Development of a New, Precise Near-infrared Doppler Wavelength Reference: A Fiber Fabry-Perot Interferometer
We present the ongoing development of a commercially available Micron Optics
E-mail: firstname.lastname@example.org, Telephone: 1 650 392 5909
Modern day Doppler radial velocity (RV) surveys require precise spectrographs to measure the minute spectral shifts in stellar spectra induced by unseen companions. To date, over 500 planets have been discovered using the RV method. Current dedicated planet hunting spectrographs can achieve 1 m s precision on bright sun-like stars with high cadence. This high precision allows for the detection of a wide range of low mass planets. The detection of these planets requires stable, accurate wavelength calibration devices for confident detections over long periods.
A new area of exploration lies in the selection of stars studied in RV surveys. The overwhelming majority of planets discovered have been found orbiting stars of similar spectral type to our own Sun (FGK). These stars are bright in the visible and generally well understood, but do not comprise the majority of stars in our Galaxy. M-dwarfs are estimated to makeup the majority (over 70 percent) of stars in the Milky Way. These small stars have low masses, resulting in larger Doppler reflex signals when gravitationally interacting with an unseen planetary mass companion. M-dwarf Habitable Zones (HZ) are also much closer to the host stars in M-dwarf systems than larger solar type stars, making detection of low-mass habitable planets more feasible. M-stars are brightest in the NIR bands however, where few precise and dense wavelength references for spectrograph calibration exist.
Iodine (I) cells have been used to obtain precise velocity measurements on F,G, and K stars  in the 400 - 700nm wavelength regime, but lack any significant spectral features in the NIR to precisely measure low amplitude (5 m s) velocities. Similarly, the popular Th/Ar emission lamp has been utilized to achieve sub m s long term precision on Solar type stars using stabilized spectrographs, but does not contain enough lines beyond 1000 nm to be an optimal NIR reference source.
With the slew of current and next generation high-resolution NIR spectrographs (e.g. HPF: Habitable-zone Planet Finder, CARMENES: Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs,  APOGEE: Apache Point Observatory Galactic Evolution Experiment, iShell, CRIRES: Cryogenic high-resolution Infrared echelle Spectrograph), development of spectrograph references rich with spectral features in this wavelength regime is therefore critical for the advancement of NIR Doppler velocimetry.
Fabry-Perot interferometers (FPI) create emission spectra by interfering light traversing separate delay paths. The resulting interference spectrum provides a rich distribution of narrow lines, ideal for use as a spectrograph reference. FPIs have been tested as wavelength references in the past with excellent short term stability, but have yet to show the required long term stability for precision measurements down to the m s level.
We present preliminary tests of a commercially available H-band (1500 - 1700 nm) Fiber Fabry-Perot
(FFP) designed by Micron Optics
In Section 2 we summarize existing NIR and optical wavelength references for comparison purposes. In Section 3 we focus on the major photonic calibration sources used in precise astronomical spectrographs, including a description of the FFP and commercial illumination source used for this study. Section 4 describes the laboratory tests performed to gauge the stability of the device and estimate the maximum achievable velocity precision. Section 5 briefly discusses the first light results on the APOGEE spectrograph. Finally, we present initial spectra of the FFP using the NIST 2-m Fourier transform spectrometer in Section 6 .
2 Summary of precise NIR/Optical wavelength references
Connes et al.  use a combination of a tunable Fabry-Perot and stabilized laser sources to measure stellar Doppler motion by varying the interferometer cavity to match the observed spectral shift. This technique, known as Absolute Astronomical Accelerometry, has demonstrated high precision ( m s) over short intervals on the varying solar spectrum but requires a vast array of fast, complex electronics to vary the interferometer cavity at the required precision and frequency.
