Over the past 20 years, miniature fiber optic spectrometers have evolved from a novelty to the spectrometer of choice for many modern spectroscopists. People are realizing the advanced utility and flexibility provided by their small size and compatibility with a plethora of sampling accessories.
The basic function of a spectrometer is to take in light, break it into its spectral components, digitize the signal as a function of wavelength, and read it out and display it through a computer. The first step in this process is to direct light through a fiber optic cable into the spectrometer through a narrow aperture known as an entrance slit. The slit vinyettes the light as it enters the spectrometer. In most spectrometers, the divergent light is then collimated by a concave mirror and directed onto a grating. The grating then disperses the spectral components of the light at slightly varying angles, which is then focused by a second concave mirror and imaged onto the detector. Alternatively, a concave holographic grating can be used to perform all three of these functions simultaneously. This alternative has various advantages and disadvantages, which will be discussed in more detail later on.
Once the light is imaged onto the detector the photons are then converted into electrons which are digitized and readout through a USB (or serial port) to a computer. The software then interpolates the signal based on the number of pixels in the detector and the linear dispersion of the diffraction grating to create a calibration that enables the data to be plotted as a function of wavelength over the given spectral range. This data can then be used and manipulated for countless spectroscopic applications, some of which will be discussed here later on.
In the following sections we will explain the inner-workings of a spectrometer and how all of the components work together to achieve a desired outcome, so that no matter what your application is, you’ll know what to look for. We’ll first discuss each component individually so that you have a full understanding of their function in the workings of a spectrometer, then we’ll discuss the variety of configurations that are possible with those components, and why each of them has a different function. We’ll even touch on some of the accessories used to make your application as successful as they can possibly be.
A spectrometer is an imaging system which maps a plurality of monochromatic images of the entrance slit onto the detector plane. This slit is critical to the spectrometer’s performance and determines the amount of light (photon flux) that enters the optical bench and is a driving force when determining the spectral resolution. Other factors are grating grove frequency and detector pixel size.
The optical resolution and throughput of a spectrometer will ultimately be determined by the installed slit. Light entering the optical bench of a spectrometer via a fiber or lens is focused onto the pre-mounted and aligned slit. The slit controls the angle of the light which enters the optical bench.
Slit widths come in a number of different sizes from 5µm to as large as 800µm with a 1mm (standard) to 2mm height. Selecting the right slit for your application is very important since they are aligned and permanently mounted into a spectrometer and should only be changed by a trained technician.
The most common slits used in spectrometers are 10, 25, 50, 100, 200 μm, etc. For systems where optical fibers are used for input light coupling, a fiber bundle matched with the shape of the entrance slit (stacked fiber) may help increase the coupling efficiency and system throughput.
The function of the entrance slit is to define a clear-cut object for the optical bench. The size (width (Ws) and height (Hs)) of the entrance slit is one of the main factors that affect the throughput of the spectrograph. The image width of the entrance slit is a key factor in determining the spectral resolution of the spectrometer when it is greater than the pixel width of the detector array. Both the throughput and resolution of the system should be balanced by selecting a proper entrance slit width.
The image width of the entrance slit (Wi) can be estimated as:
Wi = (M2×Ws2+Wo2)1/2
Where M is the magnification of the optical bench set by the ratio of the focal length of the focusing mirror (lens) to the collimating mirror (lens), Ws is the width of the entrance slit, and Wo is the image broadening caused by the optical bench. For a CZ optical bench, Wo is on the order of a few tens of microns. So reducing the width of the entrance slit below this value won't help much on improving the resolution of the system. The axial transmissive optical bench provides much smaller Wo. Thus it can achieve a much higher spectral resolution. Another limit on spectral resolution is set by the pixel width (Wp) of the array detector. Reducing Wi below Wp won’t help to increase resolution of the spectrometer.
Under the condition that the resolution requirement is satisfied, the slit width should be as wide as possible to improve the throughput of the spectrograph.
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The diffraction grating of a spectrometer determines the wavelength range and partially determines the optical resolution that the spectrometer will achieve. Choosing the correct grating is a key factor in optimizing your spectrometer for the best spectral results in your application. Gratings will influence your optical resolution and the maximum efficiency for a specific wavelength range. The grating can be described in two parts: the groove frequency and the blaze angle, which are further explained in the sections below.
There are two types of diffraction gratings: ruled gratings and holographic gratings. Ruled gratings are created by etching a large number of parallel grooves onto the surface of a substrate, then coating it with a highly reflective material. Holographic gratings, on the other hand, are created by interfering two UV beams to create a sinusoidal index of refraction variation in a piece of optical glass. This process results in a much more uniform spectral response, but a much lower overall efficiency.
