Optical Frequency Comb

Concepts

• teddington
• precision equipment
• laboratory and optical equipment
• optical frequency combs
• optical frequency ratio measurements
• optical frequency standards
• telecoms optical fibre links
• periodic optical realignment
• frequencies
• the optical frequency comb

Location

NPL Management Ltd Hampton Road Teddington TW11 0LW

Description

Comparison of optical frequency standards is essential for verifying that the clocks both locally and remotely at other national metrological institutes all agree to within their quoted uncertainties. These quoted uncertainties can be as low as ~1e-18, and so agreement between multiple clocks all operating at this level is required to verify that uncertainty. Remote clock comparisons at NPL, or between NPL and other European NMIs, are only possible at this uncertainty using optical frequency combs and telecoms optical fibre links to compare and transmit frequencies over long distances. By using an optical frequency comb with the full flexibility to cover the 500 nm to 2000 nm wavelength range, the infrastructure will be in place to enable any of the likely candidates for an optical representation of the SI second to be compared over a short cross-site fibre link. The optical frequency comb will also require dedicated branches to link with the optical clocks and oscillators already available at NPL, these include the Sr+ at 674 nm, the Sr lattice at 698 nm, the Yb+ clocks at both 871 nm and 934 nm, and an ultrastable laser at 1064 nm.

Total Quantity or Scope

Description of the procurement / requirements: Mandatory Requirements: 1. The system must fulfil all regulatory requirement for operation in the UK and in the European Union. It must be CE marked or conform to equivalent international standards for electrical safety and a declaration of conformity or certificate of compliance must be provided. 2. The Frequency Comb must be based on femtosecond fibre laser technology, with the output from the fundamental oscillator covering a wavelength range that includes 1542 nm. 3. The nominal value of the repetition rate (comb mode spacing) must be equal to or greater than 200 MHz. 4. The frequency comb must have outputs optimised for simultaneous high precision measurement of lasers at the following nominal wavelengths: 674 nm, 698 nm, 871 nm, 934 nm, 1064 nm, and two outputs at 1542 nm. 5. It must have outputs that enable high precision measurement of the frequency of any laser with wavelength in the range from 500 nm to 2000 nm, as well as the optical frequency ratio between any two such lasers. These outputs must be useable simultaneously with those outlined in 4. 6. At least one additional output must be provided from the fundamental oscillator, to be used for monitoring of the comb spectrum and for high resolution measurement of the repetition rate. This output must be useable simultaneously with those outlined in 4 and 5. 7. The spatial mode of all output beams must be the fundamental TEM00 mode. 8. All equipment required for full stabilisation of the comb degrees of freedom must be provided. This equipment must be suitable for stabilising the comb either to an ultrastable optical reference (at 1064 nm) or to an external 10 MHz reference frequency. 9. The power of the comb outputs at 674 nm, 698 nm, 871 nm, 934 nm, 1064 nm, the two outputs at 1542 nm, as well as the outputs covering the range from 500 nm to 2000 nm, must be sufficient to achieve a signal to noise ratio > 30 dB (at 100 kHz resolution bandwidth) on the optical beat note with a CW laser with 1 mW average power. At 1542 nm the power must be sufficient to achieve this for beat notes with two separate CW lasers. This specification must be achievable by all outputs outlined in 4 simultaneously. - All beat detection units required to achieve this must be included with the system. 10. When used for spectral purity transfer from a 1064 nm ultra-stable laser to a laser at any of the outputs outlined in 4, or for optical frequency ratio measurements, the excess instability introduced by the frequency comb must be no more than 5E-17 at 1 s (measured using a Lambda-type counter). 11. When used for optical frequency ratio measurements, the comb must be able to reach an accuracy of 1E-18 or better. - Accuracy here is defined by measuring an optical frequency ratio with this comb and another independent comb and comparing the results. 12. The supplier must provide a means of tuning the repetition rate of the comb, with a tuning range of at least 500 kHz. 13. The frequency comb must be provided with automatic mode-locking capabilities. 14. A control PC must be provided with suitable software for controlling the comb included. This must be compatible with other hardware running the Windows operating systems. 15. The capability to network the control PC via Ethernet must be provided. 16. The system must include suitable equipment for monitoring all lock error signals and beat notes. 17. All control and monitoring electronics must be mountable in a 19" rack. 18. A user manual in English must be made available. 19. The supplier must provide an additional ultrastable microwave reference output of the frequency comb. The frequency of this output must be at 8 GHz and be optimised for the lowest achievable phase noise. All required components must be included. Specifications: Required: 1. The power of the comb outputs at those outlined in the mandatory requirement 4 above, as well as the outputs covering the range from 500 nm to 2000 nm, should be sufficient to achieve a signal to noise ratio > 35 dB (at 100 kHz resolution bandwidth) on the optical beat note with a CW laser with 1 mW average power. At 1542 nm the power should be sufficient to achieve this for beat notes with two separate CW lasers. This specification should be achievable for the outputs outlined in 4 above, and any two wavelengths in the range 500 nm - 2000 nm simultaneously. Higher signal to noise ratios would be beneficial. - Please advise how this requirement can be met. 2. When used for spectral purity transfer from a 1064 nm ultra-stable laser to a laser at any of the outputs outlined in the mandatory requirement 4 above or for any optical frequency ratio measurements, the excess instability introduced by the frequency comb should be no more than 1E-17 at 1 s (measured using a Lambda-type counter). A better stability would be beneficial. - Please provide details of how this stability is achieved, including the beat detection setup and any proposed noise cancellation schemes. In addition, a retroreflector should be included in all beat detection setups to enable fibre link noise to be cancelled. 3. When used for optical frequency ratio measurements, the comb should be able to reach an accuracy of 1E-18 or better for sufficiently long averaging times. The minimum time required to reach this accuracy should be specified. Better achievable accuracy would be beneficial, as would being able to achieve this accuracy with a shorter averaging time. – Please advise how this requirement can be met. 4. The linewidth of all comb modes should be <1 Hz when referenced to a suitably narrow ultrastable optical local oscillator. A lower linewidth would be beneficial. - Please advise how this requirement can be met. 5. The supplier should provide a means of tuning the repetition rate of the comb, with a tuning range of 500 kHz or more. Larger tuning ranges would be beneficial. - Please advise how this requirement can be met. 6. The supplier should provide information about the long-term reliability of the system (over years), including the mean time before failure for components such as pump diodes and information on any components for which periodic optical realignment may be required. Estimates should be provided of the likely system downtime resulting from such component failures or necessary maintenance tasks. A longer time between failures and a shorter downtime would be beneficial. - Please advise how this requirement can be met. 7. The supplier should provide information about how frequently re-locking of the comb is likely to be required and how often adjustments are likely to be required to maintain the stability, accuracy and beat signal to noise specifications. Evidence that cycle-slip-free operation can be maintained over long periods should be provided. Evidence that performance can be maintained over longer periods without human intervention would be beneficial. - Please advise how this requirement can be met. 8. The system must be modular to provide flexibility for future upgrades, for example to add outputs at additional wavelengths or to upgrade beat detection systems. Details should be provided about the range of possible upgrades, with information about which upgrades would be possible in situ and which would require all or part of the system to be returned to the supplier. - Please advise how this requirement can be met. 9. The physical dimensions of the proposed system should be provided, giving separate figures for the optical system and the electronics. The maximum allowable separation between the optics and the electronics should also be provided. In addition, the details of the stablisation equipment required for phase locking, of the mode locking mechanism used and of all the supplied monitoring equipment should be provided. - Please advise how this requirement can be met. 10. If cooling would improve the performance of the system, the supplier should state this and quote separately for any equipment required. 11. The supplier should provide the option to facilitate remote relocking. All controls required for relocking should be controllable (either directly or indirectly) via ethernet on an independent PC running Windows 10. All measurements required to relock should also be remotely viewable via ethernet.

