PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spieA laser frequency comb featuring
sub-cm/s precision for routine
operation on HARPS
Rafael A. Probst, Gaspare Lo Curto, Gerardo Avila,
Bruno L. Canto Martins, José Renan de Medeiros, et al.
Rafael A. Probst, Gaspare Lo Curto, Gerardo Avila, Bruno L. Canto Martins,
José Renan de Medeiros, Massimiliano Esposito, Jonay I. González
Hernández, Theodor W. Hänsch, Ronald Holzwarth, Florian Kerber, Izan C.
Leão, Antonio Manescau, Luca Pasquini, Rafael Rebolo-López, Tilo
Steinmetz, Thomas Udem, Yuanjie Wu, "A laser frequency comb featuring
sub-cm/s precision for routine operation on HARPS," Proc. SPIE 9147,
A laser frequency comb featuring sub-cm/s precision for
routine operation on HARPS
Rafael A. Probst*
a, Gaspare Lo Curto
b, Gerardo Avila
b, Bruno L. Canto Martins
c,
Jose Renan de Medeiros
c, Massimiliano Esposito
d, Jonay I. Gonz´
alez Hern´
andez
d,
Theodor W. H¨
ansch
a, Ronald Holzwarth
a,e, Florian Kerber
b, Izan C. Le˜
ao
c,
Antonio Manescau
b, Luca Pasquini
b, Rafael Rebolo-L´
opez
d, Tilo Steinmetz
a,e,
Thomas Udem
a, and Yuanjie Wu
a,ea
Max-Planck Institut f¨
ur Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany;
bEuropean Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany;
c
Universidade Federal do Rio Grande de Norte, 59072-970, Natal, RN, Brazil;
dInstituto de Astrofisica de Canarias, Via Lactea s/n, 38200 La Laguna, Tenerife, Spain;
e
Menlo Systems GmbH, Am Klopferspitz 19a, 82152 Martinsried, Germany
ABSTRACT
We present a re-engineered version of the laser frequency comb that has proven a few-cm/s calibration repeata-bility on the HARPS spectrograph during past campaigns. The new design features even better performance characteristics. The newly arranged oscillator, filter cavities and fiber injection for spectral broadening allow robust long term operation, controlled from a remote site. Its automation features enable easy operation for non-experts. The system is being prepared for installation on the HARPS spectrograph in fall of 2014, and will subsequently become available to the astronomical community.
Keywords: Laser frequency combs, astronomical instrumentation.
1. INTRODUCTION
Laser frequency combs (LFCs) have revolutionized spectroscopy in atomic and molecular physics, enabling mea-surements of transition frequencies at unprecedented accuracy as simple standard lab devices.1 Sparked by this
success, the potential of LFCs for astronomical spectroscopy was recognized,2where they might be able to lead
to similar break throughs. High-precision astronomical spectroscopy has been lacking calibration sources to cal-ibrate astronomical spectrographs with sufficient repeatability and accuracy to meet their highest demands. For most applications, such as radial velocity (RV) measurements, the calibration is merely required to be repeatable, since only relative changes of the positions of spectral lines are of interest. However, combining results from different spectrographs can best be accomplished, if the calibration is also accurate, i.e. if it provides an absolute wavelength scale that is close to the true wavelengths. The most widely used calibration sources for high-precision spectrographs are emission lamps such as thorium-argon hollow-cathode lamps. They suffer from irregular line intensities and spacings, and from blending of lines. Being much more repeatable than accurate, they offer a short-term calibration repeatability of a few 10 cm/s, as expressed in RV. Over longer time horizons however, aging of the lamps can result in drifting calibration, degrading the repeatability to several m/s. In contrast, an LFC directly links the frequencies of its optical lines to the SI second with utmost accuracy.1 It generates a very
regular pattern of extremely narrow lines, that can be harnessed as an ideal calibrator for spectrographs, as its spectral envelope and line spacing can be tailored to match the specific needs of the application. Since the lines of a conventional LFC are far too densely spaced for being resolved by an astronomical spectrograph, special astro-combs with large mode spacings have been developed and demonstrated with great success.3–14
Areas that can profit from such highly accurate, LFC-assisted astronomical spectroscopy include the hunt for extrasolar planets by detection of periodic Doppler shifts in the spectra of their host stars. This has been a very
successful and productive method, but limited to planets that generate a sufficiently strong signal. An extrasolar Earth orbiting a Sun-like star would produce an RV modulation of 9 cm/s with a period of one year. Thorium-argon calibration, however, is not sufficiently repeatable to allow the detection of such a planet. With an LFC on the other hand, it appears feasible to calibrate a spectrograph to 1 cm/s over arbitrary time horizons, enabling future discoveries of exo-earths. This would also allow to meet the even greater scientific challenge of directly detecting the acceleration of the cosmic expansion, by measuring the change of quasar spectra at two different epochs about two decades apart. Another field that would greatly profit from the LFC is the investigation of the apparent variation of fundamental constants, that has been observed in the absorption spectra of intergalactic gas clouds illuminated by distant quasars.
