The concept and the implementation of a compact and simplified echelle spectrometer are presented, and the working principle is demonstrated by first experimental measurements. The crucial element of the setup is a cross-grating, combining an echelle grating utilizing several higher diffraction orders (5th up to 11th) and a superposed perpendicular-oriented cross-dispersing grating. Two alternative manufacturing approaches for the cross-grating are presented and discussed. The first approach combines Talbot lithography for the deep echelle grating and interference lithography for the cross-dispersing structure. As a second approach, direct laser-beam writing was applied. The compact echelle spectrometer covers a spectral range from 380 to 700 nm and offers a spectral resolution of .
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In the last few decades, the main focus of optical spectroscopic instruments has shifted from high-end research and laboratory use to industrial process control and field applications. In particular, a strong demand for spectroscopic devices has been observable in agriculture for a broad diversity of different tasks, including site specific fertilization, soil analysis, or quality control of harvested crops [1,2] and in geology, e.g., for resource exploration . In general, the shift in focus has also had an impact on instrumental specifications and tightens the requirements for already existing mutually contradictory specifications. In particular, a major challenge concerns the simultaneous detection of a broad wavelength range with a high spectral resolution while realizing a short acquisition time and a compact integration volume. Current established solutions are mostly optimized for specific characteristics and cannot fulfill all specifications. Typical widespread used spectroscopic instruments, which cover a broad application variety, are imaging spectrometers based on a concave grating mount concept. These spectrometers, originally based on the Rowland-circle concept, offer a very compact setup with a minimum number of elements, employing only an entrance slit, a concave reflection grating, and a line detector . In contrast to their compact setup, imaging spectrometers are generally accompanied by an unavoidable trade-off between detectable wavelength range and spectral resolution. In particular, for a high spectral resolution, the grating period of the implemented diffraction grating has to be small. From this requirement follows a widely extended angular distribution of the decomposed spectrum when only a single diffraction order (e.g., the 1st order) is used. To guarantee in this case simultaneously a broad spectral range, a very large line detector, and complex imaging optics are required. In order to achieve large numerical apertures, it is becoming even more challenging to find an approach with a sufficiently flattened image field and with acceptable aberrations. An alternative to the conventional imaging spectrometer employs a multiorder concept . In this case, superimposed spectra of different diffraction orders are imaged simultaneously on the line detector. For spectral separation, the data acquisition of the different orders has to be coupled with switching illumination sources, which provides adjacent spectral ranges in a series. The disadvantage of multiorder concepts concerns the expenditure of acquisition time.
In contrast, more complex systems such as echelle spectrometers, are capable of meeting sophisticated optical specifications, in particular of combining a high spectral resolution and the recording of a broad spectral range at the same time . As a disadvantage, these systems are characterized by an elaborate and complex setup and often are large in size. Echelle spectrometers are often especially designed and constructed for use in large astronomical telescopes [7–10].
For the functionality of an echelle spectrometer, the key components are two successively arranged dispersing elements: first, the echelle grating, and second, the cross-dispersing element. The echelle grating is illuminated by the incoming light and is generating a set of superimposed spectra of different high-diffraction orders. These superimposed spectra propagate to the second element, the cross-disperser, which separates the overlying spectral bands of the different diffraction orders in a perpendicular direction with respect to the orientation of the echelle grating. Finally, the resulting separated spectra are imaged in stripes onto a two-dimensional array detector. The spectral decomposition in two stages limits the options to reduce required volume, ensure low complexity, and control remaining imaging aberrations during the optical design.