While ruled gratings are the simplest and least expensive gratings to manufacture, they exhibit much more stray light. This is due to surface imperfections and other errors in the groove period. Thus, for spectroscopic applications (such as UV spectroscopy) where the detector response is poorer and the optics are suffering more loss, holographic gratings are generally selected to improve the stray light performance of the spectrometer. Another advantage of holographic gratings is that they are easily formed on concave surfaces, allowing them to function as both the dispersive element and focusing optic at the same time.
The amount of dispersion is determined by the amount of grooves per mm ruled into the grating. This is commonly referred to as groove density, or groove frequency. The groove frequency of the grating determines the spectrometer’s wavelength coverage and is also a major factor in the spectral resolution. The wavelength coverage of a spectrometer is inversely proportional to the dispersion of the grating due to its fixed geometry. However, the greater the dispersion, the greater the resolving power of the spectrometer. Inversely, decreasing the groove frequency decreases the dispersion and increases wavelength coverage at the cost of spectral resolution.
For example, if you were to choose a Quest™ X spectrometer with a 900g/mm, it would give you a wavelength range of 370 nm, with an optical resolution as low as 0.5nm. Comparably, if you were to choose a Quest™ X with a 600g/mm grating, it would instead give you up to 700nm of wavelength coverage with an optical resolution as low as 1.0nm. As this example shows, you are able to increase your wavelength coverage at the sacrifice of optical resolution.
When the required wavelength coverage is broad, i.e. λmax > 2λmin, optical signals in wavelengths from different diffraction orders may end up at the same spatial position on the detector plane, which will become evident once we take a look at the grating equation. In this case, a linear variable filter (LVF) is required to eliminate any unwanted higher order contributions, or perform “order sorting”.
For fixed grating spectrometers, it can be shown that the angular dispersion from the grating is described by
where Β is the diffraction angle, d is the groove period (which is equal to the inverse of the groove density), m is the diffraction order, and λ is the wavelength of light as shown in Figure 1-1.
Figure 1-1
By taking into account the focal length (F) of the focusing mirror and by assuming the small angle approximation, equation 2-1 can be rewritten as
which gives the linear dispersion in terms of nm/mm. From the linear dispersion, the maximum spectral range (λmax - λ min) can be calculated based upon the detector length (LD), which can be calculated by multiplying the total numbers of pixels on the detector (n) by the pixel width (Wp) resulting in the expression
Based on equation 2-3 it is clear that the maximum spectral range of a spectrometer is determined by the detector length (LD), the groove density (1/d) and the focal length (F).
The minimum wavelength difference that can be resolved by the diffraction grating is given by
where N is the total number of grooves on the diffraction grating. This is consistent with transform limit theory which states that the smallest resolvable unit of any transform is inversely proportional to the number of samples. Generally, the resolving power of the grating is much higher than the overall resolving power of the spectrometer, showing that the dispersion is only one of many factors in determining the overall spectral resolution.
It should also be noted that the longest wavelength that will be diffracted by a grating is 2d, which places an upper limit on the spectral range of the grating. For near-infrared (NIR) applications, this long wavelength limitation may restrict the maximum groove density allowed for your spectrometer.
As a grating diffracts incident polychromatic light, it does not do so with uniform efficiency. The overall shape of the diffraction curve is determined mainly by the groove facet angle, otherwise known as the blaze angle. Using this property, it is possible to calculate which blaze angle will correspond to which peak efficiency; this is called the blaze wavelength. This concept is illustrated in figure 2-1, which compares three different 150g/mm gratings blazed at 500nm, 1250nm & 2000nm.
Figure 2-1
Gratings can be blazed to provide high diffraction efficiency (>85%) at a specific wavelength, i.e. a blaze wavelength (λB). As a rule of thumb, the grating efficiency will decrease by 50% at 0.6×λB and 1.8×λB. This sets a limit on the spectral coverage of the spectrometer. Generally, the blaze wavelength of the diffraction grating is biased toward the weak side of the spectral range to improve the overall signal to noise ratio (SNR) of the spectrometer.
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In the previous sections we discussed the importance of the entrance slit and the diffraction grating in forming a spectral image of the incident light in the image plane. In traditional spectrometer (monochrometer) designs, a second slit is placed in the image plane, known as the exit slit. The exit slit is typically the same size as the entrance slit, since, the entrance slit width is one of the limiting factors on the spectrometer’s resolution (as was shown in Part 1). In this configuration a single element detector is placed behind the exit slit and the grating is rotated to scan the spectral image across the slit, and therefore measure the intensity of the light as a function of wavelength.
In modern spectrometers, CCD and linear detector arrays have facilitated the development of “fixed grating” spectrometers. As the incident light strikes the individual pixels across the CCD, each pixel represents a portion of the spectrum that the electronics can then translate and display with a given intensity using software. This advancement has allowed for spectrometers to be constructed without the need for moving parts, and therefore greatly reduce the size and power consumption. The use of compact multi-element detectors has allowed for a new class of low cost, compact spectrometers to be developed: commonly referred to as “miniature spectrometers.”