The HARPS spectrograph is one of the leading instruments in high-precision astronomical spectroscopy to date.15 For this reason, HARPS has been selected for in-field testing of a major line of experimental astro-combs,
in a collaboration comprising the European Southern Observatory, the Max-Planck-Institute of Quantum Op-tics, and Menlo Systems GmbH. A total of five measurement campaigns were conducted between January 2009 and February 2012. During these campaigns, an excellent short-term calibration repeatability of 2.5 cm/s was demonstrated on a time scale of hours.9 Other achievements include a new comb-calibrated atlas of solar lines,16
and a demonstration of exoplanet detection using the LFC.9 The LFC was also able to uncover discontinuities
in the spectrograph’s wavelength scale,7 which were not previously known from other calibration methods.
Fur-thermore, the LFC was used to study the deformation of the spectrograph’s line profile through charge transport effects on its CCD.17 An important goal for the future is to demonstrate, that the calibration repeatability on
the low cm/s scale can be maintained over the course of years.
After the highly successful test phase, the focus of the cooperation has shifted towards elaborating a fully automated and user-friendly LFC for permanent operation on HARPS. Two other LFCs have already been permanently installed on astronomical spectrographs, one in 2011 at the VTT,10, 11 and another one14 in 2013
on HARPS-N. These systems are however still not easy to use, and require constant attention from experienced operators. The installation on HARPS is taken on with the aim of making the system available to all observers on the site, irrespective of their background or understanding of the system. This requires wise engineering of all subunits to make the system stable and fail-safe enough to work maintenance free over long periods of time. It also implies well thought-out automation features to fully operate the system with minimum effort from a control room at a remote site. In this paper, we give insights into the refined design of the system, and characterize its performance. The new system has been improved in many ways and we are convinced, that once the system is installed on HARPS and available to the astronomical community, its scientific impact will even be greater than it already has been during the test phase.
2. OVERVIEW OF THE SYSTEM
A frequency comb is generated from a mode-locked laser that emits a train of femtosecond pulses. The spectrum of this pulse train consists of a very regular pattern of extremely narrow lines at frequencies fn with:
fn= f0+ n × fr (1)
Here, fris the repetition rate of the optical pulses, f0is the so-called offset frequency, and n is an integer that is
commonly referred to as the mode number. frand f0are both radio frequencies (RFs) and are phase-stabilized
to an RF-reference such as an atomic clock, directly transferring the stability and accuracy of the RF-reference to optical frequencies. The spectral lines of a frequency comb are also known as comb modes, as they represent the longitudinal modes of the laser cavity. Our approach to realizing a robust LFC for astronomy is to make extensive use of fiber laser technology, and to avoid free-space optics wherever possible. Unlike other LFCs such as titanium-sapphire frequency combs, fully alignment-free lasers and optical systems can be created from single-mode, polarization-maintaining fiber components. Such systems are also much more compact and robust than their free-space counterparts. Currently, an Er-fiber frequency comb is prepared for a space flight on a rocket,18 which highlights the remarkable compactness and resilience of such systems. However, fiber lasers
typically operate in the near-infrared, while most astronomical spectrographs are geared towards the visible spectral region. We therefore employ Yb-fiber laser technology, which operates at about 1040 nm, with the second harmonic near the center of the visible spectral range. The latter can be covered from there by spectral
Infrared Yb-fiber based frequency comb source Increase of mode spacing through mode filtering Transfer into visible range by second-harmonic generation Spectral broadening in a tapered photonic crystal fiber Spectral flattening:
Truncation of the spectrum at a constant level
SHG
Figure 1. Concept for the generation of an astro-comb as pursued in this work: An initially infrared and densely spaced frequency comb is filtered with Fabry-P´erot cavities to increase its mode spacing. The filtered comb is then frequency doubled and subsequently spectrally broadened in a tapered photonic crystal fiber. The remaining structure of the spectral envelope is flattened out with a low-resolution spectral filter, formed by a spatial light modulator. SHG: Second-harmonic generation.
