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Abstract
Frequency combs downconvert absolute optical frequency references and thereby can significantly advance time and frequency precision in satellite-based navigation systems, fundamental science, earth observation, and many other spaceborne applications. We have developed a compact and vacuum compatible double comb system, thereby minimizing volume, mass, and power consumption compared to its precursor. Apart from redundancy aspects, the two combs enable autonomous mode number determination. Here, we report on the space comb system design and experimental results of a zero-gravity parabolic space flight in connection with an iodine-referenced cw laser system.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. INTRODUCTION
Optical frequency combs, originally invented in the late 1990s for high-precision spectroscopy and optical clocks [1], have paved the way to unprecedented precision in the measurement of time and frequency as well as optical synthesis and spectroscopy [2–5]. Frequency combs can be obtained by various approaches: (i) using feedback-controlled femtosecond solid-state [1] or fiber lasers [6], (ii) soliton-based chip-scale and whispering gallery mode Kerr combs [7,8], or (iii) electro-optical modulation of cw lasers [9]. While all these methods have produced fascinating scientific breakthroughs in research laboratories, most commercial combs are based on (i). Today’s most successful technology is particularly based on the nonlinear optical loop mirror (NOLM) fiber laser, which provides important key properties as reliability, lifetime, size, stable mode lock, and superior phase-noise performance [10]. Regarding optical clocks, the frequency comb is the fundamental means to divide optical toward countable radio frequencies (RFs) with a typical ratio of ${10^6}$. Because of the large ratio the phase noise of the RF also will be strongly suppressed. Frequency comb based optical clocks using precision lattice based optical references like, for example, Sr and quantum-logic clocks, have recently reached an accuracy of ${ \lt 10^{- 18}}$ [11,12], while cesium-based (Cs-based) RF clocks are currently limited to the ${10^{- 16}}$ range. This now-available ultimate precision has opened a window of opportunities in precision timing, ranging, geodesy, and space–time research. Promising concepts for space-based applications exist in various fields; for example, as a remote sensing tool in light detection and ranging (lidar) instruments [13,14] tracing atmospheric gas concentrations in Earth and planetary observation, as future high-precision optical clock in navigation satellites [15], for fast and precise intersatellite ranging [16–20], or in fundamental physics experiments studying space–time [21–23]. For such applications, the development of compact, robust, and energy-efficient combs with advanced space technology readiness is of high importance. A first comb demonstrator (FOKUS I fiber optical comb generator under microgravity) in space was launched on experimental TEXUS sounding rockets in 2015 and 2016 for six-minute zero-gravity flights [24], reaching an altitude of up to 280 km. In the present paper, we will discuss the succeeding comb generation, FOKUS II, which was developed to eliminate some technical limitations of the earlier system, has a reduced mass, volume, and electric power consumption, with improvements in the overall level of robustness, precision, and stability. We will present the advanced technology of this instrument and elaborate on the qualification of the sounding rocket payload. Additionally, we will present results of the TEXUS 54 sounding rocket campaign, where the comb has been connected with an optical fiber to an iodine-based optical standard (JOKARUS) [25]. We will also review the implications of the experiment on the future of optical frequency combs in space.
2. SYSTEM DESIGN
The optical instrument consists of two identical fiber optical frequency combs, two beat detection units for monitoring a 1064 nm iodine-stabilized laser, and the autonomous operating comb electronics. Figure 1 depicts the fiber optical setup of the instrument. The combs are based on a figure-9 nonlinear amplified loop mirror laser using erbium-doped fiber as gain medium [10,26]. The output in the frequency domain of the comb laser can be described as $f = n \times {f_{\rm r}} \pm {f_{0}}$, where the repetition rate ${f_{\rm r}} = 100\;{\rm MHz}$ is controlled by the fiber ring temperature and a piezoelectric transducer acting on the cavity end mirror. The oscillator provides 5 mW seed light at 1560(25) nm (FWHM), which is further amplified and compressed by an erbium-doped fiber amplifier (EDFA) to 180 mW and a pulse duration of 45 fs. The spectrum is subsequently broadened to an octave-spanning supercontinuum in a highly nonlinear fiber (HNLF). This white light is coupled into a periodically poled litium niobate (PPLN) waveguide, where the red spectral wing of the spectrum around 2120 nm generates a second harmonic at 1060 nm, so that the carrier envelope offset frequency ${f_0}$ can be measured in a heterodyne f-2f self-referencing scheme on a photodiode attached to the waveguide output after suitable spectral filtering with a fiber Bragg grating (FBG) at 1060 nm. The remaining light after the FBG can be used to generate a heterodyne beat note with the device under test (DUT) in a beat detection unit (BDU). In FOKUS II, both BDUs were designed to operate at 1064.5 nm and generate two beat notes with the iodine-stabilized laser system JOKARUS [25], which was allocated in the same payload on the sounding rocket TEXUS 54.