Award Detail

Renewal Options

12. OPTIONAL ITEM: The supplier should quote separately for any frequency counters used to achieve the mandatory requirements. Any frequency counters required to monitor the stability of the comb itself should be labelled as such and offered separately to those which are used for monitoring other signals, e.g. beat notes with external CW lasers. 13. OPTIONAL ITEM: In relation to the mandatory requirement 19 above, an optional extension to shift the ultrastable microwave output to the Cs primary standard frequency near to 9.2 GHz should be quoted for separately.

CPV Codes

• 38000000 - Laboratory, optical and precision equipments (excl. glasses)

Indicators

• Options are available.

Legal Justification

The proposed procurement is very similar to the competitive tender purchase of Frequency Combs previously conducted by T&F/OFM earlier in 2021 and again in 2023, which were purchased to equip state-of-the-art test and evaluation laboratories both in Teddington and in Glasgow. Because of this, OFM have now installed and commissioned several of these systems and invested a substantial amount of time in developing a full understanding of the system. At the time of the previous procurements, the Menlo system was evaluated to provide the most appropriate solution to the T&F needs by far. By purchasing a similar system from the same supplier, the embedded expertise in this system can be exploited, and subsequently interoperability of systems can be developed. Based on the specification from the previous tender processes, the systems from other two bidders were found to be the only ones capable of demonstrably meeting the repetition rate tunability and the required flexibility in a turn-key system. None of the alternative suppliers could guarantee or demonstrate the specification at the required level. Menlo systems scored high on demonstrated performance systems integration and service. Since we have tested the market twice in the past few years with the same outcome, we are confident that Menlo Systems is the only provider that can meet the expected requirements for our purposes.