We report for the first time the fabrication procedure of monolithic interferometers for one (1-D) and two-dimensional (2-D) spatial heterodyne spectrometer (SHS) with ultraviolet curing adhesive and commercial optical elements. The interferometer alignment was achieved by a feasible alignment adjustment scheme under the conditions of monitoring the interferogram and corresponding 2-D Fourier transform for known light source. A fabricated monolithic interferometer was calibrated and tested using both artificial and natural light sources. Its performance was steadily near the design predictions. The current work provides technical know-how for turning a design into an actual monolithic SHS interferometer.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Spatial Heterodyne Spectrometer (SHS) provides an effective tool for high-resolution spectroscopic studies of diffuse sources over a pre-selected spectral range [1–3]. An SHS is of a two-beam interferometer configuration similar to that of the Michelson interferometer, a beamsplitter is retained, but the fixed diffraction gratings are used instead of the plane mirrors. It has no moving parts and can be field widened by placing prisms in the arms of the interferometer. With these features the SHS can be developped into a rugged compact instrument suitable for spaceflight and field applications . The kernel in this development is fabrication of the monolithic interferometer where all the optical elements (beam splitter, gratings and prisms) are assembled and cemented together.
Although the monolithic SHS interferometers have already shown an extensive application in space-borne atmospheric remote sensing [2–7], available information on their fabrication procedure is limited. In particular, there is a lack of feasible alignment adjustment scheme as well as definite alignment criterion for the fabrication (assembly) of the monolith. Here the “alignment” (i.e., interferometer alignment) refers to a state of the monolithic interferometer in which all the components are assembled in proper relative positions so that the SHS system can produce desired output (interferogram and corresponding two-dimensional (2-D) Fourier transform) when the input is a known light source.
So far, nearly all the reported monolithic SHS interferometers were one-dimensional (1-D) and made by a professional company in optical contacting (LightMachinery, Inc., in Ottawa, Canada) [2–7]. In an earlier fabrication, engineers of the company utilized the optical contact technique (without adhesive) to bond a monolithic interferometer [2–4]. They took a geometric approach to achieve the alignement. The alignment of the spacers, prisms, and gratings to the beamsplitter was accomplished by means of a Zerodur plate . The bottom surfaces of all the components and beamsplitter were placed on the Zerodur plate. When the components were bonded to the beamsplitter, the Fizeau fringe field between the plate and the bottom surfaces of the components was monitored as an alignment criterion. Recently, the optical adhesive cementing was used to fabricate the monolithic interferometers instead of the optical contact technique [5–8]. The applied optical adhesive (Norland ultraviolet curing adhesive) has a feature that it begins to cure only after the bonded surface is exposed to ultraviolet light. This feature appeared to provide sufficient time for engineers to accomplish the interferometer alignment. According to a recent report involving the fabrication of monolithic interferometer, the final interferometer alignment was achieved by cementing one grating to its neighboring spacer under the condition that the interferometer could produce the desired fringe frequencies when the input was a known line source .
In this paper, we report for the first time the fabrication (assembly) procedure of monolithic SHS interferometers in 1-D and 2-D format with ultraviolet curing adhesive and commercial optical elements. It can provide technical know-how for turning a design into an actual monolithic SHS interferometer under conventional laboratory conditions. A new feasible alignment adjustment scheme is utilized to set the interferometer alignment via monitoring the interferograms and corresponding 2-D Fourier transforms (i.e., power spectra) for known light source. A monolithic interferometer fabricated in this way for a 2-D SHS system has been calibrated and tested using both artificial and natural light sources. The results shows that the performance of the monolithic interferometer is steadily near the design predictions.
