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A four channel hyperspectral imager using Liquid Crystal Fabry-Perot (LCFP) etalons has been built and tested. This imager is capable of making measurements simultaneously in four wavelength ranges in the visible spectrum. The instrument was designed to make measurements of natural airglow and auroral emissions in the upper atmosphere of the Earth and was installed and tested at the Poker Flat Research Range in Fairbanks, Alaska from February to April 2014. The results demonstrate the capabilities and challenges this instrument presents as a sensor for aeronomical studies.
© 2015 Optical Society of America
1. Introduction
Optical emissions in the Earth’s upper atmosphere serve as an important source of information regarding the physical processes underlying the region and its connection with the surrounding space environment. The optical aeronomical signals contain three essential dimensions of information: spectral, spatial and temporal. The spectral dimension contains information about atmospheric composition and energies associated with chemical interactions. The spatial dimension provides information about external drivers and geospace boundaries. Temporal information is often critical for revealing the physical mechanism underlying the signal. The advantages, in aeronomical studies, of imaging multiple wavelengths simultaneously have been recognized and implemented by Semeter et al in [1,2] where they discuss the design of a Simultaneous Multispectral Imager (SMI), an instrument that acquires images in multiple wavelength bands simultaneously using only a single optical chain. Following a similar principle, the Auroral Structure and Kinetics (ASK) [3] instrument images three auroral emissions simultaneously using three separate optical chains, each with its own detector. This paper discusses a modification of the aforementioned SMI where the static interference filters are replaced by electrically tunable filters.
Electrically tunable filters are used in many applications as summarized in [4]. Tunable filters using liquid crystals have been used in many applications such as environmental analysis [5], biomedical imaging [6], landscape imaging [7], etc. and offer unique advantages that can be exploited for aeronomical applications, but their use in aeronomy has been limited [8,9]. We present a multi-channel instrument, the Liquid Crystal Hyperspectral Imager (LiCHI), which uses four electrically tunable filters, each of which is a liquid-crystal-filled Fabry-Perot etalon designed for an independently tunable wavelength region. This system is capable of measuring four wavelength bands simultaneously, with the ability to tune each individual channel arbitrarily within its free spectral range (FSR), making each channel hyperspectral in its respective wavelength region. Post-processing can be used to achieve fine spectral resolution in each channel and rapid production of high level data products [5].
The LiCHI architecture represents a compromise between the spectral range of a traditional spectrograph and the simultaneous imaging capability of the SMI. Instead of imaging only 4 static bands, the details of the spectral content around the central wavelengths in each channel can be explored via tunability. This flexible system architecture allows for the development of compact, robust, solid-state multi-spectral sensors, with fast wavelength switching and no moving parts.
LiCHI was installed and tested at Poker Flat Research Range (PFRR) in Alaska, where it was used for multispectral measurements of the aurora. Section 2 provides a background for the liquid crystal Fabry-Perot (LCFP) technology, while sections 3, 4, 5 and 6 explain the optical design, system characteristics, experimental setup and data samples respectively, and section 7 discusses the challenges posed by the system along with the solutions.
2. Background
The tunable filters used in LiCHI are Fabry-Perot etalons with liquid crystal in their gap. Liquid crystals exhibit the property of birefringence [10], which can be exploited to control the index of refraction in one polarization. The molecules of the liquid crystal are rearranged by applying an external trigger such as voltage which causes a change in extraordinary refractive index. This change in refractive index shifts the resonant wavelength that is transmitted through the etalon. The change in wavelength with voltage is quick, smooth and can be carried out in very fine steps. This gives us a complete scan of the spectral region of interest.
Two additional optical components are required to use the LCFP as a tunable filter, a polarizer and an order sorting filter. The polarizer is needed to exploit the birefringence property of the liquid crystal, since only the light polarized parallel to the direction of the liquid crystal molecules is sensitive to the change in refractive index. The order sorting filter selects and transmits only the desired order of interference out of the many orders of the LCFP.
Figure 1(a) shows the transmission function of an ideal model of the LCFP function (blue lines) and the order sorting filter function (red line). Figure 1(b) shows an example of the tuning of the etalon. A single order of the etalon normalized transmission function from Fig. 1 is captured by the order sorting filter. The etalon is first tuned to a wavelength λ1 as illustrated by the solid normalized transmission curve and then to wavelength λ2, shown as a dashed curve, by changing the voltage applied.