In this contribution, we present a simplified echelle spectrometer setup involving a cross-grating as a key element, which combines the functionality of both the echelle grating and also the cross-disperser. The concept of this cross-grating echelle spectrometer (CGES) aims at bridging the gap between classical echelle spectrometers and single-dispersive element instruments. In comparison to classical echelle concepts, the CGES allows a volume reduction of the overall setup and offers new options for the optical design, especially to reduce the size of the final focusing optics. Therefore, the CGES concept offers widened opportunities for field applications, e.g., in agriculture or geology. We present an optical design for a CGES, including transmissive optical modules for the collimator to illuminate the cross-grating homogeneously and for the final optic group to image the laterally separated spectral stripes of the different echelle orders onto a two-dimensional detector array. The reflective cross-grating is the key element of the whole concept. As a surface relief structure, the cross-grating superimposes two orthogonal-oriented gratings with highly different profile depths. We discuss two different methods for the manufacturing of the cross-grating, in particular, initially a two-step approach combining Talbot and interference lithography (IL) and second, using direct laser-beam writing. Finally, we present the implemented CGES and demonstrate the results of initial measurements.
2. SYSTEM CONCEPT AND OPTICAL DESIGN
The development focuses primarily on demonstrating the capabilities of the CGES principle as a proof of concept. With the exception of the cross-grating, only easily available optical components are implemented. As the target spectral region the visible wavelength range is selected. This simplifies the commissioning phase of the CGES system; in particular, it allows a convenient system adjustment. For a geometrical modeling of the demonstrator setup, which was then to be used to prove the grating performance, a commercial optical design software (Zemax, “OpticStudio” ) was used. To take the entire diffraction characteristics of the cross-grating into account, in particular the orientation of the diffracted beams with respect to the different wavelengths, the generalized grating equation for the two-dimensional case  was solved and implemented in an internally developed “dynamic link library” (dll), which is accessed by the optical design software.
Figure 1 shows two cross sections of the optical design. In Fig. 1(a), a horizontal cross section is displayed, comprising all optical elements, in particular the entrance slit, collimator optics, cross-grating, final imaging optics, and two-dimensional detector array. In Fig. 1(b), the vertical cross section of the final part of the spectrometer is shown, ranging from cross-grating to the two-dimensional detector. In the following and unless indicated otherwise, the “drawing plane” is related to the horizontal cross section of Fig. 1(a).
The light to be analyzed passes an optical fiber (not shown) and enters the spectrometer at the entrance slit. The divergent light cone propagates to the collimator unit, generating a parallelized ray bundle that steers in the direction of the two-dimensional grating. As a collimator, a simple achromatic doublet (Thorlabs, Inc., AC254-50-A ) with a focal length of 50 mm was selected. For the main goal, the proof of the basic principle and with respect to the expected challenges in the manufacturing technologies, the dimensions of the grating were limited to a usable area of . For the main echelle structure, a grating period of 10 μm was chosen, which is used in the 5th to the 11th diffraction order. The cross-dispersing structure is oriented in the perpendicular direction and is used in the 1st diffraction order. In order to balance between both aspects, the optical requirements and the manufacturing challenges of the grating, two different periodicities for the cross disperser were considered, in particular, a periodicity of 2 and 5 μm, respectively.
The light incident on the two-dimensional cross-grating is diffracted simultaneously in two orthogonal directions. In the setup, the two-dimensional grating is aligned in such a way that the spectral separation induced by the cross-grating is oriented in a perpendicular direction to the drawing plane. The main grating component (echelle structure) decomposes the light in the direction of the drawing plane. If only the echelle structure were present, the spectra of several higher diffraction orders would overlap in the same area. In the implementation, the grating is rotated by 35° with respect to the normal of the drawing plane (see inset of upper image in Fig. 1). This orientation guarantees that the target diffraction orders are captured by the imaging optics. The proposed layout allows a sufficiently simple adjustment of the overall system. The superposed cross-dispersing grating is oriented in a perpendicular direction and induces a separation of the overlapping echelle diffraction orders in the direction perpendicular to the drawing plane. To keep the principal ray of the 1st diffraction order of the cross-dispersing structure for a reference wavelength of in the drawing plane, the grating was positioned in a Littrow configuration with a tilting angle of with respect to the horizontal axis (see lower part of Fig. 1). Finally, the diffracted light is focused by a 30 mm achromatic lens (Thorlabs, Inc., AC254-030-A ) onto the CCD sensor of a monochrome camera (AlliedVision, Manta G-032 B ) with an active sensor area of (pixel size ; ). The longer axis of the sensor chip is oriented upward along the cross-dispersing direction. The focal length of the imaging lens was chosen to utilize a maximum area of the sensor. The period of the cross-dispersing structure (2 μm) is associated with an angle separation of for the addressed wavelength interval (380–700 nm). To focus the incoming parallel ray bundles of maximum and minimum wavelength at the opposite sides of the sensor (4.85 mm distance), a focus length slightly larger than 30 mm is necessary. For the additionally considered 5 μm periodicity of the cross-disperser, the two-dimensional detector is only partially covered. All useful diffraction orders generated at the cross-grating, which are finally combined into the total spectrum, have to be captured free of vignetting by the clear aperture of the imaging group. For this purpose, the chosen achromat offers an overall diameter of 25.4 mm and a clear aperture of 22.9 mm, respectively. That means the overall diameter is not significantly larger than the clear aperture, which allows a small installation space and relaxes the mechanical design.