broadening. Additionally, obtaining the typically required mode spacings of more than 10 GHz directly from a mode-locked fiber laser is currently not possible. We therefore begin with a state-of-the-art 250 MHz frequency comb, hereafter referred to as the source comb, and widen its mode spacing by suppressing unwanted modes using Fabry-P´erot filter cavities (FPCs). Figure 1 illustrates our overall concept for synthesizing the comb light for spectrograph calibration.
The HARPS spectrograph has a resolution of R =115000 and a wavelength coverage of 310 nm, ranging from 380 to 690 nm.15 The full-width at half-maximum (FWHM) of an optical resolution element is of 5 GHz
at its center wavelength. The optimum mode spacing for the LFC is three times this value.2 However, for
HARPS an even larger spacing of 18 GHz was chosen, such that the comb lines are completely separated on the CCD with virtually no residual overlap. This facilitates analysis of comb-calibrated HARPS data, and makes no substantial sacrifice on calibration accuracy. Figure 2 depicts the configuration of the HARPS comb. The 250 MHz source comb is referenced to a Rb atomic clock. It is filtered by three FPCs, all at 18 GHz and with a finesse of about 2000, to provide excellent suppression of unwanted modes. The free spectral range (FSR) of the FPCs is stabilized by a continuous-wave (cw) fiber laser, that itself is locked to coincide with a comb mode. The filtered comb is then amplified to 12 W of optical power in a high-power fiber amplifier. The optical pulses at the output of the amplifier are compressed to a duration of about 140 fs, and are then frequency doubled. The green spectrum is then broadened in a tapered photonic crystal fiber (PCF).19 The resulting spectrum covers
up to more than 200 nm, but its spectral envelope is strongly structured. The final step is therefore to flatten out the spectral structure by a programmable, low resolution spectral filter.20 The final output of the system is
delivered in a single mode fiber, providing the best possible beam quality. The scope of this work ends at this point, which concentrates solely on the comb system. It is worth mentioning, however, that when coupling the comb light to a multimode fiber-fed spectrograph such as HARPS, active scrambling of the multimode fibers is critical.7, 21
The whole system is remotely controlled via a sophisticated software interface, that has been carefully designed to meet the needs of astronomers. The most remarkable feature of this software is an automation function, that upon a single mouse click automatically starts up the complete system from any initial state. This goes from mode-locking and stabilizing the source comb, to locking the cw laser and the FPCs to the right resonances, and optimizing the coupling into the tapered PCF.
source comb cwSlaser 250SMHz wavemeter FPC1 RbSatomic clock 18SGHz 18SGHz 18SGHz FPC2 FPC3 power amp lock
lock lock lock
GPC SHG spectral flattening output PCF SMF
Figure 2. Overview of the astro-comb system. FPC: Fabry-P´erot cavity. Cw laser: Continuous-wave fiber laser. GPC: Combined grating and prism compressor. SHG: Second-harmonic generation. PCF: Tapered photonic crystal fiber. SMF: Single-mode fiber.
