amplifier, optical chopper, monochromator and other equipment. The probe  light  emitted from the monochromator is split into two beams. One is converted to photocurrent and input to lock-in amplifier. As a contrast, another light shines on the solar cell and the photocurrent it produces is measured by another channel of the lock-in amplifier. Then the solar cell quantum efficiency can be caculated by the ratio of the two channel results and relevant formula.

Scan tunneling spectrum

扫描隧道谱

We can obtain scanning tunneling spectroscopy with characteristic peaks by testing I-V/dI/dV at a location on the sample surface. At the characteristic peak voltage, keep the average current constant, so that the tip scan in the X, Y plane and measure the value of dI/dV with the change of X and Y, and then scanning tunneling spectroscopy is got. The electronic and chemical properties of the surface are reflected in the I-V and dI / dV-V spectra.                         

扫描隧道IV