2. Fabrication of monolithic interferometers
The present monolithic SHS interferometers were designed to observe the direct solar-irradiance spectrum around 940-nm water vapor absorption band on the ground. Table 1 lists the design parameters of the 1-D (a) and 2-D (b) monolithic interferometers to be fabricated. Each of them is comprised of a cube beam splitter, two gratings, two prisms and four spacers. The Littrow wavelength was set to ∼940 nm. The angle parameters of the prisms and spacers were calculated based on the theoretical derivation in . The solid angle gains in Table 1 are given with respect to the solid-angle field of view of the interferometer without field-widening prisms. In light of the calculation method in , the field of view of the designed interferometers with the prisms are respectively 16.4 deg (in the ϕ direction, ϕ is the angle between the incident wavevector and the dispersion plane) and 6.15 deg (in the β direction, β is the angle between the projection of the wavevector on the dispersion plane and the optical axis) for the 1-D SHS and 16.1 deg and 1.68 deg for the 2-D one. Each spacer has the same junction surface as that of the adjacent component on each side. This ensures the implementation of the monolithic assembly. The prisms and spacers were made by Union Optics Co., Ltd at Wuhan, China. A post-fabrication test at Union Optics indicated that their fabrication tolerances for the surface flatness, wedge angles and thicknesses are respectively λ/10 at 632 nm, 0.29 mrad (1 minute) and 0.05 mm. The beam splitters are a standard product of Newport with a surface flatness of ≤λ/4 at 632 nm and dimensional tolerance of 0.254 mm. The gratings are also standard products of Thorlabs (surface flatness is not available).
In order to fabricate the monolithic interferometers, a breadboard of SHS system was built in our lab. Figure 1 shows a schematic diagram of the breadboard, while Fig. 2 presents a photograph of the corresponding actual device. A bandpass-filtered neon lamp was chosen as a calibration source. The imaging lens and camera provided a real-time monitoring for the interferometric alignment. For the convenience of alignment, apart from one grating (referred to as “grating 1” hereafter) all the interferometer components in the breadboard (see Fig. 2) had been cemented together in place to form an interferometer subassembly. The final interferometric alignment in the monolith fabrication (assembly) could be achieved by cementing the grating 1 to the outer spacer (referred to as the “spacer 1” hereafter) of the interferometer subassembly under the conditions of monitoring the interferograms and corresponding 2-D Fourier transforms (i.e., power spectra). For this purpose, the interferometer subassembly was installed on a mount, as seen from Fig. 2. The grating 1 was then clamped onto the spacer 1 by a gripper that could make the grating 1 rotate slightly with respect to its normal. Since the junction surfaces (of the spacer 1 and grating 1) had been designed to be parallel each other, the rotation adjustment conformed to the structure of the interferometers designed. In fact, the optical breadboard shown in Fig. 2 represents the alignment adjustment under the conditions of monitoring the interferograms and corresponding 2-D Fourier transforms during the final cement phase.
2.1 Alignment adjustment and monitoring
In the absence of adhesive, we rotationally adjusted the grating 1 with respect to its normal (via the fine rotation adjustor on the gripper shown in Fig. 2) and monitored the interferometer output using a 938-962 nm bandpass-filtered neon lamp as a source. Figure 3 shows the interferograms (top row, a1-d1) and corresponding 2-D power spectra (bottom row, a2-d2) from the bandpass-filtered neon lamp when the grating 1 was rotationally adjusted to four different positions which represent four different interferometric alignments recorded by the imaging camera. Figure 3(a) corresponds to the basic SHS configuration in one-dimensional format. All the fringes are parallel each other (see Fig. 3(a1)) and the true and ghost spectra are overlapped (see Fig. 3(a2)). These features result from the cosine symmetry of the interferogram . Figures 3(b-d) reflect the two-dimensional SHS configurations where the overlapped true and ghost spectra in one-dimensional case are separated. Note that there are five bright lines in each of the true and ghost spectra (see Figs. 3(b2-d2)). This coincides with the five lines (942.54, 945.92, 948.67, 953.42 and 954.74 nm) in the 938-962 nm neon lamp spectrum. A dark line smaller than the Littrow wavelength is due to out-off-band leakage of the bandpass filter (at 937.33 nm). The separation between the true and ghost spectra becomes larger with increasing rotation angle. This indicates that rotating the grating 1 by small angle with respect to its normal has the same effect as rotating one grating by small angle in the basic SHS configuration about an axis (x axis) perpendicular to the system optical axis and the grooves of the grating  (a conventional method to convert the basic SHS configuration to the two-dimensional format [1,7,10–12]). As seen in Fig. 1, the structure designed for the monolithic interferometers makes it unlikely to rotate the grating 1 about the x axis. Consequently, as an alternative solution, we can achieve the 1-D or 2-D SHS configuration by rotating the grating 1 by a small angle about its normal under the condition of monitoring the 2-D Fourier transforms of the interferograms.
