Powerful DMD-based light sources with a high throughput virtual slit Arsen R. Hajian* a, Ed Gooding a, Thomas Gunn a, Steven Bradbury a

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1 Powerful DMD-based light sources with a high throughput virtual slit Arsen R. Hajian* a, Ed Gooding a, Thomas Gunn a, Steven Bradbury a a Hindsight Imaging Inc., 233 Harvard St. #316, Brookline MA * arsen.hajian@hindsight-imaging.com; phone: ; ABSTRACT Many DMD-based programmable light sources consist of a white light source and a pair of spectrometers operating in subtractive mode. A DMD between the two spectrometers shapes the delivered spectrum. Since both spectrometers must (1) fit within a small volume, and (2) provide significant spectral resolution, a narrow intermediary slit is required. Another approach is to use a spectrometer designed around a High Throughput Virtual Slit, which enables higher spectral resolution than is achievable with conventional spectroscopy by manipulating the beam profile in pupil space. Conventional imaging spectrograph designs image the entrance slit onto the exit focal plane after dispersing the spectrum. Most often, near 1:1 imaging optics are used in order to optimize both entrance aperture and spectral resolution. This approach limits the spectral resolution to the product of the dispersion and the slit width. Achieving high spectral resolution in a compact instrument necessarily requires a narrow entrance slit, which limits instrumental throughput (étendue). By reshaping the pupil with reflective optics, HTVS-equipped instruments create a tall, narrow image profile at the exit focal plane without altering the NA, typically delivering 5X or better spectral resolution than is achievable with a conventional design. This approach works equally well in DMD-based programmable light sources as in single stage spectrometers. Assuming a 5X improvement in étendue, a 500 W source can be replaced by a 100 W equivalent, creating a cooler, more efficient tunable light source with equal power density over the desired bandwidth without compromising output power. Keywords: Tunable light source, DMD, high-resolution spectroscopy, high-efficiency spectrometer, minispectrometers, programmable digital light sources

2 1. INTRODUCTION Dispersive spectrometers are widely-used in DMD-based programmable light sources as the basis for controlling the output wavelength. The majority of spectrometer designs are forced to make a compromise between throughput and spectral resolution. For high spectral resolution, a narrow slit must be used as an entrance aperture, such that the monochromatic slit images on the detector focal plane are narrow along the dispersion axis and therefore closely-spaced wavelengths can be more readily distinguished from each other. Unless the target source is inherently small, the slit also ends up blocking a large fraction of the incident flux from the source, limiting the instrument throughput and thus the number of photons collected per unit time. To achieve the necessary SNR for a particular measurement, the exposure time, collecting aperture, source luminosity, or spectrometer size must often be larger than is otherwise desirable. Due to the these characteristics, current state of the art DMD based programmable light sources, such as the OL 490 Agile Light Source 1, only use a fraction of the output from the lamp. The system is moderately large accommodate the light source and requisite cooling. An alternative is to use a spectrometer based on the High Throughput Virtual Slit (HTVS), which improves the light utilization by a factor of 5X-10X. Using this approach could reduce the light source power requirement, leading to reduced demand for cooling and reduced size, while preserving the output power. Figure 1: OL 490 Agile Light Source 2. HTVS The High Throughpt Virtual Slit (HTVS) offers a new strategy for improving dispersive spectrometer performance. Previous publications 2,3,4, 5 have included full descriptions of HTVS but we also summarize the concept here for the reader s convenience. HTVS optics operate in pupil space, altering the collimated beam in the spectrometer to change the shape of the input aperture, making it narrower along the dispersion axis independently from the orthogonal (crossdispersion) dimension. The image of a circular fiber core, for instance, is stretched into a narrow ellipse resembling a slit (Figure 2), but it still contains nearly all of the original source flux. This anamorphic transformation can be achieved while maintaining the f/ratio of the original source unlike the simple solution of altering the profile with a cylindrical lens. In essence, the HTVS optics transfer étendue between the dispersion axis and orthogonal axis; the total étendue is preserved (and the laws of optical invariance are not violated), but the étendue distribution is optimized for spectroscopy or programmed light delivery. An HTVS system can be readily implemented entirely with reflective optics, so achromatic effects are not a concern, and any spectral bandpass from deep UV to longwave IR can be covered given a suitable high-reflection mirror coating for the specified wavelength range. The net result is that a spectrometer designed around an integrated HTVS design can achieve significantly higher system throughput at a given spectral resolution than a traditional spectrometer design allowing for the use of a lower power light source to achieve the same spectral intensity output.

