How Does a Spectrometer Work?



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 vignettes 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 read out 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




1- The Slit





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, groove 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. It is important to select the right slit for your application 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 and 200 μm. 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.



2- Diffraction Grating




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.



3- The Detector





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


CCDs, BT-CCDs, and PDAs

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 Player.


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.



4- The Optical Bench




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.



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


Concave Holographic

The third 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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