In terms of the procedure described above, the final fabrication step is to cement the grating 1 to the spacer 1 under the condition of the proper interferometer alignment (see Fig. 3). The ultraviolet curing adhesive (Norland) is used for this cementing. It will not cure until exposed to ultraviolet (UV) light. This feature provides us a sufficient time to accomplish the interferometer alignment by slightly rotating the grating 1 with respect to its normal after the adhesive is applied to the bonded surfaces of the grating 1 and spacer 1. Once the desired alignment is achieved as shown in the imaging camera, the bonded surfaces are irradiated with UV light to produce the monolithic interferometer.
2.2 Fabrication of 1-D monolithic interferometer
The 1-D SHS is often applied as spaceborne spatial heterodyne imager [2–9] in which the spectrum is retrieved by taking 1-D Fourier transform of a slice along x (horizontal) direction of the imaging detector and spatial (scene) information is contained in the y (vertical) dimension of the imaging detector. In this case, the interferometer-produced fringes should have a high parallelism with respect to the grating grooves (i.e., the two arms of the monolithic interferometer have a good symmetry) . According to the design parameters given in Table 1(a) and the method described above, we fabricated a 1-D monolithic SHS interferometer with high parallelism.
Based on the breadboard shown in Fig. 2, the high parallelism can be achieved by two-step rotation adjustments to the grating 1 under different light sources. The ultraviolet curing adhesive had been applied to the bonded surfaces of the grating1 and spacer 1 before adjustments (more exactly before the interferometer subassembly and grating 1 were mounted). The first step was similar to that mentioned above with the 938-962 nm bandpass-filtered neon lamp as a source. When the grating 1 was rotationally adjusted to a position where the true and ghost spectra were overlapped (see Fig. 3(a2)), an initial alignment for the 1-D SHS interferometer was believed to be accomplished. Then the neon lamp was removed and replaced with a white-light lamp (tungsten) under the condition of maintaining this initial alignment (the grating 1 is still at the position of the initial alignment). Figure 4(a) shows the fringe pattern from the bandpass-filtered white-light lamp under the initial alignment. As seen from Fig. 4(a), a high-contrast line (band) runs vertically through image center (x = 0) which represents the location of zero path difference. The modulation is visible along this line (band) which results from a slight y tilt of one of the gratings . In order to restrain this slight y tilt, we made the second-step adjustment to the grating 1. When the grating 1 was finely rotated to a position where the near-uniform modulation along the zero-path-difference line was observed (the line showed consistent dark color, see Fig. 4(b)), the final alignment for the 1-D monolithic interferometer was accomplished. After that, maintaining the grating 1 at the position of the final alignment, the grating 1 was cemented to the spacer 1 by UV light irradiation on their bonded surface to form the monolithic interferometer for 1-D SHS. For the fixedness, the bonded monolithic interferometer was clamped on the mount for one week (see Fig. 2). A test after several months indicates that it still has a stable alignment as shown by Fig. 4(b).
In order to evaluate the fringe visibility for the 1-D SHS interferometer, we set up an extended source with a tunable single-frequency diode laser (CTL950, TOPTICA Photonics) and an integrating sphere. The output laser beam was fed into the integrating sphere via a multi-mode fiber. The output port (a 25.4-mm circle) of the integrating sphere was put on the focal plane of the collimating lens (with a focal length of 100 mm) at the entrance of the interferometer. Thus the single-frequency light from the extended source enters the interferometer. A vibration exerting on the multi-mode fiber can greatly restrain the laser speckle effect. Figure 5 gives an example of the resulting interferograms and an intensity slice through the interferogram. The small-scale noise shown in the intensity slice is due to laser speckle. The fringe visibility near the image center is ∼0.73. This value is slightly smaller than a result of 0.85 reported by Harlander et al. . The laser speckle effect might lead to the current underestimate of the fringe visibility.
2.3 Fabrication of 2-D monolithic interferometer
Since the true and ghost spectra are separated, the 2-D SHS can provide a larger spectrum bandpass than the 1-D one for a given spectral resolution [1,7,10–12]. In addition, the alignment standard of the interferometer for 2-D SHS is more relaxed than that for 1-D one. The 2-D SHS has become a favorite choice in some spectroscopic applications [10–13]. Based on the breadboard shown in Fig. 2 and alignment adjustment method described above, it was straightforward to fabricate a monolithic interferometer for 2-D SHS in the light of the design parameters in Table 1(b). After the adhesive had been applied to the bonded surfaces of the grating1 and spacer 1, the interferometer subassembly (lacking the grating 1 only) was installed on a mount and the grating 1 was clamped onto the spacer 1 by a gripper, as shown in Fig. 2. Then the grating 1 was slightly rotated about its normal (via the fine rotation adjustor on the gripper shown in Fig. 2) under the condition of monitoring the interferograms and corresponding 2-D Fourier transforms obtained using the 930-950 nm bandpass-filtered neon lamp source. In order to avoid the impact of the low-frequency noise, the final alignment for the 2-D monolithic SHS interferometer was believed to be achieved when the grating 1 was rotationally adjusted to a position where the true and ghost spectra were visibly separated (see Fig. 6).
