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FT-MIR spectrometer | SIMTRUM Photonics Store

Products / Light Analysis /Spectrometers /FT Infrared Spectrometer(900-16000nm) /FT-MIR spectrometer(2000-16000nm) /
FT-MIR spectrometer(2000-16000nm)
Overview Specifications Working Principle Software Application Q&A Related Products Order Now！

SIMTRUM'S FT-MIR spectrometer is a highly peformant, compact and reliable spectrometer that is ideal for various applications in the mid-infrared. The concentration levels of CO2 or H2O in the interferometer volume are conveniently minimized thanks to a homemade, replaceable dessicant capsule. Thanks to its permanently aligned interferometer and solid-state reference laser, the FT-MIR spectrometer offers excellent stability in both intensity and wavelength scales.

The FT-MIR can accommodate various detectors, each displaying specific characteristics. Besides pryoelectric detectors (DLATGS), we offer a selection of thermoelectrically cooled (TEC) MCT detectors that enable achieving unprecented performances while keeping the instrument extremely compact. The FT-MIR can also be mounted with a liquid nitrogen (LN2) cooled MCT detector for demanding applications that require high signal-to-noise ratio (SNR) or specific setups with very low optical throughput. In its standard configuration, the FT-MIR can reach a resolution of 2cm-1 while our high resolution modules (available on request) can deliver up to 0.5cm-1.

FT-MIR spectrometer（2-12um

FT-MIR spectrometer（2-16um）with LN2 cooled MCT detector

Features

  • Compact
  • Adjustable spectral range
  • Dynamically adjustable resolution
  • Long lifetime
  • Removable fiber adapter
  • Low power consumption
  • USB 2.0 connection

Application

  • Mid-IR Optical Spectrum Analyzer
  • Liquid, thin-film or gas measurement
  • Material identification and quantification

 

 

 

 

Principle

Permanentely aligned interferometer

The heart of the FT-Rocket is dual corner-cube (retro-reflector) interferometer. The two corner-cubes are fixed to a common swinging arm, which rotates to create an optical path difference (with respect to the beam splitter) in the two arms of the interferometer.

This type of design is called a permanently aligned interferometer. This particular arrangement of the interfeormeter is known to be the most robust against vibrations and temperature drifts. It never has to be realigned. The swinging arm of the interferometer rotates on wear-free flexure system, making this mechanical system extremely robust and durable.

 

Solid-state reference laser

For measuring the movement of the mirrors, a solid-state reference laser is coupled into the interferometer. Compared to classic HeNe lasers, the solid-state lasers that we use are more compact and have a much longer life-time. They have a very low temperature-induced wavelegnth drift and, when kept at constant temperature with a Peltier element, their wavelength can be stabilized to a few PPM, thus providing a very accurate and reproducible wavelength scale. This is crucial for ensuring a day-to-day and unit-to-unit consistency.

 

 

General Parameters

Model LA-AR01-FTMIR-060-4TE LA-AR01-FTMIR-085-4TE LA-AR01-FTMIR-120-4TE LA-AR01-FTMIR-160-LN2 LA-AR01-FTMIR-160-DLA
  Beam-splitter material CaF2 ZnSe
  Spectral Range [cm-1] 5000-1660 6600-1200 5000-830 5000-650
  Spectral Range [μm] 2-6 1.5-8.5 2-12 2-16
  Detector Type MCT (4-TE cooled) MCT(LN2 cooled) DLATGS
  Detector Peak D*[cm Hz1/2 W-1 ]  ＞1×10^11 ＞8×10^9 ＞4×10^9 ＞5×10^10 ＞2.5×10^8
  Signal-to-noise ratio ＞80000:1 ＞40000:1 ＞40000:1 ＞70000:1 ＞8000:1
  Removable fiber-optic coupler Lensed (CaF2 fiber coupler) Reflective fiber coupler (90° off-axis parabolic mirror)
  Recommended fiber IFG(1-6μm) IFG(1-6μm) orPIR(3-18μm) PIR (polycrystalline) fibers, 3-18μm
  Internal reference laser Temperature-stabilized solid-state laser @850nm
  Resolution(unapodized) [cm-1] 0.5,2,4,8(user selectable)
  Wavenumber repeatability ＜10 PPM
  Scan frequency ＞4 Hz @ 4cm-1 ＞0.4 Hz @ 4cm-1
  Power requirement 12V / 10W max 12V / 6W max