While photodetectors can be characterized in many different ways, the most important differentiator is the detector material. The two most common semiconductor materials used in miniature spectrometers are Si and InGaAs. It is critical to choose the proper detector material when designing a spectrometer because the bandgap energy (Egap) of the semiconductor determines the upper wavelength limit (λmax) that can be detected by the following relationship
where h is Plank’s constant and c is the speed of light. The product of Plank’s constant and the speed of light can be expressed as 1240 eV•nm or 1.24 eV•µm to simplify the conversion from energy to wavelength. For example, the bandgap energy of Si is 1.11eV which corresponds to maximum wavelength of 1117.117nm. InGaAs, on the other hand, is an alloy created by mixing InAs and GaAs, which have a bandgap of 0.36eV and 1.43eV respectively, and depending on the ratio of In and Ga the bandgap energy can be tuned in between those two values. However, due to a variety of factors, not all ratios of In and Ga are easily fabricated, therefore 1.7µm (or 0.73eV) has become the standard configuration for InGaAs detector arrays. It is also possible to use extended InGaAs arrays which can detect out to 2.2µm or 2.6µm, but these detectors are much more expensive and are much nosier than traditional InGaAs detectors.
The lower detection limit of a material is slightly harder to quantify because it is determined by the absorbance characteristics of the semiconductor material, and as a result can vary widely with the thickness of the detector. Another common method of lowering the detection limit of the detector is to place a fluorescent coating on the window of the detector, which will absorb the higher energy photons and reemit lower energy photons which are then detectable by the sensor. Figure 3-1 below shows a comparison of the detectivity (D*) as a function of wavelength for both Si (CCD) and InGaAs.
While currently InGaAs detector arrays are only available in one configuration, Si multi-element detectors are readily available in three different subcategories: charge coupled devices (CCDs) back-thinned charge coupled devices (BT-CCDs), and photodiode arrays (PDAs).
CCD technology allows for small pixel size (~14µm) detectors to be constructed because it eliminates the need for direct readout circuitry from each individual pixel. This is accomplished by transferring the charge from one pixel to another, allowing for all of the information along the array to be read out from a single pixel. CCDs can be constructed very inexpensively which makes them an ideal choice for most miniature spectrometers, but they do have two drawbacks. First, the gate structure on the front of the CCD can cause the incident light to scatter and therefore not be absorbed. Second, CCDs need to have a relatively large P-Si substrate to facilitate low cost production, however, this also limits the efficiency of the detector (especially at shorter wavelengths) due to absorption through the P layer.
To mitigate both of these issues in spectroscopy applications where very high sensitivity is needed, BT-CCDs are ideal. BT-CCDs are made by etching the P-Si substrate of the CCD to a thickness of approximately 10µm. This process greatly reduces the amount of absorption and increases the overall efficiency of the detector. This process also allows the detector to be illuminated from the back side (P-Si region) which eliminates the effects from the gate structure on the surface of the detector. Figure 3-2 below shows a typical comparison of the quantum efficiency between a traditional front illuminated CCD and a back illuminated BT-CCD.
While there are distinct advantages to the use of BT-CCDs in spectroscopy, there are also two major drawbacks that should be noted. First, this process greatly increases the cost of production, and second (since the detector is so thin ) there can be an etaloning effect caused from reflections off the front and back surfaces of the detector. The etaloning phenomena associated with BT-CCDs can be mitigated by a process known as deep depletion, but once again this adds additional cost to the production process.
PDA detectors are more traditional linear detectors which consist of a set of individual photodiodes that are arranged in a linear fashion using CMOS technology. These detectors, while not having the small pixel size and high sensitivity, have several advantages over CCD and BT-CCD detectors. First, the lack of charge transfer eliminates the need for a gate structure on the front surface of the detector, and greatly increases the readout speed. The second advantage of PDA detectors is that the well depth is much higher than the well depth of a CCD; a typical PDA detector well depth is ~156,000,000e- as compared to ~65,000e- for a standard CCD. The larger well depth of PDA detectors causes them to have a very large dynamic range ~50,000:1 as well as an extremely linear response. These properties make PDAs ideal for applications where it is necessary to detect small changes in large signals, such as LED monitoring.
Stay tuned for next month’s continuation – where we’ll continue our discussion on detector noise, and the role of TE cooling.
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Last month, in Part 3a, we discussed several different types of detectors and the role they play in miniature spectrometers. No discussion of detectors would be complete without covering noise sources and how they can be mitigated by the use of TE Cooling.
The main noise sources found in an array detector include: readout noise, shot noise, dark noise, and fixed pattern noise.