Fig. 1. Optics of the FOKUS II dual comb instrument. (a) Schematic of the optical setup. (b) Oscillator free space optics soldered onto a aluminum oxide ceramics bench. (c) The $(150 \times 100)\; {{\rm mm}^2}$ fiber box containing the oscillator, EDFA amplifier, and f-2f interferometer, as well as two free space optical compartments. Acronyms: BS, Beam Splitter; WDM, Wavelength Division Multiplexer; ISO, Optical Isolator; CIRC, Circulator; $\Delta \phi$, Phase Shifter; LD980, 980 nm Pump Diode.
The complete laser design is based on polarization-maintaining fiber, except a small free space region in the oscillator that is required for the actuators on repetition rate and offset phase. For increased robustness and space-readiness, the free space optics consisting of a collimator, polarizing beam splitter (PBS), electro-optic modulators (EOM), a waveplate (WP), and a piezo actuator (PZT) for the end mirror, have been soldered flux-free onto a gold-plated aluminium oxide ceramic micro-optical bench, as shown in Fig. 1. Such ceramics represents a good compromise for a thermally stable micro-optics system. Industrial-grade aluminum oxide patterned with gold wiring and pads can be ordered at printed circuit board (PCB) manufacturing services and shows a temperature coefficient that can be easily compensated by the system. Using a mask, all components, with the exception of the collimator, could be soldered onto the bench in a single alignment-free step. For the collimator placement, an induction soldering technique was used that only heated and melt a small patch of solder and hot-aligned the collimator during the process. Unlike a permanent adhesive bond, when using this method the components can be realigned by resoldering, and the solder shows much less long-term aging. Furthermore, we minimize the risk of contamination due to volatile and outgassing materials in the oscillator, which is a critical step toward vacuum compatibility and laser life span. Where soldering techniques could not be applied, an ASTM E-595 qualified [27] two-component epoxy adhesive was used. Each comb laser is housed within a $(152 \times 127 \times 36)\;{{\rm mm}^3}$ fiber box, shown in Fig. 1. The oscillator repetition rate changes with the local temperature by approximately $1.8\;{\rm KHz}\;{{\rm K}^{- 1}}$, thus a rigorous stabilization of the aluminium body is required to lock the comb to the desired frequency. This was realized by casting all fibers in space-grade silicone, which also improves the mechanical robustness and provides temperature stabilization under zero gravity. The temperature of the fiber tray was controlled with two Peltier elements at a stability level of $\le 10\;{\rm mK}$. Fast fine-tuning of the repetition rate against acoustic perturbations up to several kHz is achieved with a small piezo-stack (${\pm}10\;{\rm Hz}\;{{\rm V}^{- 1}}$) directly acting on the cavity end mirror. In the present design, ${f_0}$ is fully controlled over the complete spectral range using a proprietary EOM arrangement. The EOM is advantageous to the previous FOKUS I design, where the pump intensity has been used as an actor. In the NOLM, the variation of the pump intensity sometimes shows an undesired nonlinear response of the variation of ${f_0}$, which can limit the operating range of the NOLM as a comb. Moreover, the EOM actuator for ${f_0}$ can provide a higher bandwidth compared to pump modulation, which is always limited by the lifetime of the upper state in the gain material.
Regarding electronics, the combs are driven by two $(160 \times 134)\;{{\rm mm}^2}$ sized supply modules (OSM), as shown in Fig. 2(c). Each module contains two thermal controllers to stabilize the fiber box and the free-space bench, as well as three uncooled 980 nm 10-pin mini butterfly packages. These laser packages provide two independent chips and output fibers with an optical output power of up to 400 mW each. Using polarization-combined fibers, one package is used to pump the oscillator, the second is used for the EDFA, and the third is for redundancy only. The third PCB in Fig. 2(c) is the system’s power unit. It provides all the necessary voltages with a ripple below a few mV from a 26(4) V input and additionally can bridge power outages of more than 100 ms using four 400 mF supercapacitors. These three high-power double unit modules are placed on the opposite side of the comb module to minimize their impact on the temperature-sensitive optics. To ensure a sufficient heat transfer and cooling of all electrical components under vacuum, the PCBs are equipped with an aluminium backplane and are clamped onto the mechanics assembly using wedge-lock card retainers.
Fig. 2. Photos of the FOKUS II instrument. (a) Two frequency combs and two beat detection units are stacked on the left. In the center, the $(160 \times 67)\;{{\rm mm}^2}$ control electronics are housed. (b) Electronic and optical input/output interface at the backside of the comb modules. (c) High-power electronics, i.e., the DCDC converter and two supply modules (OSM), are separated from the rest of the system to minimize their thermal impact.
In the central section of the system, 11 $(160 \times 67)\;{{\rm mm}^2}$ (single unit) sized PCBs are arranged in vertical slots. These modules represent the entire control chain of the dual frequency comb system and provide the following functionalities: (i) six-channel photo detection, amplification and filtering; (ii) two repetition-rate phase-locked loops; (iii) two carrier envelope offset phase-locked loops; (iv) two four-channel frequency counters; (v) one 10 MHz/990 MHz reference distribution; (vi) a direct digital frequency synthesizer; and (vii) an embedded system micro controller.