Figure 2(a) shows the distribution of calculated spots for different wavelengths in the target plane for the configuration with the 2 μm periodic cross-disperser. The displayed lateral dimension correlates to the extension of the sensor chip. Each of the line series represents the spots for a specific echelle order. In particular, the figure shows seven separated spectra comprising the 5th to the 11th echelle diffraction order. For each series, 12 separated spots are depicted. The spectral distances between adjacent spots are not equally distributed but are smaller at the edges rather than in the center for each echelle order. With increasing diffraction order, the observable wavelength range shifts to shorter wavelengths and decreases. In particular, the 5th diffraction order includes the spectral interval from 620 to 700 nm on the detector, and the 9th diffraction order covers the range from 380 to 420 nm, respectively. Additionally, also the correlating spectral dispersion increases with increasing diffraction order. For example, the wavelength range from 580 to 620 nm shows an angle separation of 1.4° in the 6th order and of 1.6° in the 7th order, respectively.
Due to the system’s simplicity, it is afflicted by aberrations. Here, a significant contribution is attributed to coma, which can be observed in the upper-left and lower-right corners of Fig. 2(a). In these regions, the orientation of the coma tail is directed nearly perpendicular to the spectral distribution axis of the specific diffraction order and therefore causes only a minor negative effect on the spectral resolution. In addition to coma, a residual longitudinal chromatic aberration also remains for peripheral wavelengths. This effect is reducible by simply tilting the detector plane. For clarification, the influence of the detector tilting is illustrated by four subimages; see Figs. 2(b)–2(e). Here Figs. 2(b) and 2(d) show the magnified sections of the 5th and 11th diffraction order series, respectively, which depict resulting spots for the initial orientation in which the normal of the detector plane is parallel to the optical axis of the final imaging group. In contrast, in Figs. 2(c) and 2(e), the spots are depicted for a tilting angle of 3.5° between the normal of the detector plane and the optical axis of the achromat. In particular, for the 5th diffraction order series [Figs. 2(b) and 2(c)], it becomes clearly observable that the detector tilting provides significantly more uniform and more strongly separated spots in the observation plane compared to the initial nontilted case.
Quantitatively, the orientation of the detector plane also affects the resolving power of the system, which is defined as , where is the target wavelength and is the resolution that describes the separable wavelength range. The introduction of the tilt of the detector plane improves the resolution from initially to . In the latter case, this is transferred to a resolution of 2.1 nm for the range of 380–615 nm and 4.5 nm for the range of 380–700 nm.
3. MANUFACTURING AND CHARACTERIZATION OF CROSS-GRATINGS
The most decisive element in the setup is the cross-grating for which a suitable manufacturing technology has to be found. In general, the development of new and adequate manufacturing technologies is a long-term and evolutionary process until a qualified solution is applicable. Especially, in comparison to the manufacturing of conventional one-dimensional gratings, which are optimized with respect to spectral purity, efficiency, and resolution for many years, the structuring of the cross-grating is facing additional and new challenges. In particular, these challenges are associated with the different depths and periodicities of both perpendicular-oriented gratings. Sawtooth-like blazed profiles are targeted for both linear gratings. It has to be taken into account that the echelle structure is used in several higher diffraction orders (5th to 11th order), which leads to the necessity of a structure depth of about 1.5 μm. In contrast, the cross-dispersion uses the 1st diffraction order, making profile heights in the range of some hundred nanometers appropriate. We investigated two different technologies. At first, a combination of IL and Talbot lithography was applied, and the second approach was based on direct laser-beam writing.
