We demonstrate a temperature stabilized CMOS-compatible frequency comb based on an integrated optical micro-ring resonator. The instrument operates in the wavelength interval 1520-1600 nm with a wide free spectral range (FSR) of 200 GHz. By embedding a highly sensitive “resistive thermal device” (RTD) on the surface of the chip to provide temperature feedback to the thermal electric cooler, the bench top unit achieves wavelength stability of ~1 pm over a 24 hour period with good power stability. The new frequency comb is designed to be robust, compact and portable. Our approach reduces the cost and complexity of existing high precision frequency combs currently used in the fields of metrology, remote sensing and stellar spectroscopy where high stability is required for prolonged periods of time.
©2012 Optical Society of America
With the emergence of frequency combs used for precision metrology over the past decade, there has been a pressing need to improve on the already present techniques that are available from optical clocks to high precision spectroscopy . Of particular interest are the measurement of radial velocities that would be indicative of stellar phenomena, and the existence of exoplanets . However, the time and precision required in taking these measurements are both high, which is one of the limiting obstacles in observational astrophysics . These measurements would require a comb that has a free spectral range (FSR) that is large enough to be able to be resolved accurately and calculated for easily, but would also need a long-term stability in order to eliminate drift and to be able to allow for precise measurements over an extended period of time [2–4]. Presently, optical frequency combs are the solutions to these problems and have been able to exhibit high precision as well as stable running times in collecting measurements from astronomical sources [1–4]. However, the use of solid state high precision devices such as a Ti:Sapphire laser, which has a good precision measurement and can easily span an octave , to collect this data becomes more difficult as the size and power demands for these devices are low. These issues have led to more difficulty in maintaining a consistent thermal stability . For this reason, fiber based optical combs have been developed to overcome these issues. Thus, as the need for increased measurements within these fields rise, the drive to enhance FSR, create more compact and thermally efficient devices has arisen, leading to a more turnkey drive to optical frequency metrology .
Furthermore, through the use of micro-ring resonators, frequency sorting can be accomplished, yielding frequency combs that have a FSR that can be determined by the cavity length in the oscillator . Micro-ring resonators offer a more compact and manageable frequency comb device, as they are smaller, have less separate components and are subject to parametric non-linear effects that can allow for comb broadening . Recently we have demonstrated the generation of frequency combs from a CMOS-compatible integrated optical micro-ring resonator hyper-parameter oscillator , in this paper we will investigate the compact micro-ring resonator as turnkey frequency comb source that is capable of generating a large FSR while maintaining a thermal stability for an extended period of time. Our results show that through the use of a thermoelectric cooling (TEC) device mounted in series with a resistive thermal device (RTD) we have been able to not only maintain thermal stability, but also to rectify thermal imbalances, when they exist.
2. Device concept and fabrication
Figure 1A shows the schematic of the new device where the generation of the optical frequency comb signal arises from a combination of a wide band amplified spontaneous source (ASE) covering both C and L band between 1520 nm to 1620 nm and a micro-ring resonator. Since the ASE output is circularly polarized, a polarized beam splitter is used to linearize the output before it is passed to a polarizer which is placed at the input of the micro-ring resonator allowing for polarization selection into the resonator . The polarization can be tuned and the comb peak wavelengths can be moved between the TE and TM peaks for more versatility in measurement .
The integrated resonator device is mounted on the TEC and is controlled and relayed with the RTD. The light source used in this device could easily be replaced with a fiber laser or filtered high power source, which would allow for a broader comb bandwidth, and could allow the comb spectrum produced to span an octave [8,11]. In this work, an ASE source was used to investigate the feasibility of this approach. The ASE source has fewer components  and exhibits broad wavelength coverage (~100 nm). The temperature dependence of the RTD-TEC combination can be directly measured without knowledge of the stability of the source. This ensured that the thermal effects from the turnkey system were as a result of the thermal dependency on the TEC and not with the orientation of the modes of the laser. This also allowed for the most compact design, allowing the device to have high portability. The electronic circuitry, power supply and all the optical components are fitted inside a standard high 19” rack mount. A photograph of the fabricated device is shown in Fig. 1B.
The resonator geometry was chosen based on the criteria that 1) Q factor must be high enough to satisfy the spectroscopic resolution requirement in stellar spectroscopy; 2) wide FSR so that the teeth are well separated. The resonator used has a 135 μm radius and was fabricated with Hydex® high index core material (n = 1.7), and produced from chemical vapor deposition (CVD) onto a silica substrate . Photolithography and reactive ion etching were then able to produce a low radius-low roughness micro-ring that, when coated with silicon cladding, would still have high index and low loss . The result of this resonator was a three-dimensional upward design of resonator overtop of the bus waveguides, with a high Q (Q ~1x106) and low loss .