Fig. 1 (a) Transmission peaks of the LCFP for a gap of 10µm tuned to 6300 Å and 6364 Å, and the transmission of the order sorting filter. (b) LCFP transmission function truncated by the order sorting filter transmission function.
3. Optical design
The LiCHI optical architecture is illustrated in Fig. 2, and Fig. 3 shows the assembled instrument in the lab. This optical design is based on the SMI developed by Semeter et al [2], which used a mosaic of four interference filters along with prisms to create four different images, each corresponding to a different wavelength, on the detector chip. In LiCHI the filter and prism mosaic is replaced by the tunable filter setup (element (d) in Fig. 3) consisting of four tunable etalons with integrated prism angles, four order sorting filters each with a preselected central wavelength (CWL) and bandwidth and four polarizers. Incorporation of prism angles into the etalons serves the dual purpose of eliminating etalon effects between the glass surfaces and steering the beams to the appropriate detector quadrant. Combining these functions also reduces the total glass surface of the system, thus avoiding unnecessary reduction in total system transmission.
Fig. 3 The left panel shows the complete assembled instrument with components (a) front objective (b) field stop and other front optics (c) collimating lens (d) etalon cage with order sorting filters, polarizers and the etalon assembly (e) reimaging lens (f) camera. The right panel shows the 4-channel tunable etalon subassembly with order sorting filters and polarizers.
The front objective focuses the light into the system and provides a full field of view of ~40°. The field stop used is square in shape in order to have square image for the four square etalons. An off-the-shelf 400 mm f/2.8 Nikon lens is used as the collimator and its purpose is to limit the angles of light going into the etalons. The reimaging lens used is a 150 mm f/1 Marshall CCTV lens. The detector used is the Neo sCMOS 5.5 megapixel detector by Andor. This detector was chosen for its large chip size (16 mm x 14 mm) and small pixel size (6.5x6.5µm2) in order to give substantial chip area to image from each of the four channels while preserving spatial resolution.
The selection of optical components is quite unique to such a multichannel system. The lenses needed to have apertures large enough to accept all the light from the four etalons. Furthermore, the collimating and reimaging lenses need to be chosen such that the image at the field stop is reduced in size to fit one fourth of the chip. The image size at field stop is 24 mm x 24 mm which needed to be reduced to 8 mm x 8 mm, therefore the ratio of focal lengths of the collimating lens to reimaging lens is 3:1. Since the reimaging lens is 150 mm focal length, the collimator was chosen to have a focal length of ~400 mm. The collimator limits the incident angle on the etalon to 2-3 degrees.
4. System characteristics
LiCHI uses four LCFP etalons to create four independently wavelength tunable simultaneous images. Based on the order-sorting filters chosen for auroral observations, the four etalons together cover four non-contiguous spectral ranges in the visible and near infrared regions with CWLs of 4268 Å, 5580 Å, 6330 Å and 7320 Å with FSR of 30 Å, 130 Å, 80 Å and 80 Å, respectively, for observations of four commonly studied emission bands and lines in auroral displays. The FSR, or tuning range, is determined by the size of the gap between the plates of the etalon. The CWLs and gaps were selected based on specific scientific objectives following the methods discussed in [9]. The specifications of the four etalons are listed in Table 1 and specifications of the four filters are listed in Table 2. The full width half maximum (FWHM) in Table 1 refers to the width of the etalon transmission peak and FWHM in Table 2 refers to the bandwidth of the order sorting filters.
Table 1. Characteristics of the liquid crystal etalons along with the application of each channel.
Table 2. Characteristics of order sorting filters.
The transmission characteristics of the LCFP etalon were studied in a laboratory setting. The maximum extent of the wavelength range over which the etalon operates with the desired characteristic of transmission is determined by the coatings on the plates, and affects the applications for which the etalon can be used. Transmission measurements are made by transmitting a beam of light with known properties through the etalon and measuring the transmitted signal using a spectrometer or a monochromator. Figure 4 shows the transmission characteristic for one of the etalons with respect to wavelength, with the LC at a fixed bias voltage. The transmission is ~10% in the blue region and increases to 20-25% in the NIR region. These transmission values vary from etalon to etalon due to the differences in design. The low transmission of liquid crystal tunable filters with a Lyot architecture was also reported by Sigernes et al [8], and represents one of the major compromises in achieving the flexibility of wavelength tunability.