3. THE SOURCE COMB
The source comb, as the initial near-infrared frequency comb from which the astro-comb is generated, is a 250 MHz Yb-fiber frequency comb. It consists of two major parts: The oscillator and the f :2f -interferometer. The oscillator is a mode-locked fiber laser, that emits the femtosecond pulse train that in the frequency domain forms the source comb. A part of this pulse train is sent into the f :2f -interferometer, that measures the offset frequency and repetition rate of the comb. The other part is passed on to the FPC filter chain.
For the right choice of the oscillator design, stability and low maintenance were the highest priorities. Earlier versions of the HARPS astro-comb relied on nonlinear polarization evolution (NPE) as a mode-locking mecha-nism. Even though this provided a robust mode-lock over many months, it could not be guaranteed that the mode-lock would be kept up for years without readjusting the polarization optics inside the laser. Similarly, the signal-to-noise ratio of the offset beat of up to 35 dB (at 20 kHz resolution bandwidth) enabled a stable phase-lock of the offset frequency, but the lock could occasionally fail, requiring manual intervention. The solu-tion for us was to switch to our newly developed figure-9 lasers, which is a modified sort of figure-8 laser that employs polarization-maintaining fibers. It mode-locks by virtue of a nonlinear amplifying loop mirror.22 As an
interferometric mode-locking mechanism, this imposes a much more well-defined oscillation regime, resulting in a considerably more stable and noise-free operation. In fact, we have never seen one of these oscillators sponta-neously lose its mode-lock, and a >50 dB signal-to-noise ratio (at 20 kHz resolution bandwidth) of the offset beat has already been observed. The emission spectrum is slightly narrower than that of NPE mode-locked lasers,
9 8 0 1 0 0 0 1 0 2 0 1 0 4 0 1 0 6 0 1 0 8 0 - 5 0 - 4 0 - 3 0 - 2 0 - 1 0 0 p o w e r s p e c tr a l d e n s it y [ d B ] w a v e l e n g t h [ n m ]
Figure 3. Spectral envelope (measured at a resolution of 0.1 nm) of the mode-locked fiber laser that generates the source comb. Central wavelength: 1032 nm. 3 dB-bandwidth: 43 nm. Transform limit of pulse duration: 60 fs.
3 7 3 8 3 9 4 0 4 1 4 2 4 3 - 6 0 - 5 0 - 4 0 - 3 0 - 2 0 - 1 0 0 p o w e r s p e c tr a l d e n s it y [ d B c ] f r e q u e n c y [ M H z ] 4 6 d B 3 9 . 9 4 0 . 0 4 0 . 1 - 1 0 - 5 0
Figure 4. Offset beat of the source comb, locked at 40 MHz, and measured with an RF-spectrum analyzer. Resolution bandwidth of the measurement: 10 kHz. Signal-to-noise ratio: 46 dB. Inset: Close-up towards peak of the signal, averaged over 100 acquisitions. Full-width at half-maximum: 120 kHz.
and the FPCs and fiber amplifiers have to be adapted to this. Figure 3 shows the spectrum of the figure-9-based oscillator for the HARPS comb.
The offset frequency and repetition rate is monitored with a standard Menlo Systems XPS 800 f :2f -interfero-meter. It picks up the repetition rate of the oscillator with a photodiode. For detection of the offset frequency, it goes the usual path of generating an octave-spanning spectrum in a PCF, and then beating the short-wavelength end of the spectrum with the second harmonic of the long-wavelength end. The frequency of the resulting beat note is equal in magnitude to the offset frequency, which is why it is referred to as offset beat. Figure 4 displays the measured offset beat, which has a remarkably narrow FWHM of only 120 kHz. As a consequence, the offset beat has an excellent signal-to-noise ratio of 46 dB, which should be more than sufficient to keep the comb phase-locked over arbitrary periods of time.
The lock of the offset frequency and repetition rate is supported by two RF-synthesizers, that convert the 10 MHz signal of the RF-reference phase-coherently into a signal of arbitrary frequency. The offset beat is locked directly to the signal of one of the two synthesizers using a digital phase detector and a proportional-integral (PI) controller, keeping the offset beat in phase with the signal of the synthesizer. This scheme allows direct and easy control of the offset frequency, by adjusting the frequency of the synthesizer as desired.