The TEC and RTD array allows for a higher control of temperature fluctuations, with the platinum RTD acting as a high precision thermostat and the TEC acting as the thermo-regulator within the device . The thin-film deposition of the platinum RTD is a more reliable and smaller-scale method for creating a sensitive thermocouple capable of the required sensitivity for the TEC to function properly . The RTD, when coupled with the TEC, is capable of providing stability control over the ring and the output of the comb, as the drift due to thermal expansion could be reduced to a negligible amount.
3. Measurement results
Using the ASE source with the micro-ring resonator, a comb was attained that spanned both the C and L band from 1520 nm to 1620 nm with approximately 63 resonance peaks. Figure 2 shows the output spectrum measured using an OSA (Anritsu MS9740A). Furthermore, by manipulating the pump diode power, the comb profile can be tuned to create a more uniform and flattened comb.
A close up of the comb spectrum is shown in Fig. 3 showing the FSR of the observed comb was 200 GHz (1.6 nm), the FSR value was chosen as an acceptable spacing in order to achieve well-spaced peaks for astrophysical observations. Using a scanning laser system we have shown that the measured resonator Q-factor of 1x106 was achieved  and, for each comb peak, a 3-dB FWHM of approximately δλ~2 pm at 1550 nm was measured. The peak profiles require a spectroscopic resolution of λ/δλ ~90,000 to be fully resolved by a spectrograph. This is typical of the highest resolution spectrographs used on telescopes to detect the presence of planets around nearby stars. The high-Q factor of the fabricated device along with coherent length of ~1.2 m satisfies this requirement. Additionally, the broader FSR reduces confusion in identifying peaks during the course of long exposures. It also helps with maintaining stability since the larger spacing has less sensitivity to drift when making small incremental measurements . The results show that the 135 μm micro-ring resonator is capable of providing a good spacing for its FSR, which can yield good resolution for measurements within the astronomical field (> 40 GHz) .
To investigate the stability of the frequency comb, the output of the comb generator is monitored by a wavelength meter (HP 86120C). From the stability that is present within the device, measurements taken over a 24-hour period indicate that the device maintain its stability during this period. From Fig. 4 , the power stability is found to be very consistent over a 10-hour period showing good stability . Figure 5 show the ambient temperature and wavelength stability over a 24-hour monitoring period. From these data, it is apparent that the temperature can be stabilized within the TEC-RTD feedback component, and that the operational stability resulted in a wavelength fluctuation that was within 5 pm (min to max) with a standard deviation of less than 1 pm. The stability can further be improved by the correction factor which is proportional to the square root of the number of peaks. These results show that this device’s performance can produce a comb that is capable to measure to within 12.5 m/s for radial velocities of exoplanets [2–4]. Furthermore, the longer-term stability shows that the device can operate in the range of hours to days, where other devices have only been able to maintain for much shorter time scales .
From the temperature data in Fig. 5, the self-regulation of the TEC was noted by the response of the RTD which then allowed for the temperature to be brought back down to ambient temperature; a process that could be facilitated by ensuring that the device was indeed isolated within its environment and that proper airflow was allowed as well. These occurrences could lead to wide temperature fluctuations, but even with such fluctuations, the TEC managed to regulate the temperature and maintain thermal stability within the device.
Although the current stability result needs to be improved by approximately 100 times for it to satisfy the detection requirement of radial velocities of exoplanets [2–4], stability can be improved by an increase of the number of resonant peaks and by improving the TEC design. With the device as operated within this experiment it can be expected that by increasing the TEC-RTD feedback array to a two-stage TEC-RTD feedback, one can effectively increase the stability to a higher degree for a longer period of time. With the single stage, there was a sufficient lag within the response to the temperature reduction, which did have a pronounced effect on the wavelength stability (Fig. 5). By increasing the TEC to a dual stage TEC, this lag could be reduced, as the temperature spikes would be greatly managed and reduced as well. This should result in less extreme fluctuations while achieving thermal stabilization at a given temperature.
We have shown that a frequency comb created from an ASE light source pumping a micro-ring resonator is capable of maintaining both a good FSR spacing and a high temperature stability. Furthermore, due to the dual pumping nature of this comb to achieve C and L band emission, the comb can be tuned in order to further flatten the profile of the comb output . By having a well-spaced FSR, small linewidth, flattened profiling and good temperature stability, this device should perform well in attaining measurements of radial velocities within astronomical fields—especially with increases in stability control. Additionally, with the reduced size and cost of this device, its versatility and portability can be capitalized upon and can be incorporated into smaller observational stations, increasing the flexibility of taking astronomical measurements in optical metrology.