McMillan et al. pass reflected sunlight through a fixed Fabry-Perot cavity and observe the changing slope of the spectral features transmitted through the interferometric filter to estimate the velocity shift over time. This method allowed for precise ( m s) monitoring of solar RV variations, but requires a very bright target. Additionally, the Fabry-Perot inherently discards the majority of the stellar spectral features, significantly lowering the amount of Doppler information content.
Wildi et al. designed and tested an optical FPI for the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph, finding 10 cm s precision over a 24 hour interval. However, optical path variations induced long term large scale velocity variations at the km s level. The variable optical path difference within the cavity introduced errors of up to 3 km s from night to night. This issue hinders high-precision velocity measurements over long periods, though these issues seem to have been solved in the most recent iteration of the device (see Wildi et al., these preceedings).
Uranium Neon (U/Ne) hollow-cathode lamps have been used to obtain RV precision of 10 m s in the Y and J-bands (900 - 1300 nm) using the Pathfinder instrument  on integrated sunlight. This represents one of the few emission lamps calibrated specifically for use in NIR spectrographs for radial velocity measurements. A high density of lines is present in all commonly used NIR bands, making the lamp ideal for use as a precise reference source.
Mahadevan and Ge explored a number of atomic combinations for NIR gas cells and concluded that the a combination of commercially available HCN, CH, CO, and CO cells could conceivably provide a dense enough set of features for precise calibration in the H-band (see Figure 1), though no such cell has been deployed on a high-resolution astronomical spectrograph.
Bean et al. achieved 10 m s precision in the K-band using a NH cell on the CRIRES instrument on the Very Large Telescope (VLT) on mid-M dwarfs, which are bright in these spectral windows. This presents an interesting wavelength region as K band is densely populated with telluric absorption features from HO and CH. The wavelength coverage of this method is rather narrow however, spanning roughly 36nm. This technique required both the high resolution (R = 100,000) of CRIRES, and bright (K8 mag) targets for high precision measurements.
Blake et al.  utilize the wealth of telluric absorption features near 2.3m from HO and CH to obtain RV precisions of 50 m s on ultracool dwarfs using the NIRSPEC instrument on the Keck II telescope. There is a wide distribution of telluric lines through the NIR, including in the H-band. These lines are not static however, requiring unique reduction methods and careful atmospheric monitoring to reach sub 10 m s precision.
Anglada-Escudé et al. explored using optimized gas cells (CH, CHD, and NH) for precise RV work in the K-band. CH has an intrinsic spectrum nearly identical to CH, though the rovibrational mode differences between the two isotopes adds a slight offset between the two spectra. This offset allows for easy disentanglement of the gas cell reference lines from telluric CH lines. Passing starlight through the optimized CH cell, they were able to achieve a preliminary RV precision of m s on a bright reference star using just 6 nm of the K band on CSHELL (Cryogenic Echelle Spectrograph).
3 Astrophotonic Calibrators
Fiber Bragg gratings have been proposed as accurate, low cost wavelength calibrators for astronomical spectrographs. This method consists of passing starlight through an optical fiber with an embedded Bragg grating structure that rejects a fixed selection of wavelengths. The advantage of this technique is that the reference requires no moving parts or power, leading to a simple addition to any fiber spectrograph that will yield several distinct reference absorption lines in a desired band.
Laser frequency combs (LFC) have been tested in astronomical applications to success in recent years. LFCs represent the pinnacle of wavelength references, producing sharp emission features at known wavelengths and spacings. Despite the high cost and complexity in design, LFC development is a high priority in the spectroscopic community due to the extreme precision possibilities. The difficulty lies in creating a custom NIR LFC that is stable over both long and short intervals. LFCs have been shown to be stable to the cm s level, but are not yet at the ‘turn-key’ stage of use in astronomical instruments. An LFC spectrum taken using Pathfinder instrument on the Hobby-Eberly Telescope is shown in Figure 1. This represents the first use of such a device on an NIR astronomical spectrograph, and we are pursuing further development of a stable LFC with our NIST colleagues.