The lock of the repetition rate is a bit more intricate. We use the 4th harmonic of the repetition rate at
1 GHz and convert it down to 20 MHz by mixing it with a 980 MHz signal. This is then phase locked to the 20 MHz signal of the corresponding synthesizer. This locking scheme implicates, that if the frequency of the synthesizer is changed by a certain amount, the change of the repetition rate of the source comb is only a fourth of this value.
4. MODE FILTERING
The repetition rate of the source comb of 250 MHz is multiplied to 18 GHz by means of a chain of three identical FPCs. This means, that only one out of 72 comb modes is transmitted through the FPCs. The FPCs are stabilized in length by a cw laser, that itself is locked to one of the transmitted comb modes. At first it is unknown which set of comb modes is transmitted through the FPCs, leading to a 250 MHz ambiguity of the offset frequency of the filtered comb. This ambiguity can be resolved by measuring the frequency of the cw laser using the wavemeter included in Fig. 2.
The stabilization of the FPCs uses a Pound-Drever-Hall (PDH) scheme, that works by imprinting a sinusoidal phase-modulation on the cw laser with a frequency of 5 MHz, as indicated in Fig. 5. For locking of the cw laser
beatAdetection toAFPCs toAFPCA filterAchain servo cw laser PI controller RFAsignalA 5AMHz EOM RFAsignalA 150AMHz
AOFS sourcecomb
Figure 5. Locking scheme of the continuous-wave (cw) laser to a comb mode. The cw laser (single frequency fiber laser at 1040 nm) receives a 5 MHz phase modulation by an electro-optic modulator (EOM) to support the stabilization of the Fabry-P´erot cavities (FPCs). Since locking at a frequency of zero is challenging, the cw laser experiences a 150 MHz frequency shift by an acousto-optic frequency shifter (AOFS) before detection of the beat with the comb. The beat is then phase-locked to the frequency by which the cw laser has been shifted. Optical signals are split off or combined using fused fiber couplers.
to a comb mode, the frequency of the cw laser is shifted by 150 MHz using an acousto-optic frequency shifter (AOFS), before beating it with the source comb. The beat is then stabilized at 150 MHz by phase-locking it to the 150 MHz RF-signal that drives the AOFS. This means that the cw laser itself has no shift relative to comb mode it is locked to. It is necessary to introduce the frequency shift before the beat detection, because a phase-lock at 0 Hz is technically challenging.
Figure 6 illustrates the design and stabilization of the FPCs. The optical cavity has a plano-concave geometry, with a small flat mirror glued on a piezo tube. Two lenses on either side of the cavity ensure mode matching between the fundamental cavity mode and the in and outgoing fiber channels. The cavity mirrors are mounted inside of a mechanical structure that consists of two thick brass tubes. This construction ensures maximum mechanical stability. Additionally, the whole assembly of the FPC is enclosed in a box with very good vibration isolation from below. The cw laser is spatially overlapped with the comb in crossed polarization with a fiber-optic polarization beam splitter. This ensures perfect spatial mode matching between the two beams. After being transmitted through the FPC, the two beams are separated by a polarization beam splitter cube. The beam of the cw-laser is then picked up by a photodiode, monitoring the amplitude modulation of the transmitted cw laser. Mixing this signal with the RF-signal of the original phase modulation of the cw laser yields an error signal, that vanishes if the frequency of the cw laser is in the center of the FPC resonance. The length of the FPC is locked to this by a PI servo loop, controlling the voltage on the piezo tube of the flat mirror for fast control, and a Peltier element regulating the temperature of the FPC body to compensate slow drifts. After each FPC, the comb light is amplified in a core-pumped, Yb-doped fiber amplifier to compensate the losses in the FPC. The first FPC is also preceded by an amplifier, as it suffers greater power losses than the others, since it rejects previously unsuppressed modes.
