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Abstract
We present a hybrid fiber/waveguide design for a 100-MHz frequency comb that is fully self-referenced and temperature controlled with less than 5 W of electrical power. Self-referencing is achieved by supercontinuum generation in a silicon nitride waveguide, which requires much lower pulse energies (~200 pJ) than with highly nonlinear fiber. These low-energy pulses are achieved with an erbium fiber oscillator/amplifier pumped by two 250-mW passively-cooled pump diodes that consume less than 5 W of electrical power. The temperature tuning of the oscillator, necessary to stabilize the repetition rate in the presence of environmental temperature changes, is achieved by resistive heating of a section of gold-palladium-coated fiber within the laser cavity. By heating only the small thermal mass of the fiber, the repetition rate is tuned over 4.2 kHz (corresponding to an effective temperature change of 4.2 °C) with a fast time constant of 0.5 s, at a low power consumption of 0.077 W/°C, compared to 2.5 W/°C in the conventional 200-MHz comb design.
1. Introduction
There exists an increasing need to operate femtosecond frequency combs outside the laboratory [1,2]. Applications such as comb spectroscopy [3,4], femtosecond timing dissemination [5], low-noise microwave generation [6,7], and portable optical clocks [8,9] all require practical, fieldable frequency combs. While chip-scale combs may one day be a practical alternative [10], the current solution for fieldable combs remains fiber frequency combs with polarization-maintaining (PM) erbium (Er) fiber [11–14]. Self-referenced fiber frequency combs have already been demonstrated in moving vehicles [15], sounding rockets [16], industrial environments [17] and remote field sites [18]. As these combs become more robust and immune to environment perturbations, the principle challenge of fielding a comb shifts to the total electrical power consumption and the heat dissipation associated with their operation. Such issues might go unnoticed in the laboratory, but they constitute serious limits in the field. A typical fully self-referenced frequency comb will require two to four high-powered pump diodes for the erbium-doped fiber amplifier (EDFA) to produce the high-energy pulses traditionally needed for self-broadening. Additionally, careful temperature control of the oscillator is required to stabilize the comb repetition rate (frep) otherwise the thermal refractive coefficient of fiber (~10−5/°C) would overrun all other cavity length modulations. Overall, it is not uncommon for a typical Er fiber comb to draw > 40 W of electrical power. This power draw is particularly cumbersome in space, where electrical power is both expensive and limited [16,19]. As a point of reference, a CubeSat may have only 30 W of electrical power to distribute between all systems.
For the comb design demonstrated in Ref. [15], which is typical of many fiber systems, the power draw is equally divided between temperature control of frep and generation of the carrier-envelope offset (fCEO) signal for self-referencing. For fCEO generation, the aforementioned 200-MHz comb requires almost 2 W of pump power corresponding to almost 20 W of conditioned, DC electrical power. Temperature control is often applied to a large enclosure when only the oscillator fiber at the heart of the comb needs to be controlled. Here, we demonstrate two solutions to reduce the power consumption of a frequency comb by almost a factor of 10. First, by performing spectral broadening in silicon nitride (Si3N4, henceforth SiN) waveguides [20–22], we can greatly reduce the optical power required for self-broadening by a factor of 5. At the same time, we have redesigned the waveguide to allow efficient spectral broadening with 200-fs pulses as opposed to <80-fs pulses used in Ref. [22]. There are two important consequences of this redesign. First, <80-fs pulses from a fiber system are typically generated via nonlinear amplification in a high-gain, normal-dispersion fiber amplifier [23]. (For instance, in Ref. [22], pulses are amplified to >2 nJ before being attenuated to ~100 pJ). Broadening with 200-fs pulses greatly increases the design space available for the combs, allowing for low power linear amplifiers and truly low power operation. Secondly, the use of low gain fiber amplifiers means one can practically use low power, passively-cooled pump diodes for these amplifiers. Since active cooling can consume up to two-thirds of power draw of a pump diode, it further magnifies the energy savings from the SiN approach. As an additional point of interest, low power fiber amplifiers can be constructed from lightly doped erbium fiber, which can be made more compatible with a radiation environment in space.