Readout noise is caused by electronic noise in the detector output stage and related circuitry, which largely dictates the detection limit of the spectrometer.
Shot noise is associated with the statistical variation in the number of photons incident on the detector, which follows a Poisson distribution. Therefore, shot noise is proportional to the square root of the incident photon flux.
Dark noise is associated with the statistical changes in the number of electrons generated in a dark state. A photo detector exhibits a small output even when no incident light is present. This is known as the dark current or dark output. Dark current is caused by thermally generated electron movements and is strongly dependent on ambient temperatures. Similar to shot noise, dark noise also follows a Poisson distribution; as a result, dark noise is proportional to the square root of the dark current.
The fixed pattern noise is the variation in photo-response between neighboring pixels. This variation results mainly from variations in the quantum efficiency among pixels caused by non-uniformities in the aperture area and film thickness that arise during fabrication.
The total noise of an array detector is the root square sum of these four noise sources.
Cooling an array detector with a built-in thermoelectric cooler (TEC) is an effective way to reduce dark noise as well as to enhance the dynamic range and detection limit. For Si detectors, dark current doubles when the temperature increases by approximately 5 to 7 °C and halves when the temperature decreases by approximately 5 to 7°C. Figure 3-3 shows the dark noise for an un-cooled and cooled CCD detector at an integration time of 60s. When operating at room temperature, the dark noise nearly saturates the un-cooled CCD. When the CCD is cooled down to only 10°C by the TEC, the dark current is reduced by about four times and the dark noise is reduced by about two times. This makes the CCD capable of operating at a longer integration time to detect weak optical signals. When a CCD based spectrometer is involved in non-demanding high light level applications such as LED measurement, the dark noise reduction due to TE cooling is minimal because of the relatively short integration time used.
As a rule of thumb, when the integration time of a CCD spectrometer is set to less than 200ms, the detector is operating in a read noise limited state. Therefore, there is no significant noise reduction due to the TE cooling; although the temperature regulation under these conditions will be beneficial for long term baseline stability.
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As stated in Part 1: The Slit, a spectrometer is an imaging system which maps a plurality of monochromatic images of the entrance slit onto the detector plane. In the past 3 sections, we discussed the three key configurable components of the spectrometer: the slit, the grating, and the detector. In this section, we will discuss how these different components work together with different optical components to form a complete system. This system is typically referred to as the spectrograph, or optical bench. While there are many different possible optical bench configurations, the three most common types are the crossed Czerny-Turner, unfolded Czerny-Turner, and concave holographic spectrographs (shown in figures 4-1, 4-2, and 4-3 respectively).
The crossed Czerny-Turner configuration consists of two concave mirrors and one plano diffraction grating, as illustrated in figure 4-1. The focal length of mirror 1 is selected such that it collimates the light emitted from the entrance slit and directs the collimated beam of light onto the diffraction grating. Once the light has been diffracted and separated into its chromatic components, mirror 2 is then used to focus the dispersed light from the grating onto the detector plane.
Figure 4-1 Crossed Czerny-Turner Spectrograph
The crossed Czerny-Turner configuration offers a compact and flexible spectrograph design. For a diffraction grating with given angular dispersion value, the focal length of the two mirrors can be designed to provide various linear dispersion values, which in turn determines the spectral coverage for a given detector, sensing length and resolution of the system. By optimizing the geometry of the configuration, the crossed Czerny-Turner spectrograph may provide a flattened spectral field and good coma correction. However, due to its off-axis geometry, the Czerny-Turner optical bench exhibits a large image aberration, which may broaden the image width of the entrance slit by a few tens of microns. Thus, the Czerny-Turner optical bench is mainly used for low to medium resolution spectrometers. Although this design is not intended for two dimensional imaging, using aspheric mirrors (such as toroidal mirrors) instead of spherical mirrors can provide a certain degree of correction to the spherical aberration and astigmatism.
To minimize image aberrations, the Czerny-Turner optical bench is generally designed with an f-number of >3, which in turn places a limit on its throughput. The f-number of an optical system expresses the diameter of the entrance pupil in terms of its effective focal length. The f-number is defined as f/# = f/D, where f is the focal length of the collection optic and D is the diameter of the element. The f-number is used to characterize the light gathering power of the optical system. The relation of the f-number with another important optical concept, Numerical Aperture (NA), is that: f/# = 1/(2•NA), where the numerical aperture of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.
The relatively large f/# of Czerny-Turner optical benches, in comparison to a typical multimode fibers (NA ≈ 0.22), can cause a fairly high level of stray light in the optical bench. One simple and cost-effective way to mitigate this issue is by unfolding the optical bench as shown in figure 4-2 below. This allows for the insertion of “beam blocks” into the optical path, greatly reducing the stray light and, as a result, the optical noise in the system. This issue is not as damaging in the visible and NIR regions where there is an abundance of signal and higher quantum efficiencies, but it can be a problem for dealing with medium to low light level UV applications. This makes the unfolded Czerny-Turner spectrograph ideal for UV applications that require a compact form factor.