A. Combination of IL and Talbot Lithography
For a first proof of concept, two lithographic process steps were applied successively. A Borofloat-substrate (thickness 2 mm) covered with a 2 μm thick positive photoresist layer was used (MicroChemicals, AZ1518 ). First the cross-dispersion structure with a period of 2 μm was created using IL. In detail, a classical one-beam recording method was applied, in which the interference of both the forward-propagating and backward reflected light is used . The desired periodicity of the cross-grating was adjusted by inclining the substrate with respect to the interference pattern of the recording beams. Additionally, the space between the photoresist and the autocollimation mirror was filled with a medium of sufficient high refractive index to minimize Fresnel reflections in the exposure setup. The wavelength of the laser light source was 406 nm (Ondax; LM-405-PLR-40 ). For the structuring of the echelle grating, serial Talbot lithography was used . Therefore, a 10 μm periodic linear amplitude chrome mask with a duty cycle of 0.25 was applied (duty cycle: ratio between width of the transparent part to the grating period). The resist-coated substrate was placed in the lithography setup in the plane of the first self-image Talbot distance (: wavelength, p: mask period). As a light source, a light-emitting diode (LED) with a wavelength of 402 nm (ProLight Opto; PM2L-1LLx-LC ) was applied. The chrome mask was illuminated by a plane wave, which creates a periodic Talbot-intensity pattern in the substrate plane with a retained duty cycle. To inscribe the triangular-shaped blaze profile into the photoresist, the illumination angle was changed step by step, shifting the intensity pattern laterally in the substrate plane. To create the 10 μm periodic grating, the illumination angle was varied between 1.11° and in 22 steps of about 0.087°. On the substrate, this angle variation resulted in a lateral shift of the Talbot-intensity pattern from 4.67 to . Simultaneously with the changing of the illumination angles, the exposure dose was adapted between 1.14 and , so that a deep, triangular-shaped latent image was created in the resist. Afterwards, the double-exposed photoresist was developed so that the surface topography was formed. Finally, the surface was coated with a reflective aluminum layer.
Figure 3(a) shows a microscopic image of the cross-grating taken by differential interference contrast (DIC). Both the echelle structure, with 10 μm periodicity in the direction and the cross-dispersion structure (2 μm periodicity) in the direction are clearly observable. For a more quantitative analysis, the profile topography was also measured by atomic force microscopy (AFM) [see Fig. 3(b)], from which the profile heights were determined (see Table 1). The arrows above the upper edge of the DIC microscopy image indicate the positions of the measured profile cross sections by AFM. The echelle structure in the direction, marked by red arrows, shows a distinct sawtooth-shaped form with a height of 1.4 μm, indicating the suitability of Talbot lithography for the corresponding process step.
In the direction, along the cross-dispersion direction, two cross sections are depicted. The first, indicated by the green arrows, passes the higher structures of the echelle grating, whereas the second (black arrows) is a cross section through its middle parts, respectively. Both cross sections show a 2 μm periodicity and slight asymmetry with profile depths of 120 and 75 nm measured at the top and the bottom of the echelle structure, respectively. Thus, the cross-grating is suitable for a proof of principle of the spectrometer concept but is limited in diffraction efficiency.