The outputs of the oscillators, f-2f spectroscopies, and BDUs are sent via a fiber patchcord toward the photodetection modules. From there, they are forwarded as filtered and amplified RF signals to the respective ${f_{\rm r}}$ and ${f_0}$ phased-locked loops. Typical RF spectra and attributed phase noise are shown in Fig. 3.
Fig. 3. Typical RF spectra and SSB phase noise of the locked system measured on the ground when referenced against the internal Ln-CSAC 10 MHz oscillator: (a) phase noise (dBc/Hz) of ${f_{\rm r,A}}$, (b) phase noise (dBc/Hz) of ${f_{\rm 0,B}}$, (c) RF signal ${f_{\rm r,A}}$ at 100 kHz RBW, (d) offset beat signal ${f_{\rm 0,A}}$ at 100 kHz RBW, (e) beat signal ${f_{\rm b,A}}$ at 250 kHz RBW, and (f) beat signal ${f_{\rm b,B}}$ at 250 kHz RBW. Note that the central frequency of the iodine beat with both combs is slightly different because of the unequal comb repetition rates.
Fig. 4. Locking scheme of the FOKUS II frequency combs. The repetition rate is detected at 1 GHz and downmixed by a 990 MHz phase-locked oscillator. ${f_{0}}$ and ${f_{\rm r}}$ are then locked at 10 MHz to the Ln-CSAC cesium reference. The ${f_{{\rm r,B}}}$ lockbox of the second comb (B) uses the DDS output instead of the fixed reference to mix the error signal.
As shown in Fig. 4, the repetition rate is detected at the 10th harmonic at 1 GHz. The signal is then down-mixed with a 990 MHz phase-locked oscillator and referenced to the 10 MHz low noise chip scale atomic clock (Ln-CSAC, Microsemi Corp., Aliso Viejo, CA, USA). Similarly, the offset frequency is directly locked to the Ln-CSAC. One of the combs (B) can additionally be tuned by adjusting the ${f_{\rm r}}$ lock target frequency with a DDS frequency synthesizer, limited within a range of a few kHz offset from 100 MHz. This allows the automatization of the mode-number determination by measuring the beat signal ${f_{\rm b}}$ of the cw laser simultaneously at two different repetition rates. The absolute frequency of the DUT can be calculated using ${f_{{\rm cw}}} = n{f_{\rm r}} \pm {f_{0}} \pm {f_{\rm b}}$. Using two combs A and B, where ${f_{{\rm r,A}}} \mathbin{\lower.3ex\hbox{$\buildrel \gt \over{\smash{\scriptstyle \lt}\vphantom{_x}}$}} {f_{{\rm r,B}}}$, we get
All frequencies – ${f_{{\rm r,A}}}$, ${f_{{\rm r,B}}}$, ${f_{{\rm 0,A}}}$, ${f_{{\rm 0,B}}}$, ${f_{{\rm b,A}}}$ and ${f_{{\rm b,B}}}$ – are directly measured with the systems Pi-type frequency counter. In addition, ${f_{{\rm 0,A}}}$ and ${f_{{\rm 0,B}}}$ frequencies and signs of both combs are identical by design and control algorithm. The mode number difference $\Delta n$ can be selected during system operation by adjusting the DDS (and therefore ${f_{{\rm r,B}}}$), or be iteratively computed during data post-processing. After exclusion of any nonvalid cases the mode number can be unambiguously obtained.
For remote control and automation, a business-card sized embedded controller is used to govern the system’s high- and low-level functions. It is based on an ARM Cortex A8 processor, which running a customized Linux environment, and provides direct communications with the control electronics over the internal serial periphal interface (SPI) and controller area network (CAN) bus. The simplified block diagram in Fig. 5 depicts the experiment’s communications interfaces. A web interface allows a user to operate the two combs manually or to monitor the fully autonomous operation. For operation in the lab, the connection is established via a standard Ethernet link. For the TEXUS 54 campaign, however, it is necessary to break down the TCP/IP communications to a raw serial stream, which is transmitted from the rocket to the ground station where it is reconverted to TCP/IP on a local gateway of the ground station equipment (GSE). Due to the limited bandwidth of the S-band link of 1.9 kB/s down and 30 B/s up, a highly automatized state machine that responds to various timer and event signals from the TEXUS rocket was implemented. In this way, the operator was able to fully focus on the experiment during the 15 min flight and 6 min microgravity phase.
The assembled dual comb system, including optics, supply, cabling, and control electronics, was mounted onto a common aluminium baseplate, measuring $(328 \times 319)\;{{\rm mm}^2}$ in its maximum dimensions. Integrated water cooling pipes allow the stabilization of this baseplate and acceleration of the thermalization process during lab operation and before launch in the TEXUS 54 campaign. Typical baseplate temperatures are between 15°C and 18°C. It was possible to operate the system without any external cooling at an ambient temperature between 10°C and 35°C. In the final configuration, the instrument weighs 10 kg, within a volume of 7 L without the baseplate at a continuous power consumption of 66 W. We were thus able to effectively reduce these key parameters by a factor of two compared to the previous FOKUS I system while additionally providing one additional and fully functional comb (see Table 1). The combs, their BDUs as well as the electronic units remained fully modular allowing a straightforward assembly of the system. The design enabled us to easily access all subsystems of the instrument for testing purposes, largely improving the system integration, testing, and qualification workflow.