The finesse of an FPC can be characterized by scanning its length while monitoring the transmission signal of the cw laser on an oscilloscope. The known free spectral range of the FPC of 18 GHz divided by the FWHM of the FPC transmission is the finesse of the FPC. In order to add a frequency scale to the scan of the FPC length, we exploit the side-bands that are created by the phase modulation of the cw laser. Exclusively for this purpose, we set the frequency of the modulation to 60 MHz for better separation of the side bands. The three strong transmissions in a single scan of the cavity length are fitted with a sum of three Lorentzian functions, as demonstrated in Fig. 7. Knowing, that the separation of the two outer Lorentzians corresponds to 120 MHz, the FWHM of the central Lorentzian is determined to be 9.42 MHz, leading to a cavity finesse of 1911.
PZT FC FC FPC body PBS temperature control filtered comb out cw cavity transmission RF signal of cw modulation PI controller fast slow comb in cw in iso iso PBS Yb fiber amp
Figure 6. Design and stabilization of a single Fabry-P´erot cavity (FPC). The body of the FPC is made of two thick brass tubes. The smaller mirror of the plano-concave cavity is glued on a PZT piezo tube. The FPC is stabilized to the continuous-wave (cw) laser, whose polarization is perpendicular to that of the comb, putting into practice a Pound-Drever-Hall locking scheme. The triangle at the filtered comb output represents a core-pumped Yb-fiber amplifier that compensates losses. PBS: Polarizing beam splitter. FC: Fiber collimator. Iso: Optical isolator.
120 MHz
sample number
transmi
tted power [a.u.]
Figure 7. Measurement of the cavity finesse of an FPC. The data points show a time trace of the cavity transmission during a scan of the cavity length, sampled in equidistant time steps by an oscilloscope. The transmitted light stems from a 1040 nm cw fiber laser, modulated at 60 MHz, which creates two main modulation side bands with a separation of 120 MHz to each other. The three major lines are fitted with a sum of three Lorentzian functions (green line), from which the finesse of the cavity can be deduced.
The expected suppression of the strongest suppressed mode can be calculated from the finesse to be 103 dB after the full chain of FPCs. The subsequent SHG and spectral broadening re-amplifies the suppressed modes.23
We estimate, that in a worst-case scenario, the unwanted modes still remain suppressed to better than 51 dB. Consequently, these modes cannot shift the center-of-gravity of a calibration line, as seen by the spectrograph, by more than 2 kHz, or 1 mm/s in terms of RV.
It has been argued, that filtering a frequency comb with an FPC can shift the center of gravity of the comb modes, if the comb modes are located at a slope of the filter function instead of its center.21 To quantify this
effect, we characterize the line profile of the comb modes by beat detection with the cw laser (linewidth 35 kHz). We assume the measured beat note (FWHM: 79 kHz) as a conservative estimate for the profile of the comb modes. In combination with the measured cavity finesse, we can then calculate that the mode filtering cannot shift the centroids of the lines by more than 3 kHz for the final filtered and frequency doubled comb, which is the equivalent of 2 mm/s in RV.
5. NONLINEAR FREQUENCY CONVERSION
After mode filtering, the comb spectrum is frequency doubled and then broadened to cover a large portion of the visible spectral range. In order to drive these nonlinear optical effects, high peak intensities are required, which are hard to achieve at a pulse repetition rate of 18 GHz. We therefore employ a high-power fiber amplifier (see Fig. 8) to amplify the pulse train to 12 W of average power. This is possible by pumping the core of the Yb-doped gain fiber from the surrounding cladding, which allows the use of powerful multimode diode lasers for pumping. The unabsorbed part of the pump light is dumped at a heat sink at the end of the gain fiber. This high-power fiber amplifier must not be pumped without being seeded. Even though the amplifier is protected by an optical isolator, back-reflections also represent a potential threat. This is why the amplifier incorporates a fiber coupler that allows the monitoring of both the seed at the amplifier input and the signal traveling in back direction. If either of them reaches a critical value, the pump lasers are shut down by an interlock circuit within less than 10 µs, in order to prevent damage.