Figure 4-2 Unfolded Czerny-Turner Spectrograph
The other most common optical bench is based on an aberration corrected concave holographic grating (CHG). Here, the concave grating is used both as the dispersive and focusing element, which in turn means that the number of optical elements is reduced. This increases throughput and efficiency of the spectrograph, thus making it higher in throughput and more rugged. The holographic grating technology permits correction of all image aberrations present in spherical, mirror based Czerny-Turner spectrometers at one wavelength, with good mitigation over a wide wavelength range.
Figure 4-3 Concave-Holographic Spectrograph
In comparison with a ruled grating, the holographic grating presents up to over a 10x reduction in stray light, which helps to minimize the interferences due to unwanted light. A ruled diffraction grating is produced by a ruling engine that cuts grooves into the coating layer on the grating substrate (typically glass coated with a thin reflective layer) using a diamond tipped tool. A holographic diffraction grating is produced using a photolithographic technique that utilizes a holographic interference pattern. Ruled diffraction gratings, by the nature of the manufacturing process, cannot be produced without defects, which may include periodic errors, spacing errors and surface irregularities. All of these contribute to increased stray light and ghosting (false spectral lines caused by periodic errors). The optical technique used to manufacture holographic diffraction gratings does not produce periodic errors, spacing errors or surface irregularities. This means that holographic gratings have significantly reduced stray light (typically 5-10x lower stray light compared to ruled gratings) and removed ghosts completely.
Ruled gratings are generally selected when working with low groove density, e.g., less than 1200 g/mm. When high groove density, low stray light, and/or concave gratings are required, holographic gratings are the better choice. It is important to keep in mind that the maximum diffraction efficiency of concave holographic gratings is typically ~35% in comparison to plano ruled gratings, which can have peak efficiencies of ~80%.
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One of the most important characteristics of a spectrometer is the spectral (or optical) resolution. The spectral resolution of a system determines the maximum number of spectral peaks that the spectrometer can resolve. For example, if a spectrometer with a wavelength range of 200nm had a spectral resolution of 1nm, the system would be capable of resolving a maximum of 200 individual wavelengths (peaks) across a spectrum.
In dispersive array spectrometers, there are 3 main factors that determine the spectral resolution of a spectrometer: the slit, the diffraction grating, and the detector. The slit determines the minimum image size that the optical bench can form in the detector plane. The diffraction grating determines the total wavelength range of the spectrometer. The detector determines the maximum number and size of discreet points in which the spectrum can be digitized.
It is important to understand that the observed signal (So) is not solely dependent on the spectral resolution of the spectrometer (R) but it is also dependent on the linewidth of the signal (Sr). As a result, the observed resolution is the convolution of the two sources,
When the signal linewidth is significantly greater than the spectral resolution, the effect can be ignored and one can assume that the measured resolution is the same as the signal resolution. Conversely, when the signal linewidth is significantly narrower than the spectrometer resolution, the observed spectrum will be limited solely by the spectrometer resolution.
For most applications it is safe to assume that you are working in one of these limiting cases, but for certain applications such as high resolution Raman spectroscopy, this convolution cannot be ignored. For example, if a spectrometer has a spectral resolution of ~3cm-1 and uses a laser with a linewidth of ~4cm-1, the observed signal will have a linewidth of ~5cm-1 since the spectral resolutions are so close to each other (assuming a Gaussian distribution).
For this reason, when attempting to measure the spectral resolution of a spectrometer it is important to assure that the measured signal is significantly narrow to assure that the measurement is resolution limited. This is typically accomplished by using a low pressure emission lamp, such as an Hg vapor or Ar, since the linewidth of such sources is typically much narrower than the spectral resolution of a dispersive array spectrometer. If narrower resolution is required, a single mode laser can be used.
After the data is collected from the low pressure lamp, the spectral resolution is measured at the full width half maximum (FWHM) of the peak of interest.
When calculating the spectral resolution (δλ) of a spectrometer, there are four values you will need to know: the slit width (Ws), the spectral range of the spectrometer (Δλ), the pixel width (Wp), and the number of pixels in the detector (n). It is also important to remember that spectral resolution is defined as the FWHM. One very common mistake when calculating spectral resolution is to overlook the fact that in order to determine the FWHM of a peak, a minimum of three pixels is required, therefore the spectral resolution (assuming the Ws = Wp) is equal to three times the pixel resolution (Δλ/n). This relationship can be expanded on to create a value known as resolution factor (RF), which is determined by the relationship between the slit width and the pixel width. As would be expected, when Ws ≈ Wp the resolution factor is 3. When Ws ≈ 2Wp the resolution factor drops to 2.5, and continues to drop until Ws > 4Wp when the resolution factor levels out to 1.5.