The FFP used in this study was purchased from Micron Optics
The device requires a specialized light source for operation, as coupling a significant amount of light through the m single-mode fiber (SMF) FFP is not achievable using common broadband lamp sources. The continuum source chosen is a white light fiber laser source, discussed in Section 3.3.
Light reflecting off the dielectric mirror stack is susceptible to wavelength-dependent phase dispersion, which may alter the intensity or central wavelength of a given emission feature. The extent to which this affects the transmission spectrum is not yet known, but if the dispersion is temporally stable it will not greatly affect the long term velocity stability.
3.1 Expected Output
The theoretical FFP output spectrum can be quantified as a simple interferometric airy function:
where is the cavity delay, related to the free spectral range (FSR) and wavenumber : . is the finesse coefficient, related to the standard finesse value through:
Few wavelength references yield the ability to model the inherent emission spectrum using basic laws of interference. This ability to model the intrinsic emission line profile can allow for precise instrument profile modeling, though mirror phase dispersion and collimation issues can complicate the inherent functional form. An advantage of the Micron Optics
A common metric for estimating radial velocity information content from spectra is the quality factor Q. The Q factor represents a quantitative assessment of the quality and richness of the lines within a spectrum, or the amount of extractable Doppler information given an intrinsic emission spectrum at a given instrument spectral resolution. It is also independent of spectral flux.
The functional form of Q for a given spectrum I spanning N pixels is given by:
where I() is the intensity of the intrinsic spectrum at a given pixel , is the wavelength of that pixel, and W is the optimum weight, given as (neglecting detector noise):
The quality factor for a model FFP spectrum from 1200 to 1700 nm is shown in Figure 3 as a function of spectral resolutions. The overall velocity uncertainty for a spectral range can be determined through the quality factor:
where is the total number of detected photoelectrons in the spectrum. The high quality factor of the FFP, combined with the unresolvable lines and bright illumination source, imply the spectrum is ideal for use as a stable and precise wavelength reference for a wide range of spectrograph resolutions.
3.2 Expected Stability
The simple nature of the FFP design reduces the number of physical parameters that introduce velocity measurement error. The interferometer mirror spacing (L) and refractive index (n) of the SMF-28 fiber are both sensitive to temperature fluctuations that cause the interferometric emission line output to drift. These spectral shifts can mimic velocity shifts, thereby introducing measurement error. The phase change of the output pattern d/dT is related to the cavity width and refractive index through:
where is an unknown constant. The fraction phase change as a function of n and L can be described as:
This corresponds to a temperature dependent wavelength shift of:
For a standard SMF-28 fiber, the physical coefficient values are listed in Table 1.
Operating wavelength range (nm)
|1260 - 1640||Micron Optics
Free spectral range (nm)
We estimate the velocity stability of the device to be = 2.2 km s K at a wavelength of 1550 nm. Section 5 shows achieved control stability using the internal FFP temperature electronics with a bench-top temperature controller. The peak-to-peak temperature variation over a 24 hour period is 0.15 mK, meaning the expected velocity stability based on temperature fluctuations alone is 0.35 m s. Note that this does not include any variation due to phase dispersion from the dielectric mirror stacks, though we expect this effect to be minimal relative to the temperature sensitivity.
3.3 Illumination Source
The single-mode fibers used in the FFP reduce the number of continuum sources that have sufficient broad-band intensity to illuminate the emission lines. High power broad-band lamps do not have the required coupling efficiency to yield a practical number of photons per FFP line in reasonable integration times. We tested a broadband, 20 W Quartz Tungsten Halogen (QTH) light source optimized for multi-mode fiber delivery, but found the light loss in coupling to a SMF to be too great. Integration times needed to achieve high S/N () spectra were not practical with this source.
Super-luminescent diodes (SLEDs) can provide several mW of power through SMF fibers in the NIR, but are generally limited to narrow (50 - 100 nm) bandwidths. A combination of several SLEDs would be required to provide full coverage of the H-band.