The second principle source of energy consumption in a frequency comb is temperature control of the oscillator, which is necessary for any frequency stabilized comb. This temperature control is typically achieved by controlling the oscillator enclosure with heaters or thermoelectric coolers, often consuming several Watts of power. Typically, one will limit this power draw by compressing the size of the oscillator package to reduce the thermal mass to be controlled. Here, we take this idea to the logical extreme and use metalized fiber — as first employed in the telecom industry [24–26] — to resistively heat and control only the very small thermal mass of the optical fiber in the oscillator. By direct resistive heating of intra-cavity gold-palladium-coated fiber, which we refer to as a “fiber resistive modulator,” we demonstrate a simple and low-power temperature tuning solution consuming ~77 mW/°C.
Together these two techniques allow for a 5–10× reduction in power consumption and bring the total consumption of the comb system below 5 W. At this power, a host of new applications should be possible including battery powered operation.
2. f-to-2f self-referencing at low power
The low power comb design developed here is shown in Fig. 1(a)
Fig. 1 (a) Layout of the low power comb. EDF: erbium-doped fiber, OC: output coupler, WDM: wavelength division multiplexer, ISO: isolator, PPKTP: periodically-poled potassium titanyl phosphate, SiN: silicon nitride, NC: not connected. Note that this oscillator is pumped through the end mirror (which is transparent at 980 nm) using the passively-cooled laser diode at 980 nm. The intracavity WDM only serves to dump extra pump light, protecting the SESAM and limiting back reflection into the pump. After the amplifier, a coupler directs 90% of the output to the SiN waveguide for broadening and 10% to a PPKTP crystal for doubling. (b) 30-dB fCEO signal recorded with 22 mW of light incident on the SiN waveguide (300-kHz resolution bandwidth). (c) Spectra of the comb at the output of the oscillator (red), at the output of the amplifier (black). (d) Spectra at the output of the SiN waveguide to beat against the doubled light from PPKTP.
Spectral broadening is achieved via supercontinuum generation in an SiN waveguide fabricated by Ligentec [27] using the “Photonic Damascene” process [28,29]. The waveguide thickness (height) is 750 nm, the width is 2100 nm, and the total length is 21.05 mm. The waveguide is fully clad with silicon dioxide (SiO2). Inverse tapers on the input and output facets enlarge the mode and provide better coupling, which is estimated at −2 dB per facet. The waveguide provides anomalous dispersion at the 1560-nm oscillator wavelength, which leads to supercontinuum generation via the soliton fission process [30]. Such waveguides have been shown to provide octave-spanning spectra with as little as 100-pJ pulses incident on the waveguide facet [20–22]. Here, we require ~200-pJ pulses due to the somewhat longer pulse duration. However, it is likely that a longer and dispersion optimized waveguide would allow for broadening with as little as 100 pJ given our ~200-fs pulse width.
With 23 mW of comb light incident on the waveguide, we output roughly 156 μW of power within the “dispersive wave” centered at ~760 nm (>1% of the total output) while depositing relatively little optical power in other, unused, spectral bands, as seen in Fig. 1(d). The dispersive wave is stable for a large range of input powers and its center wavelength can be tuned by optimizing physical parameters of the SiN waveguide. The f-to-2f signal is generated by doubling 1572-nm light from the amplifier in a periodically-poled potassium titanyl phosphate (PPKTP) waveguide, combining it with the supercontinuum light in a single mode fiber and detecting on a silicon avalanche photodiode. When pumped with 2.5-mW pulses, centered at 1560 nm, the PPKTP generates 12 μW of light in a 6-nm band which, when mixed with ≈2.7 μW of supercontinuum light in the same band yields an fCEO signal with a 30-dB signal-to-noise ratio in a 300-kHz bandwidth [Fig. 1(b)]. It is worth noting that while the powers used here are low, there is still further room for optimization. As can be seen in Fig. 2(c)
Fig. 2 (a) SiN chip used in this experiment. (b) Calculated dispersion profile for the 2100-nm waveguide. (c) recorded spectrum vs incident power. The dotted line denotes 786 nm.