All of this information can be summarized by the following equation,
For example, if a spectrometer uses a 25µm slit, a 14µm 2048 pixel detector and a wavelength range from 350nm – 1050nm, the calculated resolution will be 1.53nm.
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When configuring a spectrometer for a given experiment, one of the commonly overlooked considerations is in choosing the best fiber optic cable. Although there are many different factors to consider for this choice, this section will focus on the following two key factors: core diameter and absorption.
First, we will briefly review what a fiber optic cable is and how it is used to direct light into a spectrometer. Then, we will discuss the two characteristics stated above and why they are important for determining the throughput of the fiber optic.
A fiber optic can be thought of as a “light pipe”. If you consider how the pipes in a home direct water from one location to another by guiding it through twists and turns to the desired location, you can recognize that fiber optics guide light waves in a similar fashion. Instead of directing light to a bathroom or kitchen, though, we are interested in guiding the light into a spectrometer or other optical detection system. This is achieved by a process known as total internal reflection.
In order to understand how total internal reflection is achieved, we must first look at the optical property known as refraction. Refraction arises because the speed of light varies based on the material it is traveling through. As a result, when light transitions from one medium to another, the angle at which the light is traveling is retarded relative to the interface.
The refracting power of a material is defined as
where n is the index of refraction, v is the speed of light in the medium of interest, and c is the speed of light in a vacuum. For example, the index of refraction of air is 1.000293, which shows that the speed of light in air is almost exactly the same as it is in a vacuum, whereas the index of refraction of water is 1.333, showing that light travels 25% slower in water than in a vacuum.
The relationship between the index of refraction and the angle at which light travels is defined by Snell’s law
From this equation, we can see that the refracted angle (θ2) is dependent on the ratio of the indices of the two materials (n1/n2) as well as the incident angle (θ1). As a result, by controlling the ratio of the indices, one can engineer the refracted angle such that all of the light is reflected back from the interface. This is known as total internal reflection and is the method that allows for light to be contained and guided inside of a fiber optic.
Figure 6-1 Total Internal Reflection in a Fiber Optic
Figure 6-1 illustrates how a fiber optic is designed to facilitate total internal reflection by using two different types of glass, a lower index cladding, and a higher index core in order to trap the light within the core of the fiber and guide it through the fiber optic. This ability to collect light from one place and direct it to another is the reason fiber optic cables are the ideal solution for coupling light into a spectrometer.
Since all of the light in a fiber optic is collected in the core, the diameter of the core directly correlates to the amount of light that can be transmitted. Based on this principle, it would seem intuitive that a larger core diameter will improve the sensitivity and signal-to-noise ratio of a spectrometer. While this is true to a certain extent, there are other limiting factors that need to be considered when selecting the right fiber optic.
The first thing to consider is the pixel height of the detector. As shown in previous sections, the optical bench of a spectrometer is designed to form an image of the slit onto the detector plane. If the detector pixels are only 200µm in height and you select a 400µm core fiber, 50% of the light incident on the detector is wasted. In this case, there appears to be no advantage gained from having a larger core, but there is a way to get around this issue by adding a cylindrical lens into the optical bench in front of the detector.
Figure 6-2 Signal Intensity for Various Core Diameters with a Cylindrical Lens Installed
The cylindrical lens focuses the image of the slit in the axis orthogonal to the array without distorting the image along the axis parallel to the array in the detector plane. This allows for the light from the entire core to be directed onto the pixel, greatly increasing the sensitivity of the overall setup. Figure 6-2 shows that this approach works quite well up to a 600µm core fiber.
Another important factor to consider is the absorption properties of the fiber optic. If the light is absorbed by the fiber, it will never be detected by the spectrometer.
During the traditional manufacturing process for fiber-optics, OH- ions are inadvertently doped into the glass by the plasma torches used to soften the bulb so that it can be drawn into fibers. The presence of these ions creates very strong absorption bands (known as water peaks) in the NIR, which can greatly interfere with the ability to make broad band measurements through this region. In order to avoid this when using fiber optics for NIR spectroscopy, fiber optics need to be manufactured using special low OH- plasma torches.
Figure 6-3 Comparison of Standard and Low OH- Fiber Optics in the NIR
Inversely, there are also severe absorption properties in the UV spectrum. This property arises from a photo-chemical effect known as solarization, which worsens over time with extended UV exposure especially below 290nm.
For these reasons, it is extremely important to pay close attention when selecting a fiber for a specific application. When operating in the NIR spectra, make sure to choose low OH- fibers optics (also commonly called NIR fiber optics). When working in the visible and near UV spectral region, standard fiber optics commonly referred to as UV fiber optics are acceptable. When working in the deep UV (<290nm), solarization resistant fibers generally referred to as SRUV fibers are required.