 

Other Parameters

  Fibered interface Fiber core up to ∅ 900μm, NA=0.3, SMA 905 connector
  Free-space interface ∅ 12.7mm collimated (max ~30mrad half angle)
  Interferometer type Permanently aligned, double retro-reflector design
  A/D Converter 24 bit
  Amplifier 4 gain levels low noise trans-impedance amplifier
  Operating temperature 10℃-40℃
  Communication Interface USB 2.0
  Software Interface Windows 7/10/11 API for controlling the instrument via our DLL
  Dimensions 180mm×160mm×80mm (Without Dewar)
  Weight 1800g (Without Dewar)

FT-IR spectroscopy: high resolution measurements

Introduction

Fourier Transform infrared (FT-IR) spectrometers have proven to be efficient and reliable tools for a large variety of applications targeting the near infrared (NIR) and mid-infrared (MIR) regions of the electromagnetic spectrum. One of the most critical specification of FT-IR spectrometers is their spectral resolution , as this defines the scale of the features to be distinguished e.g. in the absorbance spectrum of a gas. It is thus quite natural to wish for the highest achievable resolution when purchasing an FT-IR spectrometer. Due to their operational mode, achieving high resolution measurements using FT-IR spectrometers is however not entirely straightforward. This technical note explains the main limits of high resolution FT-IR measurements.

 

Fig .1 Simplified FT-IR setup

 

A simplified FT-IR is depicted in Fig.1. It consists of a beamsplitter, a fixed mirror and a movable mirror. Light from a purely monochromatic light source is split in two beams, assumedly in equal parts (50% beamsplitter). Each beam bounces back on either mirror (fixed or movable) before being recombined and focused onto the detector. When the movable mirror is at the same distance from the beamsplitter as the fixed mirror, both beams travel the same distance and recombine in-phase, yielding constructive interference. By displacing the moving mirror by a distance ℓ, one introduces an optical path difference (OPD) between the two beams Δ given by:Δ=2ℓ.

Upon recombination at the detector, the optical intensity as a function of OPD is given by: I(0)=0.5I0[1+cos(2πν0∆)]=IDC+IAC(∆).

Where I0 is the source intensity at wavenumber ν0 (the wavenumber is the reciprocal of the wavelength). The AC part of the interference record is labelled IAC(Δ) and is called the interferogram. For a purely monochromatic source (as considered here), the interferogram is a pure cosine function.

 

Fig 2. Interferogram of a purely monochromatic light source

 

Here the wavenumber (or equivalently the wavelength) of the source as well as its intensity can be retrieved from a direct observation of the interferogram (amplitude and period of the cosine function). For a broadband source, the interferogram IAC(Δ) and spectrum I0(ν) of the source are related via a Fourier transform operation:

I0(v)=∫∆maxIAC(∆)cos(2πν0∆)d∆

Obviously, the OPD cannot be made arbitrarily large and has to reach a value Δmax that is defined by technological design. Given the nature of the relationship between the interferogram and the spectrum (Fourier transform), it turns out that Δmax also defines the achievable spectral resolution. To a first approximation, the spectral resolution Δν of an FT-IR is given by:

∆ν=(∆max)-1

and is often expressed in cm-1. So why not simply increase the maximum OPD to enhance the spatial resolution of an instrument ? While this is true, special care has to be considered when operating at high resolution. As explained hereafter, the mirror maximum displacement is limited by the divergence of the system and the dimensions of the detector.

 

 

HR measurement: effect of beam divergence

We consider the exact same setup as in Fig.1. The beam divergence is accounted for by observing the behavior of the so called "extreme ray", which makes an angle α with respect to the "central ray" discussed previously.