In our current configuration, the entire power draw of the pump diodes—including diode drivers — is 4.6 W, well below the power draw of common battery-powered devices (an operating laptop will consume between 20 and 100 W). Indeed, we successfully powered the comb off a handheld USB charger for more than an hour while fully self-referenced. Figure 3
Fig. 3 Counted frep, and fCEO while the frequency comb was locked and powered by a USB charger. Counter gate time was 100 ms. The standard deviations calculated for the first 1000 points are 12.7 mHz for frep and 4.7 Hz for fCEO.
Space applications have an obvious need for low power combs. Our design provides that low-power alternative and presents some additional advantages from a radiation hardness perspective. While standard PM fibers are available in a radiation-hardened variety, radiation-hardened highly-doped Er PM fibers are more difficult to obtain. Here, the lower peak-power required for self-broadening relaxes the gain and dispersion requirements for the Er-doped fiber and makes this design more compatible with the available radiation-hardened fibers. Secondly, replacing traditional highly-nonlinear fiber with the SiN waveguide is attractive since the properties of nonlinear fiber when subjected to radiation are unknown whereas SiN has been exploited as a radiation hardened material for decades in electronics [31,32] and recent work has shown that this robustness extends to its optical properties as well [33].
3. Fiber resistive modulator
To demonstrate low-power temperature stabilization and tuning of the oscillator, we incorporated a 13.9-cm-long fiber coated with an alloy of Gold-Palladium (AuPd, 47% Au and 53% Pd) into the cavity. Electrical current traveling through this coating modulated the cavity length (hence frep) by direct resistive heating of the fiber. To apply the AuPd coating, the buffer coating on the fiber was stripped off so that the metal layer adhered directly to the 125-μm diameter fiber cladding. Here, we applied the coating to the Er-doped fiber portion of the cavity but the AuPd coating could equally well be applied to the intra-cavity PM1550 fiber of any length. For coating, the fiber was wound in a spiral pattern and mounted on the surface of a silicon wafer, such that the fiber did not cross over itself at any point. After mounting, the fiber was cleaned with oxygen plasma ashing to remove any organic residues on the surface. We then utilized electron-beam evaporation to deposit 10 nm of titanium (Ti) followed by 100 nm of the AuPd alloy on the exposed surface of the fiber, effectively coating roughly half of its surface area. The initial Ti layer improved the adhesion of the AuPd layer. We selected AuPd instead of Au because of the former’s higher resistivity, which ensures heating of the fiber with less ohmic loss in the connecting leads. The total resistance of the 13.9-cm coated fiber was 2.9 kΩ.
To demonstrate the fiber resistive modulation of frep, we made an electrical connection to each end of the AuPd coating with an electrically-conducting epoxy. A voltage of 1 V was then applied across the connections and increased in steps of 1 V while monitoring the current draw and frep. Figure 4
Fig. 4 Fiber resistive modulator design and results. (a) Oscillator design: in this case, only 13.9 cm of the ≈1 m oscillator is coated. (b) Repetition rate (left axis) and cavity-averaged temperature (right axis) versus input electrical power. Blue line is a linear fit giving an frep tuning constant of 77 mW/kHz. EDF: Erbium-doped fiber, frep: repetition rate, fCEO: carrier envelope offset frequency.
To measure the response time constants (τ) of the fiber resistive modulator, we applied an electrical square-wave modulation at 6 V while monitoring the repetition rate. A fit to the rising and falling edges of the response curves with exponentials yields time-constants τrise and τfall of 0.57 s and 0.47 s, respectively, as shown in Fig. 5(a)
Fig. 5 Response time constants and Bode plots. (a) The response of frep (green) and fCEO (shown in the inset in purple) to a square wave voltage modulation. The exponential fits (transparent red and violet lines) extracted the time constants (τ) of the frep response. (b) Transfer function (Bode plots) for rising (red) and falling (blue) edges.