After the final alignment was achieved, the monolithic interferometer for 2-D SHS was fabricated by cementing the grating 1 to the spacer 1 via UV light irradiation on their bonded surfaces. By using the same setup in the fringe visibility evaluation of the 1-D SHS interferometer but an output port of 25.4 mm × 3 mm (the vertical vs horizontal) for the integrating sphere, we obtain a monochromatic interferogram of the 2-D SHS interferometer for extended source as shown in Fig. 7(a). An intensity slice through the interferogram is shown in Fig. 7(b). The fringe visibility was estimated to have a value of ∼0.41 near the image center for the 2-D SHS interferometer. For the convenience of field observations, we further built an integrated 2-D SHS system containing the monolithic interferometer as shown in Fig. 8. In the following discussion on calibration and observation, only this 2-D SHS system is involved.
3. Wavelength calibration and efficiency profile
Here we present the wavelength calibration and efficiency characterization for the 2-D SHS system containing the currently-fabricated monolithic interferometer (Fig. 8). A schematic of the optical setup for this purpose is shown in Fig. 9. The tunable single-frequency diode laser mentioned above was used for a tunable monochromatic calibration source. It has a tunable range of 915-985 nm and linewidth of ∼100 kHz, providing a full coverage for the spectral range of the 2-D monolithic SHS interferometer (930-950 nm, determined by the bandpass filter). A calibrated super-precision wavemeter (WS-7, HighFinesse) was utilized for measuring absolute wavelength of the tunable calibration source. As seen from Fig. 9, the output laser beam is separated into three parts by fused fiber splitters. Two parts are delivered respectively to the wavemeter and power meter for real-time monitoring. The remaining part is transmitted to the 2-D SHS system (see Fig. 8) via an integrating sphere which makes the light distribution more homogeneous in the output fiber.
As the first step of the calibration, the tunable laser was scanned about wavelength over the bandpass of the 2-D SHS yielding a series of monochromatic interferograms. Figure 10(a) shows an example of the laser line interferograms (The contamination from the laser speckle was obvious since no vibration was applied here). As a preprocessing, each of the interferograms was firstly apodized with a Gaussian function to prevent sharp-edge induced ringing in the Fourier transform, and then oversampled by a factor of five by zero-padding the interferogram. The preprocessed interferograms were subsequently 2-D Fourier transformed to yield a sequence of power spectra associated with the different laser lines. An example of the power spectra of the laser lines is shown in Fig. 10(b).
With the positive y-frequency (fy) spectrum, we determined the position (i.e., abscissa fx and ordinate fy in the 2-D spectral domain) of the spectral peak corresponding to each of the laser line spectra (see Fig. 11). All the position points fall on a straight line (that is a linear least square fit to the points). The straight line is determined by the interferometer alignment in the monolith fabrication. Note that the spectrum should be retrieved by taking the intensity values along the straight line. The abscissas (fx) of all the spectral peak positions were combined with the corresponding wavelength values measured by the wavemeter to obtain the wavelength calibration (stretch and shift) as shown in Fig. 12. According to the linear least square fit, the Littrow wavelength for the 2-D SHS is 940.24 nm, while the theoretical spectral resolution is 0.173 cm−1. The detector-determined spectral range was estimated to be 10458-10813 cm−1 (924.8-956.2 nm).
The overall efficiency profile of the 2-D SHS (including contribution from the bandpass filter and other optical elements) can be obtained by combining the spectral peak intensity values taken along the fitted straight line (see Fig. 11) with corresponding power values measured by the power meter. Figure 13 shows the overall efficiency profile (normalized). The individual peaks (colored) represent the instrumental line shape functions at spectral line positions that the laser is tuned to. They all have a full-width at half-maximum (FWHM) of ∼0.27 cm−1 in terms of our estimate. The FWHM value represents the actual spectral resolution for the 2-D SHS which is ∼1.56 times the theoretical spectral resolution (0.173 cm−1). It is equivalent to a resolving power of ∼39400. As seen in Fig. 13, if the data with a relative efficiency less than 30% are abandoned, the final spectral range for the 2-D SHS is between ∼931 and 949 nm, being ∼60% of the detector-determined spectral range (924.8-956.2 nm).
