[Instrument usage tips] Basic principles of Mozza spectrometer

[Instrument usage tips] Basic principles of Mozza spectrometer

The core part of any spectrometer is the spectroscopic system, which generally consists of three parts: a collimation system, a dispersion system, and an imaging system. Its main function is to separate the light to be measured in a certain space according to a certain wavelength.

As shown in Figure 1, the collimation system generally consists of an incident slit and a collimating objective lens. The incident slit is located on the focal plane of the collimating objective lens. The light emitted by the light source and illumination system passes through the slit and is illuminated by the collimating objective lens, and is transformed into a parallel beam and projected onto the dispersion system.

Figure 1. Spectrometer principle

The function of the dispersion system is to decompose the incident single beam of composite light into multiple beams of monochromatic light. Multiple beams of monochromatic light pass through the imaging objective lens and are imaged on the focal plane of the lens in order of wavelength; in this way, the single beam of composite light successfully becomes the image of multiple beams of monochromatic light after passing through the spectroscopic system. At present, the main dispersion systems include material dispersion (such as prism), multi-slit diffraction (such as grating) and multi-beam interference (such as interferometer).

The function of the detection and receiving system is to convert the spectral energy received on the focal plane of the imaging system into an easily measurable electrical signal, and measure the wavelength and intensity of the corresponding spectral components, thereby obtaining the characteristic parameters of the light or substance being studied, such as the composition and content of the substance, the temperature of the substance, the movement speed of the star, etc.

At present, the receiving system of spectroscopic instruments can be divided into visual system, spectrophotographic system and photoelectric system.

According to the working principles of modern spectroscopic instruments, spectrometers can be divided into two major categories: classic spectrometers and new spectrometers. Classic spectroscopic instruments are based on the principle of spatial dispersion; new spectroscopic instruments are based on the principle of modulation.

The infrared spectrometer Mozza (FASTLITE.Inc) described in this article is a new spectrometer based on acousto-optic modulation for wavelength selection.

The spectroscopic element inside Mozza is not a grating or prism structure in a traditional spectrometer, but a bandpass filter (Acousto-optic Tunable Filter, AOTF) based on the acousto-optic modulation of TeO2 crystal. The center frequency of this narrowband filter is selected by controlling the frequency of the acoustic wave loaded on the crystal.

Therefore, by scanning the frequency of the acoustic wave, different frequency components of the incident light pulse can be selected and their optical intensity values ​​can be recorded with a detector. Finally, the acoustic frequency is reconstructed into the optical frequency to obtain the complete incident light spectrum. Because Mozza has the advantages of both wide measurement range (1-5um) and high resolution, it has become an increasingly indispensable measuring instrument in infrared spectroscopy research.

Mozza spectrometer

The following describes the working principle of the Mozza spectrometer in conjunction with Figure 2:

Figure 2. Mozza basic schematic diagram

As shown in Figure 2, which is the basic schematic diagram of Mozza, the incident laser pulse is divided into two beams through the beam splitter, the transmitted light passes through the AOTF crystal (loaded with a specific acoustic frequency), and the intensity information of the output narrow-band diffracted light is recorded by a single-point detector (MCT);

The reflected light from the beam splitter is directly recorded by another single point detector (MCT). If the energy stability of the incident light pulse changes, the intensity of the reference light can be used to correct the intensity change of the diffracted light caused by the change of the incident light energy, so that the normalized intensity recorded by the spectrometer remains stable.

However, when a laser beam to be measured is incident on an acousto-optic crystal, in addition to the diffracted beam we need for measurement, there will also be a transmitted beam that is not needed for measurement, and the energy of this transmitted beam is several orders of magnitude greater than the diffracted beam. In order to eliminate the transmitted beam and improve the signal-to-noise ratio, Mozza also uses the following technologies:

• Since the diffracted beam and the transmitted beam through the acousto-optic crystal have different exit directions, slits can be used to filter out most of the transmitted light;

Since the diffracted beam and the transmitted beam through the acousto-optic crystal have different exit directions, slits can be used to filter out most of the transmitted light;