Table 1. Budget Comparison of the Two FOKUS Instrument Generationsa
3. QUALIFICATION
To qualify the mechanical robustness of the system for launch and operation on sounding rockets, particularly of the micro-optical bench, the system was subjected to a series of vibration tests within the facilities of the IABG mbH in Ottobrunn, Germany. The test parameters were in accordance with the requirements by Airbus SE for a suborbital flight on a TEXUS sounding rocket. The system was subjected to a random noise vibration pattern, as shown in Fig. 6, for 120 s on each axis at an integrated RMS level of 8.8 g. While the comb was not powered during active vibration, its function has been verified in between each individual axis shaking. Additional resonance search sweeps between 10 Hz and 2 kHz at 0.5 g were performed before and after each run to detect possible changes or damages in the mechanical structure. The response measurements at two control points on the system verified the intrinsic stability under substantial vibrational loads and, moreover, no potentially damaging resonances have been observed. Both combs operated nominally after these tests without the need for any realignment of the optical setup.
Fig. 6. Vibration qualification of the FOKUS II instrument. The vibration of the shaker is shown in the upper left panel and was applied to all axes x, y, and z. The excitation of the system shown here was measured on the comb optics unit with an acceleration sensor in all dimensions and for all directions of shaking. The secondary acceleration does exceed the original excitation of the shaker for frequencies above 500 Hz by a factor $\le 20$ due to internal resonances, but stays within the expected limits for a vibration robust setup.
With respect to environmental temperature requirements, the full system and also individual submodules were tested in an in-house climate chamber with rapid temperature change cycles between 10°C and 45°C over several hours. This step was necessary since temperatures during the transport to, and storage at the launch site can vary between 15°C and 35°C. System operation in the lab was usually performed with an external water chiller to reduce thermalization time. Swift ambient temperature changes, as they will occur after liftoff and touchdown of the rocket, were also simulated by controlling the chiller’s target temperature. In this way, the PID parameters of the comb’s temperature-control loops could be fine tuned and optimized for the upcoming mission.
To further increase the frequency comb’s technology readiness level for future space missions, the system was readily designed for in-vacuum operation. All electronic PCB modules were tested under operation in a vacuum chamber at Menlo Systems (Planegg, Germany), at a vacuum level of $\le 10\;{\rm mbar}$, sufficient to confirm the survival and thermal design of the modules. The comb optics modules, supplied by an OSM pump and DCDC module, were also successfully operated in vacuum for several hours. An additional systemwide vacuum test comprising multiple launch and flight simulations has been performed at Airbus Trauen in January 2018. The test uncovered that, within the first 20 min after a rapid evacuation, the systems repetition rate cannot be locked reliably. After further investigation, we suspect that expanding microbubbles in the silicone cast exert local force on the tightly packed fibers and splice connections in the optics. After 20 to 30 min under vacuum, the system became stable and could be successfully locked in the repetition rate and offset frequency. Accordingly, in the short flight time of roughly 18 min, where the experiment should begin with the micro-g phase 70 s after liftoff, fast pressure changes for the comb had to be excluded for the TEXUS 54 mission. For the payload, it was decided that the instrument would be operated under a cylindrical dome, pressurized at 1.2 bar.
4. TEXUS 54 SOUNDING ROCKET CAMPAIGN
The TEXUS 54 campaign began on May 5, 2018, at the Esrange Space Center close to Kiruna, Sweden. Within the first days, the FOKUS II system was unpacked, tested, and integrated into the VSB-30 vehicle. FOKUS II was connected with an optical fiber to another payload module, the frequency reference JOKARUS, which was developed by Humboldt University of Berlin and collaborators [25]. JOKARUS uses a micro-integrated extended cavity diode laser (ECDL) at a wavelength of 1064 nm. A spectroscopy module together with locking and control electronics allows the stabilization of this laser on the hyperfine component of the rovibronic transition R(56)32-0 ${{\rm a}_1}$ of molecular iodine using a modulation transfer scheme [28]. One auxiliary port of the JOKARUS master oscillator was connected via fiber to FOKUS II, where the signal was split 50/50 and then fed into the two beat detection units of the comb. As shown in the measurement scheme in Fig. 7 and in Fig. 3, with the two combs A and B running at slightly different repetition rates, the cw beat signal is detected at different frequencies, allowing an autonomous determination of the combs’ mode numbers.