3 Figure 2. Illustration comparing the effect of a slit versus HTVS for achieving high spectral resolution from a round input aperture like an optical fiber core. Using the unaltered fiber as the spectrometer s input aperture (a), the resulting spectrum has plentiful signal (area under the curve) but poor spectral resolution (wide emission lines). Using a slit in the traditional fashion (b) narrows the spectral line profiles, but at the cost of reduced signal at the spectrometer s focal plane. With HTVS optics integrated into the spectrometer design (c), the fiber image has been reshaped into a tall narrow ellipse such that the spectral lines are narrow but contain the same total flux as in the initial case (minus a few percent due to the not-quite-perfect reflectivity of the optical components). 3. APPLICATION TO TUNABLE LIGHT SOURCES Tunable light sources are often the limiting factor in modern analytical microspectroscopy experiments. Live cell imaging, single molecule spectroscopy and nanophotonic structure analysis can all require high flux of the desired passband combined with excellent out-of-band rejection. With increasing interest in multiple fluorophores, often involving SWIR wavelengths, or complex photonic structures, researchers require greater wavelength flexibility and a broader spectral range than can be obtained through traditional filter wheels. Combining an ultrabroadband source with a

4 tunable spectrometer offers the highest flexibility possible, as both the center wavelength and passband width can be tuned continuously over the range of the light source. Traditionally, two broad categories of ultrabroadband sources are used. Incoherent sources such as QTH and Xe arc lamps are inexpensive and have long been available. More recently, supercontinuum laser sources from vendors such as NKT and Fianium, which generate white light in an optical fiber via ultrafast laser excitation, have become popular. A third approach, which produces much higher UV flux than supercontinuum sources, is the plasma laser-driven light source commercialized by Energetiq. While these laser-based systems are excellent for microspectroscopy, their relatively high cost precludes their use for many applications. We will now discuss how to best use inexpensive incoherent sources in microspectroscopy. There is a significant mismatch between the large physical arc/filament size in broadband sources such as QTH and Xe lamps and the étendue of traditional spectrometers using spherical or toroidal optics. While these incoherent sources can easily generate many hundreds of watts of power, the size of the illumination source increases commensurately. Increasing the source power beyond ~100 W does not typically deliver much more power to the sample, because the white light output cannot be coupled efficiently through the spectrometer. Typical output fluxes of W QTH lamps through commercial f/4 imaging spectrometers with narrow entrance and exit slits are on the order of tens of µw per nm. This is extraordinarily poor coupling efficiency. In addition, with high power incoherent sources, heating becomes a severe problem, and cold mirrors or externally cooled filters must be used to divert excess heat away from the entrance slit. While the conservation of étendue means that it s not possible to efficiently image an extended source onto a small area such as a spectrometer slit, the pupil space reformatting made possible by the HTVS provides an effective workaround. High coupling efficiency eliminates the need for high power sources, which in turn eliminates the need for external cooling and waste heat rejection. In summary, the use of a HTVS-equipped spectrometer in a DMD-based tunable light source results in a decrease in the size, permits a significant reduction of the light source power, reduces the thermal load on the instrument, and still delivers the same output power.

5 4. REFERENCES [1] Gooch and Housego, [2] Meade, J., Behr, B.B., Hajian, A.R., and Schmidt, B., Simultaneous High-Resolution and High-Throughput Spectrometer Design Based on Virtual Slit Technology, in Imaging and Applied Optics Technical Papers, OSA Technical Digest (online), paper AM2A.2, (2012). [3] Meade, J., Behr, B.B., and Hajian, A.R., A new high-resolution, high-throughput spectrometer: first experience as applied to Raman spectroscopy, Proc. SPIE 8374, Next-Generation Spectroscopic Technologies V, 83740V (2012) [4] Meade, J., Behr, B.B., Bismilla, Y., Cenko, A.T., and Hajian, A.R., In-depth performance analysis of the HyperFlux spectrometer, Proc. SPIE 8572, Advanced Biomedical and Clinical Diagnostic Systems XI, 85720V (2013) [5] Behr, B.B., Meade, J.T., Hajian, A.R., and Cenko, A.T. High-performance hyperspectral imaging using virtual slit optics, Proc. SPIE 8726, Next-Generation Spectroscopic Technologies VI, (2013)