• Since the diffracted beam (P polarization) and the transmitted beam (S polarization) of the acousto-optic crystal have different polarizations, a broadband polarizer can be used to filter out most of the transmitted light;

Since the diffracted beam (P polarization) and the transmitted beam (S polarization) of the acousto-optic crystal have different polarizations, a broadband polarizer can be used to filter out most of the transmitted light;

• However, after the above two methods, it is still possible that transmitted light or its scattered light enters the MCT. In order to further improve the signal-to-noise ratio, Mozza uses a lock-in detection technique. Mozza records the signal measured by the MCT when the modulated sound wave is loaded on the acousto-optic crystal and when the modulated sound wave is not loaded, and then the difference operation is performed between the two to eliminate the influence of the remaining transmitted light on the detection signal. Therefore, Mozza can achieve extremely high signal-to-noise ratio.

However, after the above two methods, there may still be transmitted light or scattered light entering the MCT. In order to further improve the signal-to-noise ratio, Mozza uses a lock-in detection technique.

Mozza recorded the signals measured by the MCT when modulated acoustic waves were loaded on the acousto-optic crystal and when no modulated acoustic waves were loaded, and then the difference operation was performed between the two to eliminate the influence of the remaining transmitted light on the detection signal. Therefore, Mozza can achieve extremely high signal-to-noise ratio.

In addition, the sensitivity and resolution curves of Mozza are shown in Figure 3 below. It can be seen that Mozza has extremely high sensitivity and resolution in the 1.2um-3.0um band range.

Figure 3, Mozza sensitivity (black) and resolution (blue) curves

Figure 4. Mozza measures dark spectrum

In summary, based on the working principle of Mozza, it has the following advantages:

• Based on AOTF, mid-infrared lasers from several Hz to MHz and even continuous light can be measured;

Based on AOTF, mid-infrared lasers from several Hz to MHz and even continuous light can be measured;

• There is no need to replace slits and gratings, and infrared pulses (1~5um) of several optical octaves can be directly and accurately measured;

There is no need to replace slits and gratings, and infrared pulses (1~5um) of several optical octaves can be directly and accurately measured;

• Based on MCT single-point detector, extremely high spectral resolution (5cm-1) and sensitivity (0.5pJ/5cm-1) can be achieved, as shown in Figure 3;

Based on MCT single-point detector, extremely high spectral resolution (5cm-1) and sensitivity (0.5pJ/5cm-1) can be achieved, as shown in Figure 3;

• Based on the optical path design and simulation algorithm, it is possible to compensate for laser energy fluctuations and reduce the requirements for pulse energy stability during the scanning process;

Based on the optical path design and simulation algorithm, it is possible to compensate for laser energy fluctuations and reduce the requirements for pulse energy stability during the scanning process;

• Dynamic range can reach 40dB;

Dynamic range can reach 40dB;

• Extremely high signal-to-noise ratio - locked detection mode

Extremely high signal-to-noise ratio - locked detection mode

In actual use, Mozza has the following shortcomings.

• Mozza is based on scanning measurement. If the repetition frequency of the signal light to be measured is low, the measurement time will be very long. At this time, the stability of the light is required. It is not very convenient for some occasions that require fast scanning speed, such as CEP measurement of low repetition frequency light.

Mozza is based on scanning measurement. If the repetition frequency of the signal light to be measured is low, the measurement time will be very long. At this time, the stability of the light is required. It is not very convenient for some occasions that require fast scanning speed, such as CEP measurement of low repetition frequency light.

• Mozza uses MCT to detect optical signals. As can be seen from Figures 3 and 4, when the wavelength to be measured is greater than 5um, the noise measured by Mozza is very large.

Mozza uses MCT to detect optical signals. As can be seen from Figures 3 and 4, when the wavelength to be measured is greater than 5um, the noise measured by Mozza is very large.

But the flaws are not hidden, because Mozza has the advantages of wide measurement range (1-5um) and high resolution, making it an increasingly indispensable measuring instrument in infrared spectroscopy research.

We will introduce the specific usage of Mozza in the next issue.