 

 

Fig 3. FT-IR setup with a divergent source

 

 

The two extreme rays (reflected from either the fixed or the moving mirror) hit the lens with an angle α, unlike the central ray which hits the lens at normal incidence. They are thus focused on another point on the detector. Moreover, their OPD is shorter than for the central ray:

∆ext=2ℓcos(α)

The larger the angle α, the greater the difference with the central ray OPD Δcen=2ℓ. Consider now the case where the difference in OPD between the central ray and the extreme ray is equal to one half of the source wavelength, that is:

∆cen-∆ext=2ℓ[1-cos(α)]=λ/2

In this scenario, when the central rays are in phase, then the extreme rays are out of phase (and vice versa). Consequently, the intensity over the detector surface follows the profile shown in Fig. 4.

 

Fig 4. Intensity over the detector surface due to a highly diverging beam

 

 

Since the detector yields a single value that corresponds to the average intensity received on its surface, the signal detected in this case corresponds to the average optical intensity only, and all information regarding the interference signal vanishes. Practically speaking, the interferogram will start losing contrast as the moving mirror is scanned as shown in Fig. 5.

 

 

Fig 5. Loss in interferogram contrast due to a diverging beam

 

 

This effect is naturally existing in all interferometers based instruments (such as FT-IR) and cannot be avoided. It can however be properly managed by appropriately trading-off the parameters involved in equation (6), namely :

  • Δν: the effect is more pronounced for high resolution measurements due to the larger OPD required.
  • α: the effect is more pronounced for a highly diverging beam.
  • λ: the effect is more pronounced at short wavelengths (large wavenumbers).

 

For most applications in solids and liquids, the size of the observable features is typically broader than 2cm-1, and high resolution (HR) measurements performed at 0.5cm-1 are usually not required. In addition, these would prove challenging due to the added contrast loss described in this document. HR measurements might still be a viable option for specific applications, such as e.g. laser characterization, where the highly collimated laser beam prevents the dramatic loss of the interferogram contrast.


We fully appreciate and value the multiple benefits that a dedicated, performant and reliable software can bring to your application. Automatic data collection, parameters changes, status diagnosis and many other essential tasks should be implemented as simply and as efficiently as possible in order to get the most out of your spectrometer. This philosophy led to the development of a multi-threading, cross-platform and versatile software application, the digital acquisition system or AoDAQ.

The AoDAQ simultaneously takes care of:

1. Handling communication with the FT-IR via USB

2. Processing raw signals to deliver a spectrum

3. Running a TCP Ethernet server

The AoDAQ can be installed on all sorts of computers, from desktop machines to embedded, low-power single board computers. Thanks to the hosting of a TCP server, the instrument data and parameters can be accessed locally and/or remotely. All communication with the instrument eventually reduces to a set of TCP/IP commands that allow to quickly acquire data, adjust parameters, monitor the instrument status etc. using the programming environment of your choice. 

 


Case1：Textile Characterization

Experimental setup - NIR：

  • LL-AR01-NIR: NIR light source with QTH bulb
  • LA-AR01-FTNIR-025: FTIR spectrometer with 2TEC 0.9-2.5µm InGaAs detector
  • LA-AR01-FIB-NIR-600-100: Low-OH multimode silica fiber with a 600µm core diameter
  • Other Accessories: SMA fiber coupler with CaF2 lens

NIR setup (reference measurement) using the LL-AR01-NIR and an optical fiber collimator for free-space acquisition

 

Experimental setup - MIR：

  • LA-AR01-FTMIR-120-4TE: FTIR spectrometer with 4TEC 2-12µm detector,common-path output
  • Other Accessories: Parabolic reflector for diffuse reflectance measurement

MIR Textile characterization using a common path configuration

 

Experimental result：

The experimental parameters in both setup were identical in order to have a fair comparison between these two methods. The instrument resolution was set to 4cm-1, with Norton-Beer weak apodization function. The baseline was acquired with 64 averages, which corresponds to ~12s of acquisition time. The white reference for the NIR is made of Teflon, while the MIR reflectance standard is made of gold. The sample spectra are then measured using 32 averages, ~6s per scan.The absorbilities of pure cotton, pure synthetic fiber, and blended fabrics containing 55% cotton and 45% polyester were measured respectively.