Due to the short mission duration of about 13 min, the system operation was primarily automated with the remaining possibility to manually override parameters at any point in time. In the hours prior to launch, the system was thermalized and in stable operation. An event signal is send from the rocket 40 s prior to liftoff (T-40s), which activates a safe mode in the state machine, disabling the ${f_{\rm r}}$ and ${f_{0}}$ lock of both combs, while the pump lasers stay active and the fiber oscillator remains mode-locked. This step is necessary since a stable lock of the repetition rate could not be guaranteed when encountering heavy vibrations and shocks during the ascent stage, potentially causing an overreaction of the locking electronics that could drive the comb into an undesired state. However, completely disabling the pump lasers was not an option during ascent as it would have substantially changed the temperature in the fiber tray. To counteract the thermal drift occurring during the unlocked launch phase, a very small temperature change was initiated in the two fiber boxes at liftoff. This temperature change was simulated and tested in the integrated payload, so that when the microgravity phase at ca. T $+$ 70 s begins, the repetition rate could quickly be stabilized close to the new target temperature. The two comb frequencies for each comb are then relocked sequentially in the following procedure: (i) Guide the repetition rate toward the target by fine-adjusting the fiber temperature (100 MHz); (ii) when within ${\pm} 10 \; {\rm Hz}$ of the ${f_{\rm r}}$ target, engage the piezo lock on the cavity end mirror; (iii) shift the ${f_{0}}$ beat within a bandpass window ($10 \pm 5 \; {\rm MHz}$) by adjusting the ${f_{0}}$ actuator temperature; (iv) detect the ${f_{0}}$ sign and direct the ${f_{0}}$ frequency toward the target (10 MHz); and (v) engage ${f_{0}}$ lock when within ${\pm}1\;{\rm MHz}$. As soon as the two combs are fully stabilized, the cw beat can be correctly measured by the counter.
The microgravity phase lasts approximately six minutes from T + 70s to T + 430s, at which point the rocket descents into the atmosphere and begins to tumble. During reentry, the combs remained active and locked as long as possible, even during some harsh deceleration shocks. At T + 780s, a shutdown signal is given by the service module, initiating backup of the experiment log files and safely turns off all lasers and the embedded system before touchdown at T + 810s. The complete flight sequence has been successfully tested several times in connection to the JOKARUS system on the ground before and after payload integration.
TEXUS 54 was launched on May 13, 2018, at 08:30 UTC (mission time 0 s). Figure 8 depicts the onboard measurements during the space flight, as discussed below. The data of Comb A is shown in red, the data of Comb B is in blue. The first panel shows the difference in $\Delta f$ of JOKARUS’ acquired absolute frequency to the reference value at 1064 nm of
Fig. 8. Top: In-flight measurement showing the flight altitude, total acceleration, and absolute frequency measurement subtracted by the reference value. After 470 s comb B was unlocked and cannot be compared to literature values. Middle left: In-flight deviation of the repetition rate ${f_{\rm r}}$ and ${f_{0}}$ frequency from the lock target. Middle right: Stability as ADEV of the ground and zero-g flight data for the iodine beat with comb A compared to the LnCSAC 10 MHz RF clock as characterized in the lab with an ultrastable reference. Bottom: Post-flight measurement of the cw frequency after recovery.
The second and third panels in Fig. 8 show the deviation from the target frequency for ${f_{\rm r}}$ and ${f_{0}}$. Prior to liftoff, both combs were firmly locked and thermalized for several hours until T-40s, when the safe mode and temperature jump was initiated as planned. From 0 to 43 s the vehicle ascends with its two stages at peak acceleration of $118\;{{\rm ms}^{- 2}}$ (12 g). During this stage, the rocket is gyroscopically stabilized by initiating a rotation along its roll axis, resulting in a 3.5 Hz spin rate at burnout. Two yo-yos are ejected from the vehicle to reduce the rotation down to ${\lt} 50 \; {\rm mHz}$ at T + 57s. The remaining spin is eliminated by a reaction control system (RCS), such that at T + 72 a microgravity level of less than ${10^{- 4}}\;{\rm g}$ is achieved. At this point, the FOKUS master state machine is reactivated and starts to relock the combs as described above. At T + 125s the repetition rate ${f_{{\rm r,A}}}$ of comb A was locked and at T + 155s the ${f_{{\rm 0,A}}}$ of comb A was locked as well, immediately starting the measurement of the JOKARUS output, which was at this time already stabilized. Comb B was fully relocked at 165 s after µg was achieved, requiring slightly more time. This can be explained by the fact, that comb B is positioned at the bottom of the optics stack and directly affected by the temperature drift of the base plate. During the flight, not only the comb system but also the TEXUS service module located underneath dump their thermal load into the base plate, the latter providing communications and a power interface including high-power DCDC converters and batteries for the FOKUS II and JOKARUS modules. The thermal load exceeded previous simulations significantly and, as a consequence, the fiber tray thermal controller required more time to catch up and stabilize the temperature. However, apart from redundancy, comb B’s primary objective has been to provide a second measurement to enable a direct mode number determination. For this, even a short measurement of a few seconds in a fully determined state is sufficient. Both combs remained fully locked for the remaining experimental phase, ending at T + 425s with the end of microgravity. At this point in time, JOKARUS’ control loops were deactivated for re-entry. We observed that comb B’s repetition rate shortly lost phase at peak decelerations of $120\;{{\rm ms}^{- 2}}$ (12.2 g), and lost lock completely from T + 508s until shutdown. Comb A was stable up to a $\ge 55\;{{\rm ms}^{- 2}}$ (5.6 g) shock from the pneumatic heat shield release of the rocket at T + 575s. It quickly relocked after the parachutes were deployed and enabled another measurement with the also restabilized JOKARUS system from T + 659 to T + 785s. At 785 s, the payload shutdown signal was given and all systems were turned off for security as scheduled for touchdown, which occurred at T + 810 with an impact shock of greater than $206\;{{\rm ms}^{- 2}}$ (21 g). The payloads touched ground 93 km away from the launcher on a mountain plateau. It was recovered and returned to Esrange by a helicopter 4 1/2 h after touchdown. The payload was disintegrated from the rocket structure, inspected for any damages, and powered up again. At 15:20 UTC the launch day (T + 24700s) both combs and JOKARUS were again fully stabilized and another control measurement was performed, as shown in the bottom panel of Fig. 8.