4. Ground-based solar spectra observed by the 2-D SHS
The currently-fabricated 2-D SHS system (Fig. 8) was coupled to an equatorial telescope via a fiber to observe the direct solar-irradiance spectra around 940-nm water vapor absorption band at our atmospheric observation site on the campus of Wuhan University in Wuhan (30.5°N, 114.4°E), China . An image of the equatorial telescope is shown in Fig. 14. It can keep tracking the solar disk. A low-pass filter (>900 nm) was put in front of the telescope to restrain the visible solar light. All of the interferograms were taken with a ∼10−3 s exposure time.
The acquired interferograms were preprocessed following the way mentioned in section 3. The spectra were then recovered using the 2-D Fourier transform. The spectral intensity was obtained by taking the intensity values along the alignment-determined straight line in the 2-D spectral space, while the wavenumber (wavelength) registration (spectral position) was set by the wavelength calibration result (stretch and shift). Based on the overall efficiency profile shown in Fig. 13, an instrument efficiency correction was performed in the spectrum retrieval. However, no flat field or phase corrections was used for the interferogram data processing. Figures 15 and 16 show two examples of the ground-based observed solar spectra (red) around 940-nm water vapor absorption band by the current 2-D SHS. The simulated solar spectra using MODTRAN 6 were also plotted for comparison . The main input parameters (water vapour total column and solar zenith angle) for the simulation were estimated from the local radiosonde data as well as the local time and geographic latitude of the SHS observations. As seen from Figs. 15 and 16, although the two spectral examples show a difference in water-vapor absorption due to different column water vapour amounts, the spectral features of the SHS solar spectra and associated MODTRAN results match very well in the observed spectral range of 10540-10740 cm−1. This indicates that the performance of the fabricated monolithic interferometer is well near the design predictions.
5. Summary and conclusion
Interferometer in SHS consists of beamsplitter, gratings and prisms. Its monolith that assembles these optical elements in place enables the SHS to be readily developped into a compact (portable), inexpensive and rugged instrument for field and space applications. We report for the first time the fabrication procedure of monolithic interferometers for 1-D and 2-D SHS with ultraviolet curing adhesive and commercial optical elements. It provides technical know-how for turning a design into an actual monolithic SHS interferometer under conventional laboratory conditions.
A breadboard of SHS system was first built. In this breadboard, all the interferometer components apart from one grating were first cemented together to form an interferometer subassembly. After the adhesive had been applied to the bonded surface, the interferometer subassembly was installed on a mount and the remaining grating was clamped onto its outer spacer by a gripper. The interferometer alignment adjustment was made by slightly rotating the grating about its normal under the condition of monitoring the interferograms and corresponding 2-D Fourier transforms for known light source. This rotation adjustment conformed to the structure of the monolithic interferometers designed. It has the same effect as rotating one grating by small angle in the basic SHS configuration about x axis . Once the desired alignment was achieved, the bonded surface was irradiated with UV light to produce the monolithic interferometer. The wavelength calibration and efficiency characterization were conducted for a 2-D SHS system containing the currently-fabricated monolithic interferometer using a tunable single-frequency diode laser (as light source). The retrieved spectral position lies in a straight line (corresponding to one of the true and ghost spectra) in the 2-D spectral space that is determined by the interferometer alignment, while the corresponding spectral intensity is obtained by taking the intensity values along the straight line. The current 2-D SHS system was coupled to an equatorial telescope to observe the direct solar-irradiance spectra around 940-nm water vapor absorption band. The spectral features of the SHS-observed solar spectra and corresponding results simulated using MODTRAN 6 matched very well in the SHS spectral range of 10540-10740 cm−1. This indicates that the performance of the fabricated monolithic interferometer is well near the design predictions.
Although the current monolithic interferometers were designed to work in the water vapor absorption band around 940-nm, our fabrication method can readily be applied to those monolithic interferometers with different target spectra. In addition, the bonded monolithic interferometers in the present way might be suited for space applications after further mechanically fastened.
National Natural Science Foundation of China (NSFC) (No. 41521063 and 41327801); Meridian Space Weather Monitoring Project (China).
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