One of the seven SI units is being measured with an accuracy of 10-14 - 10-15 and it is the most correctly measurable unit. That is why time and frequency measurements are used in order to increase accuracies of the other units. Nowadays, technologically improved countries establish their own time and frequency systems and support the works on the subject to improve time and frequency standards. With the improving technology, the need for time and frequency knowledge in avionics, space and defence systems has become a must for every preceding country in science and technology. Improvement of frequency stabilised lasers is very important and useful for development of optical clock and also for length measurements with nanometer uncertainty and displacement measurements with picometer uncertainty. In National Metrology Institute (UME) of Turkey Time, Frequency and Wavelength laboratory was developed since 1993.
The information related main activity and results of this time frequency laboratory are given below, presented as two items.
- Time Scale Generation and Traceability
In Time and Frequency Laboratory of UME, time keeping and dissemination systems were developed using commercial available 5 Cs clocks (5071 A) and 2 multichannel GNSS (TTS-3 and TTS-4) receivers. This laboratory is a member of BIPM TAI club since 1994. UTC (UME) time scale is generated with an uncertainty better than 2x10-14. 10 MHz and 1PPS signal is used as reference for calibration laboratory, time dissemination system and femtosecond COMB for laser frequency measurement. For analysing of measurement uncertainty of developed system 2 of UME atomic clocks are compared same time with time interval counter and GPS Common View (CV) method using 2 single channel GPS receiver. In parallel one of Cs clock which is reference is compared with PTB reference clock using GPS CV method with uncertainty less than 6 ns.
- Time Dissemination
Time Dissemination System was developed for distribution generated time from Cs atomic clock among local area networks (LANs), wide area networks (WANs), and internet/intranet by using of network time protocol (NTP) at stratum - 1 level. By using this system time dissemination is realized with an uncertainty better than 5 ms for LAN and better than 50 ms for WAN.
The wavelength laboratory main activities have been presented in the following within the 8 basic items.
- Frequency Stabilized Lasers
The different lasers with different wavelengths have been developed and they have been stabilized to the Rb and Cs atoms, I2 and CH4 molecules with a stability of 1x10-11 - 1x10-14. The parameters influencing to the frequency of the He-Ne/I2, He-Ne/CH4 gas lasers, Nd-YAG/I2 solid state laser and the ECDL/Rb,Cs semi-conductor lasers have been investigated and analyzed. The absolute frequency of the He-Ne/I2 (633 nm) stabilized on the f line of the iodine molecules has been measured and compared both with the BIPM (473 612 353 602.0 ± 1.1 kHz) and the UME (473 612 353 600.6 ± 1.1 kHz) Ti:Sa Comb systems.
Absolute frequency (88 376 181 000 253 ± 23 Hz) of the He-Ne/CH4 laser (3390 nm) has been measured by the PTB frequency chain system. Developed ECDL lasers have been locked to the Cs (852 nm) and Rb (780 nm) D2 atomic lines and to the 2-photon S-D lines of Rb atoms (778 nm).
- High-Resolution Laser Spectroscopy
Using the frequency tunable lasers, selective reflection on the D2 lines of Cs atoms, wave mixing, laser pressure on the resonances, optical pumping on the Zeeman levels, coherent population trapping (CPT) effects have been investigated. Research on the polarization and Faraday effects on the S-D 2-photon transitions of Rb atoms have been carried out. Influence of the light intensity and gas pressure on the absorption resonances of the I2 and CH4 molecules have been investigated.
Under EMRP project “EMF&SAR Traceable Measurement of Field Strength and SAR for the Physical Agents Directive” the free space microwave-atom-laser interaction and radio-optical coherent resonances have been investigated and applied for electrical field strength measurements. Recently laboratory is attended to the new EMRP project “Compact and high-performing microwave clocks for industrial applications” and under this projects the investigation of CPT resonances in Cs and Rb atomic gas in pump-probe optical configuration in progress.
- Absolute Frequency Measurements of Lasers using Femtosecond COMB
Absolute frequency measurement of frequency stabilised He-Ne/I2, Nd-YAG/I2 and ECDL/Rb,Cs lasers was measured by femtosecond Ti:Sa COMB (530 - 1100 nm) that is externally triggered by the 10 MHz signal of Cs atomic clock. In the laboratory Yb fiber based femtosecond COMB working in the 600 nm - 1600 nm range and generating 33 fs pulses has been developed. The repetition and offset frequency of the developed Yb COMB system has been locked to the 10 MHz signal of Cs atomic clock. Homemade UME Yb fiber comb was compared with commercially available Ti:Sa fs comb in UME using frequency stabilised Nd:YAG laser.