The achieved cross-grating profile offers a suitable basis for further technology developments. The basic advantages of the proposed manufacturing approach are, on the one hand, associated with IL being one of the few technologies allowing the manufacturing of small periodic gratings with asymmetric groove profiles. On the other hand, Talbot lithography allows the generation of moderate deep and asymmetric profiles but is limited to relatively large periodicities. Additionally, both lithography methods offer the potential to generate gratings with high spectral purity, in particular offering a high groove-spacing uniformity. Hereby, it should be mentioned, that the mask used for Talbot lithography can also be manufactured by IL, assuring spectral purity for the final echelle structure. Furthermore, it is possible to enhance the initial insufficient IL-generated resist profile depth by using the selectivity of reactive ion beam etching (RIBE). Depending on the different etching components and process characteristics, the etching rates for resist and substrate are controllable, so that an adapted structure transfer with enhanced depth can be achieved .
B. Direct Laser-Beam Writing
In a second approach, the cross-grating was manufactured by direct laser-beam writing. In contrast to the first approach, using the point-wise illumination by a laser spot that is laterally scanned across the substrate, this method enables the direct creation of the required two-dimensional grating structure. A -wafer (thickness 1 mm) was covered with a 6 μm thick photoresist layer (MicroChemicals, AZ4562 ) by spin-coating. For the lithography process, a commercial laser system was applied (Heidelberg Instr. Mikrotechnik GmbH, DWL 2000 ). The working wavelength was 413 nm (Krypton Ion Laser), and the nominal minimum structure size measured was 0.7 μm. The final surface relief structure of the cross-grating forms during the development process. The writing regime addresses, successively, bands of 160 μm width, in which each band is created line by line. The bands were oriented parallel to the echelle structure. The overlay accuracy (stitching error) measures approximately 200 nm. Due to the stitching effects between two adjacent bands, disturbing periodicity errors are introduced. They are associated with a decreasing spectral purity, the occurrence of “ghosts,” and a nominal resolution decay. In order to generate a distinct profile also in the cross-dispersion direction and due to resolution restrictions of the writing laser system, we chose a larger period of 5 μm for the cross-dispersion grating instead of the 2 μm periodicity as before. Figure 4 shows the result of an AFM measurement of the final cross-grating profile, which again enables the determination of the profile height (see Table 1). In Fig. 4(a) a three-dimensional representation of a surface area is displayed. Both the echelle structure with a period of 10 μm and the cross-dispersing structure with 5 μm periodicity are clearly observable. The cross section parallel to the axis [Fig. 4(b)] shows a well-defined blaze-like structure with a profile depth of 1.57 μm. The cross section in the perpendicular direction, parallel to the axis, shows the profile of the cross-dispersing structure [Fig. 4(c)]. Here, also a sawtooth-like profile appears, having a depth of . The measurement of the cross-dispersing structure indicates the same profile shape, regardless of whether taken along the upper or the lower part of the echelle structure. In comparison to the previously discussed manufacturing method using the combination of Talbot and IL, direct laser writing obviously offers an advantage concerning local profile accuracy. But, due to unavoidable stitching errors, the grating structure manufactured by direct laser-beam writing will suffer from periodicity imperfections and therefore decreases the spectral purity leading to spectral resolution constraints.
4. EXPERIMENTAL DEMONSTRATION OF THE CGES WORKING PRINCIPLE
Finally, the working principle of the CGES concept was demonstrated experimentally. Therefore, the cross-grating manufactured by the combination of IL and Talbot lithography is integrated in the setup. Figure 5(a) shows the measured continuous partial spectra of a white LED captured by the two-dimensional detector. Clearly, the orientation of the different diffraction orders is visualized on the detector. The 6th up to the 9th diffraction orders show pronounced signals, whereas the partial spectra of the 5th and the 11th orders appear very weak. The bandwidth on the detector covers the range from 640 to 700 nm for the 5th order and 380 nm to 405 nm for the 11th order, respectively. The weak signal at the lowest usable diffraction order can be attributed to both the weak light intensity emitted at the long, red spectral range and to a grating efficiency decrease in the long wavelength range. Since an LED source with an initial wavelength of about 450 nm, which excites the converting phosphor, was used, also the 10th and 11th diffraction orders covering the spectral bands around 400 nm and below are not or are only weakly present at the figure. Further restrictions arise from the UV absorption of the glass fiber and the absorption of the flint glasses in the collimator and imaging optics.