From the acquired frequency data shown in Fig. 8, the statistics of the frequency difference measurement $\Delta f =$${f_{{\rm cw\,measured}}} - {f_{{\rm JOKARUS}}}$ can be derived. Immediately before flight comb A and B show ${\mu _{\Delta {\rm f,A}}} = 0.4$ kHz, ${\sigma _{\Delta {\rm f,A}}} = 9.2$ kHz, and ${\mu _{\Delta {\rm f,B}}} = 6.6$ kHz, ${\sigma _{\Delta {\rm f,B}}} = 10.3$ kHz. During the zero-g phase the difference frequencies stay in the same order with ${\mu _{\Delta {\rm f,A0g}}} = 10.9$ kHz, ${\sigma _{\Delta {\rm f,A0g}}} = 9.9$ kHz, and ${\mu _{\Delta {\rm f,B0g}}} = - 7.7$ kHz, ${\sigma _{\Delta {\rm f,B0g}}} = 16.8$ kHz. Post-flight measurements followed this trend with ${\mu _{\Delta {\rm f,Apf}}} = 11.7$ kHz, ${\sigma _{\Delta {\rm f,Apf}}} = 10.1$ kHz, and ${\mu _{\Delta {\rm f,Bpf}}} = 11.3$ kHz, ${\sigma _{\Delta {\rm f,Bpf}}} = 8.1$ kHz.
FOKUS’ accuracy and stability is inherently limited by the RF reference used, which represents the main contribution for the mean offset and standard deviation observed here. This can also be seen from the ADEV of the LnCSAC compared to flight and ground stability of the iodine beat measured with comb A.
The LnCSAC’s frequency accuracy is given with a maximum offset ${\pm}5 \times {10^{- 11}}$ (14 kHz). Aging of the device is specified to $\le 10 \times {10^{- 8}}/{\rm month}$. Additionally, a magnetic sensitivity of $\le 9 \times {10^{- 11}}/{\rm G}$ and a temperature sensitivity of ${\pm}5 \times {10^{- 10}}$ over the operating temperature are specified, resulting in a possible uncertainty of $\le 15\;{\rm kHz}$ at 0.6 G and 31 kHz for a 9 K temperature change during the mission, which is well within the observation of the experiment visible in the linear drift in the bottom plot of Fig. 8. Nevertheless, comb B shows a nonstatistical deviation from the mean value during flight between 250 s and 400 s. This is due to less efficient nonlinear broadening of this comb leading to weaker beats from the f-2f interferometer and the iodine BDU. Therefore, the measurement of comb B was slightly less accurate compared to A. It is, however, precise enough to derive the mode numbers for both combs during this mission.
5. CONCLUSION AND OUTLOOK
To enable future space optical clocks and raise the technology readiness level of the frequency comb technology, we have developed a compact and lightweight dual comb system qualified for in-vacuum operation and demonstrated its high robustness on ground and on a sounding rocket flight. The two combs are able to operate autonomously at different repetition rates, allowing mode number determination without the necessity for any additional hardware or user interaction. While the power, mass, and volume budget already have been substantially improved compared to its predecessor, some potential remains for advancements, particularly regarding the central electronics stack. For the system design, we chose a highly modular and flexible setup. Considering higher integration density of the control electronics would allow a much more power-efficient design and further reduce the requirement for the total volume. We currently estimate that for a space-qualified single comb unit based on a compact optics module and five $(140 \times 160)\;{{\rm mm}^2}$ PCB, the overall size, weight, and power (SWAP) can be reduced to a box-sized module with about 6 L volume, 7 kg weight, and 30 W power consumption. Such a system could presumably be placed on a compact technology test satellite or mounted on an external experimental platform on the International Space Station. A demonstrated long-term remote operation in space would not only prove the feasibility and reliability of the system, but could, in connection with a suitable optical reference based on, for example, iodine or rubidium and a laser terminal for time transfer, also be a precursor system for next-generation satellite clocks and intercontinental time transfer techniques. At the current state of technology, we are convinced that long-term operation of fiber-based combs even in demanding space environments can be achieved and have the potential to enable some of the earlier-mentioned breakthrough observations relying on precision clocks and timing in space.