This work is funded by the City University of Hong Kong Applied Research Grant, Project Number 9667039. STC thanks Amonics Ltd for their help and discussion on the assembly and temperature control of the instrument.
References and links
2. T. Steinmetz, T. Wilken, C. Araujo-Hauck, R. Holzwarth, T. W. Hänsch, L. Pasquini, A. Manescau, S. D’Odorico, M. T. Murphy, T. Kentischer, W. Schmidt, and T. Udem, “Laser frequency combs for astronomical observations,” Science 321(5894), 1335–1337 (2008). [CrossRef] [PubMed]
4. C. H. Li, A. J. Benedick, P. Fendel, A. G. Glenday, F. X. Kärtner, D. F. Phillips, D. Sasselov, A. Szentgyorgyi, and R. L. Walsworth, “A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1,” Nature 452(7187), 610–612 (2008). [CrossRef] [PubMed]
5. W. C. Swann, J. J. McFerran, I. Coddington, N. R. Newbury, I. Hartl, M. E. Fermann, P. S. Westbrook, J. W. Nicholson, K. S. Feder, C. Langrock, and M. M. Fejer, “Fiber-laser frequency combs with subhertz relative linewidths,” Opt. Lett. 31(20), 3046–3048 (2006). [CrossRef] [PubMed]
6. T. R. Schibli, K. Minoshima, F.-L. Hong, H. Inaba, A. Onae, H. Matsumoto, I. Hartl, and M. E. Fermann, “Frequency metrology with a turnkey all-fiber system,” Opt. Lett. 29(21), 2467–2469 (2004). [CrossRef] [PubMed]
8. L. Razzari, D. Duchesne, M. Ferrera, R. Morandotti, S. Chu, B. E. Little, and D. J. Moss, “CMOS-compatible integrated optical hyper-parametric oscillator,” Nat. Photonics 4(1), 41–45 (2010). [CrossRef]
9. P. Z. Dashti, Q. Li, and H. P. Lee, “All-fiber narrowband polarization controller based on coherent acousto-optic mode coupling in single-mode fiber,” Opt. Lett. 29(20), 2426–2428 (2004). [CrossRef] [PubMed]
10. H. Lin, “Waveband-tunable multi-wavelength Er-doped fiber laser,” Appl. Opt. 49(14), 2653–2657 (2010). [CrossRef]
11. P. Del’Haye, T. Herr, E. Gavartin, M. L. Gorodetsky, R. Holzwarth, and T. J. Kippenberg, “Octave spanning tunable frequency comb from a microresonator,” Phys. Rev. Lett. 107(6), 063901 (2011). [CrossRef] [PubMed]
12. M. Razik, A. Budnicki, and K. M. Abrams, “ASE source at 1550 nm,” Proc. SPIE 5576, 135–138 (2004). [CrossRef]
13. M. Ferrera, L. Razzari, D. Duchesne, R. Morandotti, Z. Yang, M. Liscidini, J. E. Sipe, S. Chu, B. E. Little, and D. J. Moss, “Low-power continuous-wave nonlinear optics in doped silica glass integrated waveguide structures,” Nat. Photonics 2(12), 737–740 (2008). [CrossRef]
14. D. D. Nelson, J. B. McManus, S. C. Herndon, J. H. Shorter, M. S. Zahniser, S. Blaser, L. Hvozdara, A. Muller, M. Giovannini, and J. Faist, “Characterization of a near-room-temperature, continuous-wave quantum cascade laser for long-term, unattended monitoring of nitric oxide in the atmosphere,” Opt. Lett. 31(13), 2012–2014 (2006). [CrossRef] [PubMed]
15. M. T. Tinker and J. B. Lee, “Thermal and optical simulation of a photonic crystal light modulator based on the thermo-optic shift of the cut-off frequency,” Opt. Express 13(18), 7174–7188 (2005). [CrossRef] [PubMed]
16. A. Gusarov and F. Liegeois, “Experimental study of a tunable fiber ring laser stability,” Opt. Commun. 234(1-6), 391–397 (2004). [CrossRef]
17. T. M. Fortier, A. Bartels, and S. A. Diddams, “Octave-spanning Ti:sapphire laser with a repetition rate >1 GHz for optical frequency measurements and comparisons,” Opt. Lett. 31(7), 1011–1013 (2006). [CrossRef] [PubMed]