Fig. 4 This figure shows the transmission characteristics of one of the etalons with respect to wavelength, at a fixed voltage. The different panels are (a) Spectra of a tungsten lamp as measured through the etalon transmission function (b) Spectra of the tungsten lamp as measured directly, without the etalon (c) Transmission of the etalon obtained by dividing the spectra in (a) by the spectra in (b).
5. Experimental setup
LiCHI was installed under a large dome at the scientific facility at the Poker Flat Research Range (PFRR) near Fairbanks, Alaska for the initial tests. The location of the facility and the array of optical instruments available there to supplement the observations make PFRR an ideal location for testing of new aeronomical instruments. Figure 5 shows LiCHI installed in a dome in the science building at PFRR. LiCHI was tested from 23rd of February 2014 to 30th of April 2014. The aim of these tests was to demonstrate the abilities and understand the challenges posed by such a system in measuring aeronomical targets.
Fig. 5 LiCHI installed in a dome in the science building at Poker Flat Research Range near Fairbanks, Alaska.
For the field test, LiCHI was pointed to the magnetic zenith and was run remotely for a four hour period each night. It was tested for three specific applications as listed in Table 1, for the four emissions (4278 Å, 5577 Å, 6300 Å, 7320 Å), shown in Fig. 6. These applications and related methodologies are detailed in [9]. Although only four emission lines for which our instrument has been designed have been shown here, the auroral spectrum consists of other emission lines for which the reader is referred to [11]. The 5577 Å channel was tuned to two wavelengths, on-band to measure the oxygen emission line and off-band to measure the background. The 6300 Å channel was tuned to a background wavelength and to the 6300 Å and 6364 Å oxygen emission lines. The 4278 Å channel was tuned to 4258 Å and 4278 Å wavelengths for rotational temperature measurements [12]. The 7320 Å channel was tuned to the O + 7320/7330 Å multiplet [3] and to a background wavelength.
Fig. 6 The auroral spectrum showing (a) the emission lines at 4278 Å (N2 + 1NG), 5577 Å (O) and the doublet at 6300 Å - 6364 Å (O) embedded in a broad background (spectrum from Poker Flat, AK, courtesy of Jeff Baumgardner, CSP, Boston University). (b) the 7320-7330 Å O + emission lines (spectrum measured with the spectrograph HiTIES [13]).
6. Data samples
A bright auroral event was observed and measured by LiCHI on April 17th 2014 above Poker Flat Research Range (PFRR) in Alaska. Figure 7 shows a set of six panels with images captured by LiCHI with exposure time of 30 seconds each. In each panel, the plot on the left shows the DMSP measurements and the passband of LiCHI in two of its spectral channels. The image on the top right is the data from DASC with the superimposed black box showing the corresponding LiCHI field of view. The image on the bottom right is corresponding measurements made by LiCHI, with the channels marked in yellow and the DMSP slit position marked by a dotted white line.
Fig. 7 Shown here are images from an auroral event, on April 17th 2014, which was captured by LiCHI in the 5577 Å and the 6300 Å channels with exposure time of 30 seconds. LiCHI made on-band and off-band measurements in both channels. The 6 panels labeled (a)-(f) show the temporal progression of the event as it was captured by three instruments – LiCHI, Digital Meridian Spectrograph (DMSP) and the Digital All Sky Camera (DASC), all collocated at Poker Flat Research Range. Each panel shown above comprises three figures. The left figure in each panel shows the DMSP measurements and the passband of LiCHI. The figure on the top right in each panel shows the image captured by the DASC with the black box showing the LiCHI field of view. The figure on the bottom right of each panel shows the corresponding image captured by LiCHI with the channels labeled in yellow and the DMSP slit position marked by a dotted white line.
The data from the DMSP and the DASC are used to verify the measurements made by LiCHI. The data samples seen in Fig. 7 demonstrate the ability of LiCHI to capture spatial and spectral information in multiple channels simultaneously. When the 5577 Å channel is on band (panels b and c), i.e. to include the 5577 Å emission line, the arc within the black box in the all sky image is also seen in LiCHI data, whereas for off band, i.e. to exclude 5577 Å, the features in the all sky image are not captured (panels a, d and e) by this channel. Analogous measurement results can be seen in the on band (panel f) and off band data from the 6300 Å channel.