Funding
Deutsches Zentrum für Luft- und Raumfahrt (50WM1343, 50WM1850).
Acknowledgment
The authors thank Alexandra Bleicher for her work on suitable solder alloys, and the team at Menlo Systems for substantial engineering support of the system. The successful TEXUS 54 payload integration and launch involved a concentrated effort of various scientific and technical teams, in particular from DLR, SSC, Airbus, Moraba, and OHB. The measurements with the iodine reference would not have been possible without the cooperation with the JOKARUS team of Markus Krutzig at the Humbold University Berlin. We are also indebted to Thomas Driebe and his colleagues at the DLR RFM for their continuous support over many years regarding the development of space combs.
Disclosures
The authors declare no conflicts of interest.
REFERENCES
1. R. Holzwarth, T. Udem, T. W. Hänsch, J. C. Knight, W. J. Wadsworth, and P. St.J. Russell, “Optical frequency synthesizer for precision spectroscopy,” Phys. Rev. Lett. 85, 2264–2267 (2000). [CrossRef]
2. N. R. Newbury, “Searching for applications with a fine-tooth comb,” Nat. Photonics 5, 186–188 (2011). [CrossRef]
3. J. Ye and S. T. Cundiff, eds., Femtosecond Optical Frequency Comb: Principle, Operation and Applications (Springer, 2005).
4. T. Fortier and E. Baumann, “20 years of developments in optical frequency comb technology and applications,” Commun. Phys. 2, 153 (2019). [CrossRef]
5. N. Picqué and T. W. Hänsch, “Frequency comb spectroscopy,” Nat. Photonics 13, 146–157 (2019). [CrossRef]
6. B. R. Washburn, R. W. Fox, N. R. Newbury, J. W. Nicholson, K. Feder, P. S. Westbrook, and C. G. Jørgensen, “Fiber-laser-based frequency comb with a tunable repetition rate,” Opt. Express 12, 4999–5004 (2004). [CrossRef]
7. M. H. P. Pfeiffer, C. Herkommer, J. Liu, H. Guo, M. Karpov, E. Lucas, M. Zervas, and T. J. Kippenberg, “Octave-spanning dissipative Kerr soliton frequency combs in Si3N4 microresonators,” Optica 4, 684–691 (2017). [CrossRef]
8. T. Herr, V. Brasch, J. D. Jost, C. Y. Wang, N. M. Kondratiev, M. L. Gorodetsky, and T. J. Kippenberg, “Temporal solitons in optical microresonators,” Nat. Photonics 8, 145–152 (2014). [CrossRef]
9. K. Beha, D. C. Cole, P. Del’Haye, A. Coillet, S. A. Diddams, and S. B. Papp, “Electronic synthesis of light,” Optica 4, 406–411 (2017). [CrossRef]
10. W. Hänsel, H. Hoogland, M. Giunta, S. Schmid, T. Steinmetz, R. Doubek, P. Mayer, S. Dobner, C. Cleff, M. Fischer, and R. Holzwarth, “All polarization-maintaining fiber laser architecture for robust femtosecond pulse generation,” in Exploring the World with the Laser (Springer, 2018), pp. 331–340.
11. B. J. Bloom, T. L. Nicholson, J. R. Williams, S. L. Campbell, M. Bishof, X. Zhang, W. Zhang, S. L. Bromley, and J. Ye, “An optical lattice clock with accuracy and stability at the 10−18 level,” Nature 506, 71–75 (2014). [CrossRef]
12. S. M. Brewer, J.-S. Chen, A. M. Hankin, E. R. Clements, C. W. Chou, D. J. Wineland, D. B. Hume, and D. R. Leibrandt, “27Al+ quantum-logic clock with a systematic uncertainty below 10−18,” Phys. Rev. Lett. 123, 033201 (2019). [CrossRef]
13. A. Schliesser, M. Brehm, F. Keilmann, and D. W. van der Weide, “Frequency-comb infrared spectrometer for rapid, remote chemical sensing,” Opt. Express 13, 9029–9038 (2005). [CrossRef]
14. W. C. Swann and N. R. Newbury, “Frequency-resolved coherent lidar using a femtosecond fiber laser,” Opt. Lett. 31, 826–828 (2006). [CrossRef]
15. U. Hugentobler, D. Voithenleitner, M. Heinze, V. Klein, and M. Plattner, “OCTAGON: optical clocks in satellite navigation (German),” technical report (Technische Universität München, 2011).