We can also calculate the fixed point for the fiber resistive modulator based on the method described in Ref. [34]. using the data in Fig. 5(a) and inset. The fixed point, ffix, is calculated as
which yields 4.5 THz, similar to most fiber stretchers as would be expected.The principle advantage of the fiber resistive modulator approach is that it allows for compensation of ambient temperature swings with modest power consumption, but there are also comb design advantages provided by the speed of this approach. Typically, cavity length modulation in a fiber oscillator is delegated to several modulators. Fast modulation, >1 Hz, is often handled by a fast small throw PZT or EOM [35,36]. Slow modulation to counteract temperature drift is handled by temperature feedback or translating optics typically having several seconds to minute response times. Often an additional, large dynamic range PZT modulator, is required to bridge the gap in timescales between fast and slow modulators. Direct resistive heating of the fiber offers sufficient speed and dynamic range to potentially replace both slow and intermediate range modulators while requiring no free space alignment or bulky associated optics, thus reducing system complexity, size and weight.
4. Discussion
Here, we demonstrate a novel low-power frequency comb design that greatly reduces the power requirements of a fully self-referenced comb. Table 1
Table 1. Comparison of electrical power consumption between a conventional 200-MHz comb design (left column), and two optimized 100-MHz designs, first only with fiber resistive modulator (middle) and second fully optimized using passively-cooled pumps (right). It should be noted that the fully optimized design assumes the use of a fiber resistive modulator for temperature tuning. All the values are based on actual measurements. The temperature tuning values are based on a 3 °C offset from ambient temperature. Measured power draw for the pump diodes includes the diode controllers (Wavelength Electronics LDTC1040 and LDTC2E for the oscillator and amplifier pump respectively and two LDTC1024’s for the passively-cooled pumps [27]). The power consumption of the locking electronics including the PZTs has not been included here, although for the PZT the consumption is negligibly low (< 5 mW)
Though not chip scale, fully self-referenced hybrid fiber/waveguide frequency combs can be made compact and low power. While direct electrically-pumped combs could one day improve on these power consumption levels, the size and power draw shown here is already quite low. At these levels, the size and power consumption of any larger system employing the comb will likely be limited by the surrounding electronics and not the comb itself.
Acknowledgments
We thank Ivan Ryger and John H. Lehman for discussions about deposition of nichrome and gold on fiber optics, and Isaac Khader for assistance in the measurements. Work of U.S. Government, not subject to copyright.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. S. A. Diddams, “The evolving optical frequency comb [Invited],” J. Opt. Soc. Am. B 27(11), B51–B62 (2010). [CrossRef]
2. N. R. Newbury, “Searching for applications with a fine-tooth comb,” Nat. Photonics 5(4), 186–188 (2011). [CrossRef]
3. I. Coddington, N. Newbury, and W. Swann, “Dual-comb spectroscopy,” Optica 3(4), 414–426 (2016). [CrossRef]
4. K. C. Cossel, E. M. Waxman, I. A. Finneran, G. A. Blake, J. Ye, and N. R. Newbury, “Gas-phase broadband spectroscopy using active sources: progress, status, and applications,” J. Opt. Soc. Am. B 34(1), 104–129 (2017). [CrossRef] [PubMed]
5. H. Bergeron, L. C. Sinclair, W. C. Swann, C. W. Nelson, J.-D. Deschênes, E. Baumann, F. R. Giorgetta, I. Coddington, and N. R. Newbury, “Tight real-time synchronization of a microwave clock to an optical clock across a turbulent air path,” Optica 3(4), 441–447 (2016). [CrossRef] [PubMed]