Absorbance measured using the NIR setup (top) and MIR setup (bottom)

Case 2:Laser characterization

Experimental setup：

Pay Attention:Whether the laser source to be characterized is continuous wave (CW) or pulsed, the following limits must be watched at all time. Failing to respect these power limitations will result in loss of warranty and may lead to breakage of the detector .

 

Laser sources can be measured by directly shining laser light into the spectrometer via an optical fiber or through the free-space input port. There are however two key limiting aspects to take into account.

Maximum input power:

  • The CW, average or peak power of pulses longer then 1μs must not exceed 25mW
  • The peak power of pulses shorter than 1μs must not exceed 100W
  • For repeated irradiation with pulses shorter than 1μs, the equivalent CW irradiation, i.e. average power over the pulse-to-pulse period should be less than the CW maximum power according to equation:(Equivalent CW radiation power) = (pulse peak power)(pulse duration)(repetition rate)

Minimum repetition rate:

Pulsed lasers (or pulsed sources in general) can only be measured if their repetition rate is faster that the FTIR modulation frequency. Typically, only lasers with repetition rates above 10 kHz can be measured.

 

Case 3：Transmission Measurement

Transmission spectroscopy is a widespread analysis technique that consists in measuring the absorption produced by a given sample when a beam of infrared light goes through them. Transmission spectroscopy applies mostly to liquids and gases, yet pressed powders or thin-films might also be characterized using this methodology. Besides spectroscopic measurements, transmission measurements are also useful to evaluate the properties of optical components, such as windows and filters.

Experimental Setup：

  • LA-AR01-FTMIR-120-4TE: FTIR spectrometer with 4TEC 2-12µm detector,common-path output
  • LL-AR01-MIR : 1-25 μm MIR Source

1.What resolution should I use for my application?

In FTIR, resolution is traded-off with two other experimental metrics that are acquisition time and signal-to-noise ratio (SNR). Increasing resolution, meaning reducing the resolution parameter number, will result in longer acquisition time, and poorer SNR. In general, it is recommended to work at the "worst" possible resolution, that is the limit resolution that allows to distinguish the features of the sample or substance that you are characterizing. Liquids and solids present broader features than gases or gas mixtures and can generally be probed with standard resolution instruments (down to 2cm-1). Gas analysis or light sources characterization (typically lasers) usually benefit from a sharper resolution of 0.5cm-1.

 

2.How is the equivalent wavelength resolution calculated?

Due to its working principle, FT-IR provides uniformly sampled spectra in the form of wavenumber (ν) within a given spectral range, with the unit of cm-1. Wavenumber is simply defined as the reciprocal of wavelength (λ). The resolution of a FT-IR is a constant wavenumber (Δν), but varies with the wavelength (Δ λ) due to the inverse relationship between these two units. The conversion between wavenumbers resolution and wavelengths resolution is Δλ=λ2 · Δν, as shown in the following figure:


 

3.What is the acquisition rate of this spectrometer?

The acquisition rate varies with the resolution. At the standard resolution of (4cm-1), the scanning rate is ~5Hz, and at the high resolution of (0.5cm-1), the scanning rate is ~1Hz, that is, the scanning rate is inversely proportional to the resolution.

 

 

4.What are the differences between DLADTGS detectors and MCT detectors?

DLADTGS is based on the pyroelectric effect. When exposed to infrared radiation, its temperature will change, causing the polarization inside the crystal to change. MCT is a bandgap type photoconductive detector. DLADTGS has a wider spectral response range (up to 18-20 μm), and MCT detectors utilize thermoelectric cooling to achieve higher sensitivity, which can suppress dark current and improve their signal-to-noise ratio.

 

 

5.How much better is the performance of LN4 cooled detectors than that of MCT cooled detectors?

The SNR of the LN4 cooling system is approximately ten times that of the MCT cooling system.

 

 

6.Why is the performance of the Fourier transform spectrometer superior to that of the dispersive grating spectrometer?

The performance of a grating spectrometer mainly depends on the slit and detector. Due to the different dark noise of each pixel point, additional noise will be introduced during measurement. FTIR spectrometers are generally superior to dispersive grating spectrometers due to their higher light throughput, better signal-to-noise ratio, higher spectral resolution and wider wavelength range. These advantages make FTIR spectrometers the preferred choice in many applications, especially in places where high sensitivity and accuracy are required.