16. I. Coddington, W. C. Swann, L. Nenadovic, and N. R. Newbury, “Rapid and precise absolute distance measurements at long range,” Nat. Photonics 3, 351–356 (2009). [CrossRef]
17. X. Wang, S. Takahashi, K. Takamasu, and H. Matsumoto, “Space position measurement using long-path heterodyne interferometer with optical frequency comb,” Opt. Express 20, 2725–2732 (2012). [CrossRef]
18. R. Pierce, J. Leitch, M. Stephens, P. Bender, and R. Nerem, “Intersatellite range monitoring using optical interferometry,” Appl. Opt. 47, 5007–5019 (2008). [CrossRef]
19. J. Lee, Y.-J. Kim, K. Lee, S. Lee, and S.-W. Kim, “Time-of-flight measurement with femtosecond light pulses,” Nat. Photonics 4, 716–720 (2010). [CrossRef]
20. H. Linz, D. Bhatia, L. Buinhas, M. Lezius, E. Ferrer, R. Förstner, K. Frankl, M. Philips-Blum, M. Steen, U. Bestmann, W. Hänsel, R. Holzwarth, O. Krause, and T. Pany, “Infrared astronomy satellite swarm interferometry (IRASSI): overview and study results,” Adv. Space Res. 65, 831–849 (2020). [CrossRef]
21. S. Schiller, G. M. Tino, P. Gill, C. Salomon, U. Sterr, E. Peik, A. Nevsky, A. Görlitz, D. Svehla, G. Ferrari, N. Poli, L. Lusanna, H. Klein, H. Margolis, P. Lemonde, P. Laurent, G. Santarelli, A. Clairon, W. Ertmer, E. Rasel, J. Müller, L. Iorio, C. Lämmerzahl, H. Dittus, E. Gill, M. Rothacher, F. Flechner, U. Schreiber, V. Flambaum, W.-T. Ni, L. Liu, X. Chen, J. Chen, K. Gao, L. Cacciapuoti, R. Holzwarth, M. P. Heß, and W. Schäfer, “Einstein gravity explorer – a medium-class fundamental physics mission,” Exp. Astron. 23, 573–610 (2008). [CrossRef]
22. H. Selig, C. Laemmerzahl, and W.-T. Ni, “Astrodynamical space test of relativity using optical devices I (ASTROD I)—mission overview,” Int. J. Mod. Phys. D 22, 1341003 (2013). [CrossRef]
23. K. Bongs, Y. Singh, L. Smith, W. He, O. Kock, D. Świerad, J. Hughes, S. Schiller, S. Alighanbari, S. Origlia, S. Vogt, U. Sterr, C. Lisdat, R. Le Targat, J. Lodewyck, D. Holleville, B. Venon, S. Bize, G. P. Barwood, P. Gill, I. R. Hill, Y. B. Ovchinnikov, N. Poli, G. M. Tino, J. Stuhler, and W. Kaenders, “Development of a strontium optical lattice clock for the SOC mission on the ISS,” C.R. Phys. 16, 553–564 (2015). [CrossRef]
24. M. Lezius, T. Wilken, C. Deutsch, M. Giunta, O. Mandel, A. Thaller, V. Schkolnik, M. Schiemangk, A. Dinkelaker, A. Kohfeldt, A. Wicht, M. Krutzik, A. Peters, O. Hellmig, H. Duncker, K. Sengstock, P. Windpassinger, K. Lampmann, T. Hülsing, T. W. Hänsch, and R. Holzwarth, “Space-borne frequency comb metrology,” Optica 3, 1381–1387 (2016). [CrossRef]
25. V. Schkolnik, K. Döringshoff, F. B. Gutsch, M. Oswald, T. Schuldt, C. Braxmaier, M. Lezius, R. Holzwarth, C. Kürbis, A. Bawamia, M. Krutzik, and A. Peters, “JOKARUS—design of a compact optical iodine frequency reference for a sounding rocket mission,” EPJ Quantum Technol. 4, 9 (2017). [CrossRef]
26. W. Hänsel, R. Holzwarth, R. Doubek, and M. Mei, “Laser with non-linear optical loop mirror,” Patent EP2637265B1 (September 11, 2013).
27. “Standard test method for total mass loss and collected volatile condensable materials from outgassing in a vacuum environment,” ASTM E595-15 (ASTM International, 2015).
28. K. Döringshoff, F. B. Gutsch, V. Schkolnik, C. Kürbis, M. Oswald, B. Pröbster, E. V. Kovalchuk, A. Bawamia, R. Smol, T. Schuldt, M. Lezius, R. Holzwarth, A. Wicht, C. Braxmaier, M. Krutzik, and A. Peters, “Iodine frequency reference on a sounding rocket,” Phys. Rev. Appl. 11, 054068 (2019). [CrossRef]
29. K. Döringshoff, “Optical frequency references based on hyperfine transitions in molecular iodine,” Ph.D. thesis (Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2018).
30. Bureau International des Poids et Mesures, “Recommended values of standard frequencies for applications including the practical realization of the metre and secondary secondary representations of the definition of the second: iodine (532 nm),” last updated August 2007, https://www.bipm.org/utils/common/pdf/mep/M-e-P_I2_532.pdf.