The illumination source chosen is a broadband (500 - 2500 nm) supercontinuum source (SCS) that outputs 100 mW integrated across all wavelengths. The unit is an off-the-shelf source developed by SuperK Photonics
The SCS is one of the few available broadband sources capable of achieving significant output through the FFP. The output raddiance is several orders of magnitude higher than common QTH lamps. This high power concentrated within a fiber is required for the FFP system, as the device is an inherent filter of 95% of incoming light. Furthermore, the addition of necessary optical instruments that reduce overall throughput along the fiber path (such as integrating spheres) will further decrease the intensity of the FFP lines. The stability of the SCS is quoted to be 0.3dB (7%). We measure the output stability to be roughly consistent with this value using a Ge diode and power meter (see Figure 4).
4 Laboratory Tests
The major components of the FFP system were carefully selected to maximize instrument efficiency and temperature control. The areas of concern were precise temperature control, high intensity fiber fed illumination source, and interferometer mirror coatings. Finesse and free spectral range were chosen for optimum line spacing and throughput in the NIR. A bench-top Stanford Research Systems
4.1 Temperature Stability
We expect temperature variations in the FFP to be the dominant contribution to the velocity error. Therefore, the temperature control precision of the FFP will determine the overall stability of the interferometer cavity.
To estimate the achievable control precision of the temperature controller, we first connected a Vishay
Secondly, we replicated the temperature electronics within the FFP unit using an identical Quality Thermistors
Finally, the temperature feedback loop parameters in the temperature controller were optimized for the thermal response of the FFP and the ambient temperature fluctuations and the FFP was placed inside thermally insulated foam wrap for 24 hours. Setting the temperature to 298 K, the absolute peak-to-peak temperature variations of the cavity were measured to be 0.15 mK (Figure 5). This high precision control, nearly at the electronics limit of the controller, demonstrates the exquisite thermal contact between the internal TEC and thermistor, as well as the fast thermal response of the FFP as a whole. This peak-to-peak variation corresponds to a velocity error contribution of 35 cm s due to temperature fluctuations alone, though this cannot currently be measured using any available NIR grating spectrograph.
4.2 Narrow-band Laser Scan
The temperature sensitivity of the FFP spectrum allows for precise, high resolution sampling of the transmission line profiles.
A narrowband Acronym Fiber Optics
5 Commissioning on APOGEE
The Apache Point Observatory Galactic Evolution Experiment (APOGEE), as part of the Sloan Digital Sky Survey III (SDSS-III), is surveying 100,000 red stars spanning a large range of galactic latitudes to study the chemical and kinematical history of all Milky Way populations. APOGEE uses a recently commissioned 300-fiber R22,500 H-band fiber-fed spectrograph. Such an instrument is a prime candidate for the addition of a stable, precise wavelength reference source to augment its existing calibration toolbox of traditional Th/Ar and U/Ne Hollow Cathode Lamps and enable high radial velocity precision. One of the APOGEE ancillary science programs includes monitoring several hundred M-stars throughout the Galactic plane. For instruments with these resolutions, the output of a high finesse (30) FPI will be nearly indistinguishable from an LFC with similar emission line spacing. We estimate the intrinsic FFP line widths to yield emission lines with widths 6-10% larger than the instrument PSF. A block diagram of the FFP setup on APOGEE is shown in Figure 7.
The H-band FFP achieved first light on APOGEE in Spring 2012. A portion of the first light image is shown in Figure 8. The device allowed for precise characterization of many spectrograph properties, including instrument drift in both spatial and spectral directions, detector persistence, wavelength solution stability, and PSF variations. A more thorough discussion of the analysis will be presented in a separate publication.