The major problem posed by a target such as the aurora is photon starvation due to the low transmission of the system. At the outset, half of the light is lost due to the presence of polarizers in the system. Additional losses come from the liquid crystal etalon structure. Even with highly sensitive sCMOS imaging chip, exposure times required to obtain a high SNR for photon starved signals are longer than the millisecond temporal scales exhibited in a dynamic auroral event. As can be seen from Fig. 7, LiCHI successfully captured images in 5577 Å and 6300 Å channels which had adequate SNR, as opposed to the 4278 Å and 7320 Å channels which did not have adequate SNR without further processing or a longer exposure time on stable auroral features.
7. Challenges and solutions
Using a system like LiCHI in the field presents some unique challenges due to its low transmission and sensitivity to external stimuli. This section discusses these challenges and possible solutions. Some of these challenges have been resolved, and some resolutions have been planned as future modifications to the instrument.
7.1 Low transmission and photon starved signals
As discussed in section 6, photon starved signals present a difficult target for LiCHI due to the low transmission of the system. The challenge of photon starvation can be overcome by modifying the design such that the light rejected by the polarizers is collected within the system or by using etalons which do not require polarizers [14].
The low transmission of the system poses another problem, i.e. stars cannot be used to point the instrument since the time needed to record them is long enough for the stars to significantly change their position. For this, two approaches were used. For the first approach, a white light camera with its optical axis parallel to LiCHI’s was mounted on the same base as LiCHI. Images of stars from this camera were used to determine the pointing direction for LiCHI. In the second approach, the etalon assembly was removed from the system to take images of stars which were then used for determining the pointing direction of LiCHI.
7.2 Variations in calibration
While running LiCHI in Alaska, a discrepancy was noted in the voltage to wavelength calibration. The probable cause of this is the difference in ambient temperature between Boston, where the instrument was calibrated, PFRR, where the instrument was installed for field test. This happens because the liquid crystals are sensitive to any external stimulus applied, which in this case is temperature. A temperature controller and heater are used to maintain the temperature of the liquid crystals, and use a feedback thermistor to determine the heat to be supplied to the etalons. The thermistor feedback seems to have malfunctioned during this field test, resulting in the error in voltage to wavelength calibration. This problem can be solved by recalibrating the instrument in the new location, using lamps of known spectral intensity. But this becomes more challenging if the instrument is being remotely operated. Since LiCHI was running remotely, our solution was to use the nightglow for calibration on a night without an auroral event. Figure 8 shows the calibration curve for the 5577 Å channel. In this example, with the knowledge that the nightglow peaks at 5577 Å inthis wavelength band and the peak transmission of the channel is at 5577 Å, the etalon was tuned over the FSR of the channel and the nightglow was recorded. The means of the images, in raw data counts, were plotted to obtain the wavelength to voltage calibration. Similar calibration can be carried out for other channels. The auroral data presented in section 6 of this paper were obtained after this recalibration.
Fig. 8 Plot of mean values of the images of nightglow taken by LiCHI on the night of March 13th 2014, in the absence of an auroral event, for recalibration of voltage vs wavelength. The x axis at the bottom of the plot shows the voltages in data numbers and the x axis on the top shows the central wavelengths corresponding to the voltages.
8. Conclusion
Initial field results and description of LiCHI, a new four-channel hyperspectral imager using LCFP etalons has been presented, as tested at the Poker Flat Research Range near Fairbanks, Alaska. The main advantages and challenges of using such an instrument for aeronomy are summarized as follows.
Advantages:
- • The capability to make simultaneous measurements in multiple wavelengths is useful for dynamic events
- • Wavelength tunability allows exploration of spectral content around the central wavelengths
- • The system requires no mechanical moving parts, thus reducing the possibility of error
Challenges:
- • Low transmission makes it difficult to measure ephemeral photon starved signals
- • Low transmission precludes the online use of stars to determine the pointing direction
Acknowledgments
We would like to acknowledge Glenn Thayer at the Boston University Scientific Instrumentation Facility for his fantastic role in building LiCHI. We would also like to express our gratitude to Kevin Abnett and the rest of the Poker Flat Research Range team for their support during our research campaign. This work was sponsored by the National Science Foundation under grants AGS 0960078 and AGS 124467.
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