After the power amplifier, the optical pulse train enters a free-space region. Being heavily chirped by about 30 m of optical fiber, the optical pulses need to be re-compressed. We introduce the necessary amount of anomalous dispersion by sending the beam through a combined grating and prism compressor, that allows to control the linear and quadratic chirp independently. The resulting 140 fs pulses are then frequency doubled in a 3 mm long LBO crystal, yielding 400 mW of green average power.
For spectral broadening, the green light is coupled into a tapered PCF, very similar to the one described by Stark et al.19. The fiber has a 2 µm core diameter at both ends. In between, it is tapered down to a core
diameter of about 540 nm, which is maintained over 20 cm. The resulting spectrum depends strongly on the amount of coupled light. In the past, drifting alignment of the PCF coupling was an issue, requiring frequent readjustments. This problem has been solved by actively stabilizing the fiber coupling with a piezo-driven mirror before the fiber (TEM FiberLock). Additionally, we have improved the passive stability of the fiber coupling by connectorizing the fiber ends with FC/APC connectors. This involves processing the ends of the PCF by collapsing the air-holes around fiber core and polishing the end facets. This has the additional benefit of greatly facilitating the handling of the PCF, that can now easily be exchanged as a simple patch cable.
Figure 9 shows the broadened 18 GHz comb spectrum over the course of two hours. The spectral bandwidth at 20 dB below the peak of the spectrum is 162 nm, with a coupled optical power of 165 mW. With further optimization of the taper geometry, we should be able to extend the 20 dB-bandwidth to 235 nm as demonstrated by Stark et al.19.
6. SPECTRAL FLATTENING
The broadened comb spectrum covers much of the visible spectral range, but its envelope has a pronounced structure. In order to make best use of the LFC as a calibrator, it is helpful to remove this spectral structure, equalizing the signal level of all lines on the spectrograph CCD. This allows to collect a maximum number of photons in a single exposure of the comb, without saturating the CCD with any of the calibration lines. Min-imizing the effect of photon noise, this improves the attainable calibration accuracy. We provide the necessary
(a)
(b)
99:1 pumpW1 pumpW2 seedW monitor back-reflection monitor interlock heatW sink beam dump fiberWlock LBO grating-prismcompressor transmissionmonitor
HWP HWP PCF seedW in iso iso Yb3+ -doped fiber toW(b) from power amp out pumpW combiner
Figure 8. Nonlinear frequency conversion. (a) Cladding-pumped power amplifier that provides the power to drive the nonlinearities for frequency conversion. The interlock shuts down the pump diode lasers if the amplifier seed or feedback fall or rise to critical values. This amplifier is represented by a triangle in Fig. 2. (b) Pulse compression, followed by second-harmonic generation and spectral broadening. An active feedback loop stabilizes the PCF coupling. HWP: Half-wave plate. LBO: Lithium triborate crystal. PCF: Tapered photonic crystal fiber.
2 0 4 0 6 0 8 0 1 0 0 1 2 0 t i m e [ m i n ] - 4 0 - 3 5 - 3 0 - 2 5 - 2 0 - 1 5 - 1 0 - 5 - 1 0 - 2 0 - 3 0 4 5 0 5 0 0 5 5 0 6 0 0 p o w e r s p e c tr a l d e n s it y [ d B ] w a v e le n g th [ n m ] p o w e r s p e c t r a l d e n s i t y [ d B ]
Figure 9. Shape and temporal evolution of the 18 GHz comb spectrum after spectral broadening. Line plot on the left: Spectrum of the broadened 18 GHz comb at the beginning of the measurement. At 20 dB below its peak, the spectrum ranges from 437 to 599 nm. The further development of the spectrum is shown in the color plot to the right. Spectral resolution of the measurement: 0.5 nm. Temporal resolution: 1 min.