 

 

7.Can an FTIR spectrometer be used to measure pulsed laser?

Sure, but the power of the laser must be limited to avoid irreversible damage to the detector. The average power shall not exceed 25mW, and the peak power of pulses shorter than 1µs shall not exceed 100W. It is recommended to use a set of fixed or variable attenuators to adjust the optical power to avoid detector saturation. Secondly, the repetition frequency of the laser must exceed 25khz to avoid numerical artifacts (aliasing) in the measured spectrum.

 

 

8.Can the concentration of the substance being measured be obtained directly?

No. A spectrometer can only provide the measured spectrum. Users must obtain the concentration of substances in the sample through specific algorithms and calibration data.

 

 

9.Can this spectrometer be used for mineral identification?

The FT-NIR spectrometer is highly suitable for mineral identification. FT-NIR is capable of generating high-quality and high-resolution (better than 1nm) reflection spectra within the spectral range of 900-2550nm (SWIR) within a few seconds.


Related products

Model Wavelength Range Resolution Detector  
  FT-NIR spectrometer 900-2500 nm 2/4/8 cm-1(user selectable) Extended type InGaAs PIN photodiode, 2TE cooled
  FT-MIR spectrometer 200-16000 nm 0.5/2/4/8cm-1(user selectable) MCT (4-TE cooled)
MCT (LN2 cooled)
DLATGS
  Fiber coupled FT-IR spectrometer 200-16000 nm 2/4/ 8cm-1 (user selectable) InGaAs (2-TE cooled)
MCT (4-TE cooled)
MCT (LN2 cooled)
  VIS-NIR-FIB spectrometer 350-2500 nm <1.5 nm silicon array detector (3648 pixels) 16-bit ADC.extended range InGaAs photodiode, 2TE cooled, 24-bit ADC.
  VIS-NIR-DR spectrometer 360-2500 nm 5 nm Extended range InGaAs detector

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Compare Model Drawings & Specs Availability Reference Price
(USD)
LA-AR01-FTMIR-060-4TE
Spectral Range：2-6μm，Detector Peak D*：＞1×10^11cm Hz^1/2 W^-1，Signal-to-noise ratio：＞80000:1,Resolution：0.5/2/4/8cm-1(user selectable) 		4-6week $27429.00
LA-AR01-FTMIR-085-4TE
Spectral Range：1.5-8.5μm，Detector Peak D*：＞8×10^9cm Hz^1/2 W^-1，Signal-to-noise ratio：＞40000:1,Resolution：0.5/2/4/8cm-1(user selectable) 		4-6week $27429.00
LA-AR01-FTMIR-120-4TE
Spectral Range：2-12μm，Detector Peak D*：＞4×10^9cm Hz^1/2 W^-1，Signal-to-noise ratio：＞40000:1,Resolution：0.5/2/4/8cm-1(user selectable) 		4-6week $27429.00
LA-AR01-FTMIR-160-LN2
Spectral Range：2-16μm，Detector Peak D*：＞5×10^10cm Hz^1/2 W^-1，Signal-to-noise ratio：＞70000:1,Resolution：0.5/2/4/8cm-1(user selectable) 		4-6week $28160.00
LA-AR01-FTMIR-160-DLA
Spectral Range：2-16μm，Detector Peak D*：＞2.5×10^8cm Hz^1/2 W^-1，Signal-to-noise ratio：＞8000:1,Resolution：0.5/2/4/8cm-1(user selectable) 		4-6week $26880.00

LA-AR01-FTMIR-160-DLA - Parameter

LA-AR01-FTMIR-160-LN2 - Parameter

LA-AR01-FTMIR-120-4TE - Parameter

LA-AR01-FTMIR-085-4TE - Parameter

LA-AR01-FTMIR-060-4TE - Parameter

LA-AR01-FTMIR-160-DLA - Download

LA-AR01-FTMIR-160-LN2 - Download

LA-AR01-FTMIR-120-4TE - Download

LA-AR01-FTMIR-085-4TE - Download

LA-AR01-FTMIR-060-4TE - Download

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Compare Model Drawings & Specs Availability Reference Price
(USD)

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