Stay tuned for next month’s segment – where we’ll continue our discussion on fiber optic bundles and probes.
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For many spectroscopic applications, proper sampling requires more than just a simple fiber optic patch cord. In cases that require you to measure various samples simultaneously or those that require improved signal to noise ratio (as in the case of weak signals), the use of fiber optic bundles are required. In this section, we will discuss the advantages and disadvantages of some common fiber optic bundle configurations.
A fiber optic bundle is defined as any fiber optic assembly that contains more than one fiber optic in a single cable. The most common example of a fiber optic bundle is known as a bifurcated fiber assembly. The goal of using a bifurcated fiber assembly is either to split a signal or to combine signals. Figure 7-1 shows an example of a typical bifurcated fiber assembly.
Figure 7-1 Example of a Bifurcated Fiber Assembly
Some of the most common applications for bifurcated fiber assemblies are those that require you to direct light from a sample into two different spectrometers. This is generally used to extend the spectral coverage of the measurement, either to maintain higher resolution, or to cover an extended range. For example, if someone is looking to make a broadband measurement from 350 – 1700nm, they need to use both an InGaAs and a Si detector array. By using a bifurcated fiber assembly with one UV fiber and one NIR fiber to direct light into each spectrometer, they can make a simultaneous measurement. Figure 7-2 shows an example spectrum of this type of measurement.
Figure 7-2 Spectrum of a Tungsten Halogen Lamp from 350 – 1700nm
A bifurcated fiber can also be used to couple the signal from multiple samples into the same spectrometer. When using a bifurcated fiber in this fashion, only one sample can emit light at a time, or special care should be taken to make sure the signals do not have spectral overlap.
The same basic principal and applications can be scaled up to trifurcated and quadfurcated fiber assemblies as well. An example of a trifurcated fiber assembly is shown in Figure 7-3 below.
Figure 7-3 Trifurcated Fiber Assembly
Another common bundled fiber optic assembly is called a “round to slit” configuration. This configuration consists of multiple small core fibers (typically 100µm) that are put into one fiber assembly with fibers bundled tightly in a circular fashion on one end, and stacked linearly on top of each other on the other end. The end with fibers stacked linearly on top of one another form a pattern to match the entrance slit of the spectrometer, as shown in Figure 7-4 below.
Figure 7-4 “Round to Slit” Fiber Optic Bundle
This configuration allows for much higher throughput into the spectrometer, as opposed to simply using a larger core fiber. As shown in Figure 7-5 below, when a large core fiber is placed in front of the entrance slit of a spectrometer, the majority of the light is vinyetted and doesn’t make it into the spectrometer. By contrast, when the smaller fibers are stacked along the entrance slit, significantly more light enters into the spectrometer. This allows for much higher sensitivity and signal to noise, while maintaining resolution, since the slit can remain relatively narrow.
Figure 7-5 Comparison of Stacked Fiber to Single Large Core Fiber
When using a fiber optic assembly with a slit configuration, it is important to remember two important details. First, in order to get any benefit from the fiber stacking, a cylindrical lens must be used to prevent the vast majority of the light to be imaged above and below the detector. Second, it is important to properly align the fiber stack to the entrance slit, which can be done by shining light into the round end of the assembly and monitoring the signal as the fiber is rotated in the SMA905 connection port. When peak signal is achieved, the fiber can then be screwed down to lock the position. One very common application using this kind of fiber optic assembly is NIR transmission spectroscopy, where there are very few photons and photon energy is extremely low. An example of a transmittance setup is shown below in Figure 7-6.
Figure 7-6 Example Transmittance Setup Utilizing a “Round to Slit” Fiber Bundle
By combining various combinations of single, round, and stacked configurations with regular, bifurcated, trifurcated, and quadfurcated fiber assemblies, there are countless options available to suit any application. In the next section, we will discuss how to combine fiber bundles with other various opto-mechanical components to create more specific applications.
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Now that we understand the basics of fiber optic cables and bundles and how they can be used to collect and direct light, we will explore how fiber optics can be packaged and combined with different opto-mechanical components to construct fiber optic probes. Fiber optic probes are the ideal solution for analyzing large or awkwardly shaped samples, monitoring real-time kinetic reactions, sampling in vivo, and any other application where it is difficult to bring the sample to the spectrometer. The flexibility and user-friendliness of fiber optic probes has made them one of the most widespread tools in modern spectroscopy. In this section, we will briefly discuss four of the most common fiber optic probes: reflectance probes, dark-field reflection probes, transflectance dip probes, and Raman probes.