6 NIST FTS Scan
A high resolution (R750,000) scan of the FFP was taken using the NIST 2-m Fourier transform spectrometer (FTS). The goal of this scan was to measure the absolute wavelengths of the FFP emission lines to high accuracy (15 - 20 m s) by calibrating against a standard CH reference cell. This would allow for an accurate determination of both line centers and temperature-to-wavelength conversion.
The initial spectra taken using the SCS and FFP were unexpectedly noisy (S/N ), even after combining a large number of coadded frames. This high noise level is attributed to the unstable pulsation rate of the Q-switching laser that is contained in the SuperK
We instead opted to use a more narrow-band 1550 nm Superluminescent light emitting diode (SLED) source to illuminate the FFP. The SLED produced mW of power across 1500 - 1600 nm, sufficient for scans of the FFP at S/N with 30 coadds. An example FTS scan across the operating wavelengths of the SLED is shown in Figure 9.
To accurately measure the line wavenumbers in the FFP spectrum, a scan of the reference CH cell was taken directly before and after each FFP scan to precisely determine the wavenumber correction factor of the spectrograph. This was done without replacing the output fiber of the SLED or the illumination fiber to the FTS. A high S/N scan of the FFP using the wavelength solution from the CH cell is shown in Figure 10. This scan allowed for precise determination of the FFP line spacing across the SLED continuum wavelengths. The line spacing varies in regions where the interferometric line intensity deviates significantly from the SLED continuum curve (see Figure 11). The origins of this deviation are unclear at this point, though the likely cause is non-uniform phase dispersion of the FFP near these wavelengths. This is not expected to effect the device stability, so long as the dispersion remains static over time.
A known issue with these SLEDs is periodic spectral ringing, possibly due to reflection fringing at internal fiber mating interfaces. This ripple effect is imprinted on the FFP transmission function (see Figure 9), though this has little effect on the FFP line profiles. Repeated tests using the FFP and NIST FTS are planned for the second half of 2012. These tests are aimed at estimating day-to-day line stability to high accuracy, characterizing phase dispersion effects on line centers and intensities, and determining line profiles for the entire operating range of the FFP (1260 - 1640 nm). Upcoming tests will utilize a 1 W super-continuum source with a more stabilized pulsation rate. This should greatly reduce noise associated with varying repetition rate, and allow for a complete scan of the FFP transmission lines from 900 - 2000 nm.
|Stanford Research Systems
|Testing equipment (power meter, NIR diodes, safety glasses, precision resistor)||$5k|
This device represents a simple, economic solution to an important issue in precise Doppler spectroscopy. Combining astronomical instrumentation with commercial photonic devices can yield a wealth of scientific return. The output of the FFP is similar to that of an LFC but at a small fraction of the cost. Few NIR wavelength references exist that are precise to the m s level, insensitive to both vibration and pressure changes, rigid in design, and relatively inexpensive to construct. The FFP satisfies all these goals in a compact unit. Other high resolution NIR fiber spectrographs could easily be converted into precise radial velocimeters with the addition of such a device. A full list of all components used in testing and commissioning the device is shown in Table LABEL:tab:ffp_components. Our goal here is to present the initial performance report and discuss the properties of the FFP device, as well as the unique illumination source used. Further testing will be required to fully characterize both the stability, and intrinsic transmission spectrum of the FFP. A detailed analysis of the APOGEE data, as well as new results from the upcoming NIST tests will be presented in a separate refereed paper.
Acknowledgements.This work was partially supported by the Center for Exoplanets and Habitable Worlds, which is supported by the Pennsylvania State University, the Eberly College of Science, and the Pennsylvania Space Grant Consortium. We acknowledge support from NSF grant AST-1006676, the NASA Astrobiology Institute (NAI), and PSARC. This research was performed while SLR held a National Research Council Research Associateship Award at NIST. NSO/Kitt Peak FTS data used here were produced by NSF/NOAO. Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the U.S. Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/. SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, University of Cambridge, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.
- Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose.
- 41 GHz corresponds to 0.33 nm spacing at 1550 nm.
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