6. J. Davila-Rodriguez, F. N. Baynes, A. D. Ludlow, T. M. Fortier, H. Leopardi, S. A. Diddams, and F. Quinlan, “Compact, thermal-noise-limited reference cavity for ultra-low-noise microwave generation,” Opt. Lett. 42(7), 1277–1280 (2017). [CrossRef] [PubMed]
7. J. Millo, M. Abgrall, M. Lours, E. M. L. English, H. Jiang, J. Guéna, A. Clairon, M. E. Tobar, S. Bize, Y. Le Coq, and G. Santarelli, “Ultralow noise microwave generation with fiber-based optical frequency comb and application to atomic fountain clock,” Appl. Phys. Lett. 94(14), 141105 (2009). [CrossRef]
8. J. H. Burke, N. D. Lemke, G. R. Phelps, and K. W. Martin, “A compact, high-performance all optical atomic clock based on telecom lasers,” Proc. SPIE 9763, 976304 (2016). [CrossRef]
9. T. Schuldt, K. Döringshoff, E. V. Kovalchuk, A. Keetman, J. Pahl, A. Peters, and C. Braxmaier, “Development of a compact optical absolute frequency reference for space with 10−15 instability,” Appl. Opt. 56(4), 1101–1106 (2017). [CrossRef] [PubMed]
10. T. J. Kippenberg, R. Holzwarth, and S. A. Diddams, “Microresonator-based optical frequency combs,” Science 332(6029), 555–559 (2011). [CrossRef] [PubMed]
11. L. C. Sinclair, J.-D. Deschênes, L. Sonderhouse, W. C. Swann, I. H. Khader, E. Baumann, N. R. Newbury, and I. Coddington, “Invited article: a compact optically coherent fiber frequency comb,” Rev. Sci. Instrum. 86(8), 081301 (2015). [CrossRef] [PubMed]
12. N. Kuse, C.-C. Lee, J. Jiang, C. Mohr, T. R. Schibli, and M. E. Fermann, “Ultra-low noise all polarization-maintaining Er fiber-based optical frequency combs facilitated with a graphene modulator,” Opt. Express 23(19), 24342–24350 (2015). [CrossRef] [PubMed]
13. 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,” Appl. Phys. B 123(1), 41 (2017). [CrossRef]
14. S. Droste, G. Ycas, B. R. Washburn, I. Coddington, and N. R. Newbury, “Optical frequency comb generation based on erbium fiber lasers,” Nanophotonics 5(2), 196–213 (2016). [CrossRef]
15. L. C. Sinclair, I. Coddington, W. C. Swann, G. B. Rieker, A. Hati, K. Iwakuni, and N. R. Newbury, “Operation of an optically coherent frequency comb outside the metrology lab,” Opt. Express 22(6), 6996–7006 (2014). [CrossRef] [PubMed]
16. 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(12), 1381–1387 (2016). [CrossRef]
17. P. J. Schroeder, R. J. Wright, S. Coburn, B. Sodergren, K. C. Cossel, S. Droste, G. W. Truong, E. Baumann, F. R. Giorgetta, I. Coddington, N. R. Newbury, and G. B. Rieker, “Dual frequency comb laser absorption spectroscopy in a 16 MW gas turbine exhaust,” Proc. Combust. Inst. 36(3), 4565–4573 (2017). [CrossRef]
18. S. Coburn, G. Rieker, K. Prasad, S. Ghosh, C. Alden, N. Newbury, I. Coddington, E. Baumann, G.-W. Truong, K. Cossel, and R. Wright, “Methane detection for oil and gas production sites using portable dual-comb spectrometry,” 71st International Symposium on Molecular Spectroscopy (2016). [CrossRef]
19. J. Lee, K. Lee, Y.-S. Jang, H. Jang, S. Han, S.-H. Lee, K.-I. Kang, C.-W. Lim, Y.-J. Kim, and S.-W. Kim, “Testing of a femtosecond pulse laser in outer space,” Sci. Rep. 4(1), 5134 (2015). [CrossRef] [PubMed]
20. A. S. Mayer, A. Klenner, A. R. Johnson, K. Luke, M. R. E. Lamont, Y. Okawachi, M. Lipson, A. L. Gaeta, and U. Keller, “Frequency comb offset detection using supercontinuum generation in silicon nitride waveguides,” Opt. Express 23(12), 15440–15451 (2015). [CrossRef] [PubMed]
21. A. Klenner, A. S. Mayer, A. R. Johnson, K. Luke, M. R. E. Lamont, Y. Okawachi, M. Lipson, A. L. Gaeta, and U. Keller, “Gigahertz frequency comb offset stabilization based on supercontinuum generation in silicon nitride waveguides,” Opt. Express 24(10), 11043–11053 (2016). [CrossRef] [PubMed]
22. D. R. Carlson, D. D. Hickstein, A. Lind, S. Droste, D. Westly, N. Nader, I. Coddington, N. R. Newbury, K. Srinivasan, S. A. Diddams, and S. B. Papp, “Self-referenced frequency combs using high-efficiency silicon-nitride waveguides,” Opt. Lett. 42(12), 2314–2317 (2017). [CrossRef] [PubMed]