The most basic fiber optic probe is a reflectance probe, which in its simplest form consists of a bifurcated fiber where the distal (bundled) end is placed in a metal sheath instead of a SMA connector, as shown in figure 8-1. This setup allows for one of the bifurcated ends to be connected to a light source, such as a fiber coupled tungsten halogen lamp, while the other is connected to a spectrometer. In this setup, the light from the lamp will travel through the 1st bifurcated end to the distal end of the probe and reflect off of the sample. The reflected light from the sample will then travel from the distal end to the 2nd bifurcated end and into the spectrometer for analysis.
Figure 8-1 Fiber Optic Reflectance Probe
It is important to note that before reflection data can be collected by the spectrometer, the system must be calibrated by taking a reference scan. This reference scan is taken by placing a white light reflectance standard, such as PTFE, at the same geometry from the probe as will be used in the actual measurement. This will allow the spectrometer to measure the ratio between a “perfect” white light reflector and the sample of interest in order to determine which wavelengths of light are reflected and which are absorbed.
When measuring reflection, there are two standard geometries that are employed: 0o and 45o normal to the sample. When measuring at 0o, the probe will pick up the specular (mirror like) component of the reflected light as well as the diffuse component, but when measuring at 45o, the majority of the specular light is not collected by the probe. This is an important consideration for applications such as colorimetry and NIR spectroscopy, where the specular component can distort the spectrum and skew the results.
A slightly more complex approach to the design of reflectance probes is to employ a round-to-slit fiber optic bundle. As described in the previous section, this is one common approach to overcoming the issue of weak photon energy in the NIR. In many reflection probes designed to work in the NIR, this method is applied by stacking 6 fibers on the bifurcated end attached to the spectrometer and employing a 6-around-1 configuration on the distal end. The 6 outer fibers are going to the slit configuration on the spectrometer and the center fiber connects to the light source in the other bifurcated end, as shown in figure 8-2 below.
Figure 8-2 Fiber Optic Reflection Probe with Slit-to-Bundle Configuration
Reflectance probes can also be scaled up to trifurcated and quadfurcated designs in order to increase the spectral range over which the reflection data is collected.
Specular reflection does not contain any useful information for NIR spectroscopy, but it can typically be removed by measuring the sample at a 45o angle. However, if the sample cannot be measured at a 45o angle, such as when working in a field or production setting, dark-field illumination (a method borrowed from microscopy) can be used. The dark-field probe works by illuminating the sample with an annulus of 7 fibers. The diffusely reflected light is then collected by a bundle of 7 fibers in the center of the probe which directs the light to the spectrometer in a slit configuration, as shown in figure 8-3 below. The specular components of the light are further reduced by the use of a lens at the distal end of the probe to redirect the light away from the center fiber bundle.
Figure 8-3 Dark-field Fiber Optic Probe
While reflection probes can be used to measure liquids, they are primarily designed for the measurement of solids. When measuring liquid samples, a dip probe is generally the probe of choice, since it can be submerged into the sample, allowing for kinetic data to be collected. The design of a fiber dip probe is very similar to that of a reflection probe, though special effort is taken to guarantee that it is liquid tight and inert. The key functional difference is the presence of a cavity which, when immersed, fills with the liquid sample. This cavity contains an optically transparent window placed at the distal end of the fiber and a small mirror placed at the bottom of the cavity to reflect the transmitted light back through the sample and into the collection fiber as shown below in figure 8-4. This setup is commonly referred to as a transflectance, due to the fact that this method combines transmission and reflection, doubling the optical path length.
Figure 8-4 Fiber Optic Transflectance Dip Probe
It is important to note that transflectance measurements can also be made using a dark-field reflectance probe configuration. Figure 8-3 shows an adaptor which can be placed over the dark-field probe to enable transflectance measurements in liquids and slurries.
The last probe that we will discuss in this section is called a Raman probe, which is used to measure the inelastic scattering of light off of a sample. Raman scattering is a nonlinear effect resulting in the shift in wavelength from a known monochromatic source. This shift is equal to the vibrational frequency of the molecular bonds in the material. As a result, a Raman probe must be capable of directing and focusing the monochromatic excitation source (typically a laser) to the sample, collecting the scattered light and then directing it to the spectrometer. Figure 8-5 shows a typical design for a Raman probe.
Figure 8-5 Typical Design of a Raman Probe
Since a pure signal is extremely important to Raman spectroscopy, a narrow band-pass filter is placed in the optical path of the excitation source before it reaches the sample. It is also important to note that since the Raman effect is extremely weak, the signal must be collected at a 0o angle normal to the sample. As discussed earlier, this causes interference from specular reflections, which in this case is referred to as Rayleigh scattering. Therefore, it is essential to filter the collected signal through the use of a long pass filter before it is directed to the spectrometer.
The Raman probe is a perfect example of how fiber optics can be combined with other optical components to enable simple and flexible measurement of even the most complicated spectroscopy.
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