broadened/comb/in CCD spectrometer computer-controlled/ feedback/loop LCOS/SLM BS/90:10 OSA// spectrograph variable/air-gap attenuator pol./0° pol./90°
Figure 10. Spectral flattening scheme. The spectral transmission is controlled by adjusting the voltage on the LCD pixels of the SLM, altering the polarization between a pair of two crossed foil polarizers. Pol.: Polarizer. BS: Non-polarizing beam splitter. LCOS: Liquid crystal on silicon. SLM: Spatial light modulator. OSA: Optical spectrum analyzer (here taking the place of the astronomical spectrograph).
spectral reshaping by spatially separating the spectral components with a grating and a lens, and adaptively attenuate the individual spectral components by means of a spatial light modulator (SLM). We have demon-strated this technique before,20 with the SLM being a pixelated liquid crystal device operated in transmission.
We have now taken this concept one step further by using a reflective liquid crystal-on-silicon (LCOS) device (Holoeye PLUTO). It consists of a liquid crystal layer with transparent electrodes on one side, and reflective silver electrodes on the other side. Each pair of opposed electrodes represents a pixel. Controlling the degree of birefringence of the liquid crystal, every pixel rotates the polarization of the incident light depending on its voltage. The spectral components are then recombined into a single beam by passing again through the same beam path in opposite direction. The whole arrangement has single mode fibers at its input and output, where a pair of crossed polarizers translates the altered polarization of the spectrum into a spectrally resolved intensity change. The setup is depicted in Fig. 10. The reflective design of the setup makes it more compact, reduces the number of optical components, and facilitates alignment by having fewer degrees of freedom.
The broadened input spectrum is flattened by attenuating spectral components that are in excess of optical power. As a standard choice, the spectrum is truncated at 20 dB below its peak, but any other level compatible with the maximum attenuation of 27 dB can be selected. The spectrally reshaped output is distributed among a small CCD spectrometer and the astronomical spectrograph to be calibrated. The CCD spectrometer is in a computer-controlled feedback loop with the SLM, designed to adaptively keep the spectrum flat. The algorithm controlling the feedback is largely identical to the one presented by Probst et al.20. For test purposes, an optical
spectrum analyzer (OSA), which consists of a scanning monochromator, takes the place of the astronomical spectrograph. It measures the power spectral density in mW/nm, and thus has a spectral response that is quite different from that of the CCD spectrometer. Additionally, the split ratio of the beam splitter has a wavelength dependence. A spectrum is therefore not flat as measured by the OSA, when it appears flat on the CCD spectrometer. Recording a spectrum simultaneously with the OSA and the CCD spectrometer allows to generate a look-up table that quantifies this effect. The software of the SLM control uses the look-up table to compensate the different response of the OSA. Much in the same way, the algorithm can also be adapted to the spectral response of any astronomical spectrograph. The test result with the OSA is shown in Fig. 11, although this measurement was not performed on the astro-comb itself, but on a test bench using a standard 250 MHz comb.
4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 7 5 0 - 3 0 - 2 0 - 1 0 0 w i t h o u t f l a t t e n i n g f l a t t e n i n g a c t i v e p o w e r s p e c tr a l d e n s it y [ d B ] w a v e l e n g t h [ n m ]
Figure 11. Flattening of a 250 MHz broadened comb spectrum, demonstrating the capabilities of the spectral flattening setup. Red Line: Spectrum with the spatial light modulator at full transmission for all wavelengths. Black line: Spectral flattening active, truncating the spectrum at 20 dB below the previous maximum. The spectra have been recorded using the OSA in Fig. 10 at a resolution of 0.5 nm.
7. CONCLUSION
In summary, we have presented the design of a robust and user-friendly LFC for calibration of the HARPS spectrograph, that should provide a repeatability and accuracy of better than 1 cm/s. The calibration results that are achieved in practice, however, will also depend on parameters other than the astro-comb, such as the spectrograph itself, the coupling of the comb light to the spectrograph, and the handling of effects on the spectrograph CCD and software pipeline. The installation of the LFC on HARPS is planned for November 2014, which will be followed by instrumental tests. The LFC will then become available for routine use within the normal observation schedule of the HARPS spectrograph.
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