23. J. M. Dudley and J. R. Taylor, Supercontinuum Generation in Optical Fibers (Cambridge University Press, 2010).
24. B. White, J. Davis, L. Bobb, H. Krumboltz, and D. Larson, “Optical-fiber thermal modulator,” J. Lightwave Technol. 5(9), 1169–1175 (1987). [CrossRef]
25. H. G. Limberger, N. H. Ky, D. M. Costantini, R. P. Salathe, C. A. P. Muller, and G. R. Fox, “Efficient miniature fiber-optic tunable filter based on intracore Bragg grating and electrically resistive coating,” IEEE Photon. Technol. Lett. 10(3), 361–363 (1998). [CrossRef]
26. A. A. Abramov, B. J. Eggleton, J. A. Rogers, R. P. Espindola, A. Hale, R. S. Windeler, and T. A. Strasser, “Electrically tunable efficient broad-band fiber filter,” IEEE Photon. Technol. Lett. 11(4), 445–447 (1999). [CrossRef]
27. Disclaimer, “The use of tradenames in this manuscript is necessary to specify experimental results and does not imply endorsement by the National Institute of Standards and Technology.”
28. R. Sun, J. Cheng, J. Michel, and L. Kimerling, “Transparent amorphous silicon channel waveguides and high-Q resonators using a damascene process,” Opt. Lett. 34(15), 2378–2380 (2009). [CrossRef] [PubMed]
29. M. H. P. Pfeiffer, A. Kordts, V. Brasch, M. Zervas, M. Geiselmann, J. D. Jost, and T. J. Kippenberg, “Photonic Damascene process for integrated high-Q microresonator based nonlinear photonics,” Optica 3(1), 20 (2016). [CrossRef]
30. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78(4), 1135–1184 (2006). [CrossRef]
31. D. Neamen, W. Shedd, and B. Buchanan, “Thin Film Silicon on Silicon Nitride for Radiation Hardened Dielectrically Isolated Misfet’s,” IEEE Trans. Nucl. Sci. 22(6), 2203–2207 (1975). [CrossRef]
32. G. Q. Lo, A. B. Joshi, and D. L. Kwong, “Radiation hardness of MOSFETs with N2O-nitrided gate oxides,” IEEE Trans. Electron Dev. 40(8), 1565–1567 (1993). [CrossRef]
33. V. Brasch, Q.-F. Chen, S. Schiller, and T. J. Kippenberg, “Radiation hardness of high-Q silicon nitride microresonators for space compatible integrated optics,” Opt. Express 22(25), 30786–30794 (2014). [CrossRef] [PubMed]
34. B. R. Washburn, W. C. Swann, and N. R. Newbury, “Response dynamics of the frequency comb output from a femtosecond fiber laser,” Opt. Express 13(26), 10622–10633 (2005). [CrossRef] [PubMed]
35. E. Baumann, F. R. Giorgetta, J. W. Nicholson, W. C. Swann, I. Coddington, and N. R. Newbury, “High-performance, vibration-immune, fiber-laser frequency comb,” Opt. Lett. 34(5), 638–640 (2009). [CrossRef] [PubMed]
36. W. C. Swann, E. Baumann, F. R. Giorgetta, and N. R. Newbury, “Microwave generation with low residual phase noise from a femtosecond fiber laser with an intracavity electro-optic modulator,” Opt. Express 19(24), 24387–24395 (2011). [CrossRef] [PubMed]