Figure 5(b) shows the same LED spectrum for the grating produced by direct laser-beam writing. In comparison to the first grating, due to the larger period of the cross-dispersing grating, the separation of the higher diffraction orders is smaller, resulting in a more horizontal orientation of the spectral orders. The second main difference is the occurrence of significant ghosts, as shown in Fig. 5(c), where a vertical cross section of the detected diffraction pattern is depicted. The position of the cross section is indicated by the red line in Fig. 5(b). For a more detailed study of the CGES concept, the discrete emission spectra of different spectral lamps were acquired. Especially, the spectra of a mercury-cadmium lamp (Hg-Cd lamp) and a helium lamp (He lamp) were measured successively with different acquisition times. The spectral lines on the measured images were cropped and manually recomposed using different acquisition times, in the range of 10–640 ms, for each shown spectral line to create Fig. 6. Due to this method, it is possible to show many spectral lines originating from different sources in a single image, regardless of different diffraction efficiencies of the grating orders, quantum efficiencies of the detector, and intensity of different spectral lines. The different diffraction orders (5th to 10th) are clearly observable, but in contrast to the continuous spectra of the LED, the spectra show the discrete behavior from single-emission lines. The specific spectral lines are labeled with table values. The shortest detected discrete wavelength (388.9 nm, He line) is found in both the 10th and in the 11th order. Respectively, the longest wavelength (667.8 nm, He line) appears in the 5th and 6th order. A closer look at the measured signal distribution of the single lines show partially frayed or blurred spots, which can be attributed to the aberrations of the imaging system. The aberrations are dominated by a combination of coma and astigmatism, and their size and orientation significantly depend on the field position. Axial position and tilting angle of the detector also influence the aberrations. In particular, the orientation of the aberration affects the achievable spectral resolution significantly. Especially when the tail of the aberration is oriented along the spectral distribution of the specific diffraction order, the blurred spots from adjacent spectral lines overlap so that the spectral resolution is locally decreased. If the aberration tail is oriented perpendicular to the spectral direction, only minor effects of this contribution influence the resolution. Examples for both cases can be observed in the 10th order for the emission line of 404.7 nm, where the spot-broadening occurs in the direction of the spectrum and at the 7th order at 546.1 nm, where the aberration tail points mainly in the perpendicular direction to the spectrum. The spectral resolution of the CGES setup can be evaluated by the analysis of closely adjacent spectral lines. In particular, the constituent wavelength pair 577 nm (Hg) and 579 nm (Hg) is recognized as clearly separated. On the left of Fig. 6, a magnified section of the recorded spectra is displayed. This detailed view shows the aberrated but also clearly separated spots of the neighboring spectral lines from which a resolution of can be estimated.
We present the basic concept of a CGES, in which both the functionality of the main echelle grating, working in several higher diffraction orders, and the cross-disperser are combined in a single element, the cross-grating. This approach provides an opportunity to overcome at least partially the classical conflict in spectrometer design between the necessity of simultaneously fulfilling the optical specification of high spectral resolution, large accessible wavelength range, and compact setup integration without moving parts. The presented implementation aimed at the demonstration of the basic working principle and employs mainly easily accessible optical components. For an optimized demonstration of the performance potential of the CGES concept, two aspects have to be improved. First, the manufacturing technology of the cross-grating has to be further developed to allow the tailoring of specific grating requirements. In particular, the grating profile has to be tuned to increase diffraction efficiency, and the lateral accuracy of grating periodicity has to be enlarged to suppress ghost imaging. Second, the surrounding optical components for collimation and imaging have to be optimized with respect to the new spectroscopic concept.
Carl Zeiss Spectroscopy GmbH; Fraunhofer-Gesellschaft (project Miro); Thuringian Ministry for Economic Affairs, Science and Digital Society (2015-0024); Federal Ministry of Education and Research, Founding Program Research at Universities of Applied Science (13FH027IX5).
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