- Atomic Frequency Standards
A wide range of industrial fields requires stable and reliable frequency or timing signals than can be provided only by atomic frequency standards. It is well known that optical clocks are the most accurate and stable clocks available nowadays. However, their performances go well beyond what is required for most industrial applications. Optical clocks are still at the level of laboratory prototypes in terms of reliability, size, cost and power consumption. The overall aim of the project is to develop and produce 3 types of commercial viable microwave clocks which match the performance and compactness needed for upcoming demanding industrial and technical applications. Testing of the clocks in different operating situations is necessary. The tests concern the behaviour of the clocks or clocks components in more demanding environments. Selected clock components will undergo two kinds of tests simulating demanding environments: mechanical and electromagnetic compatibility (EMC) and interference (EMI) tests. The mechanical and electromagnetic tests of optical clocks will be performed by National Metrology Institute of Turkey (TÜBİTAK - UME). The temperature and humidity tests of optical clocks will be carried out in the temperature-humidity test chamber. The EMC and EMI tests will be done in a semi-anechoic and screen chamber rooms. In addition to this responsibility of the TÜBİTAK - UME contains scientific research in work packages.
- High Power Laser Characterization
In the Project “Development of High Power Laser System”, a directed, target tracking laser system with a power of more than 20kW will be developed. Project participants are Tubitak BILGEM, ASELSAN, Tubitak MAM, Bilkent University and TÜBİTAK UME.
In this Project UME is responsible for the characterization of the high power lasers which will be devoloped. In this extent, M2 factor and laser power measurement systems will be established according to international standards, providing onsite measurement service and providing support for the standardized measurement system for other project participants.
Laser characterization is an important subject for all metal industry and the medical sector. The accuracy of the measurement is of high importance for the reliable use of systems and for the design phase of the laser.
- Gauge Block Length Measurement with Laser Interferometer
“1 meter, is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second” (1983).
‘Metre’ is determined according to velocity of light and today this definition is used. The velocity of light is accepted as a universal constant and the length and distance measurements are realizied in terms of wavelengths of the accepted lasers.
By the use of interferometer systems the meter, which is the length standard, is transferred from the frequency standards (wavelength standards) to gauge blocks. The frequency stabilized lasers are the frequency standards or in other words they are determined as wavelength standards.
a) Köster Long Gauge Block Interferometer
Köster Interferometer is a system which can measure the length of the gauge blocks with a resolution of 10-9 meter. In this system the laser beams from three different stable lasers are transferred with fiber cables into the interferometer where the gauge block is placed. The interference fringes observed at the CCD camera are analyzed and the lengths of the gauge blocks are obtained. By using this system, the gauge blocks with lengths between 125 mm - 1000 mm are calibrated. 1 meter gauge block can be measured with an uncertainty of 200 nm in this system.
The lasers which are used in the interferometers are the frequency stabilized lasers. National Metrology Institute TÜBİTAK UME has designed and produced the necessary mechanical and electronic systems for the laser frequency stabilization. Stable Lasers brought to the commercial format in time, produced and made continuous improvements in this regard are as follows: 1) the stabilized He-Ne laser which is locked to energy transitions of iodine molecules, He-Ne laser (He-Ne/I2), 2) the stabilized Nd:YAG laser which is locked to energy transitions of iodine molecules, Nd:YAG laser (Nd:YAG/I2), 3) the stabilized External Cavity Diode lasers (ECDL) which are locked to energy transitions of Cs or Rb atoms, (ECDL/Cs) laser or (ECDL/Rb) laser. The stability systems of this lasers are made in TÜBİTAK UME.
TÜBİTAK UME designed and produced the Long Gauge Block Köster Interferometer measuring the length of the gauge blocks in the primary level. As a result, research in this field and contributing to the development aims to have the necessary infrastructure and knowledge. Also from various sectors across the country longe gauge block calibration demands, TÜBİTAK UME's Long Gauge Block Köster Interferometer aimed to correct the deficiencies in this area.
b) Short Gauge Block Interferometer
The aim of this project is to design and construct a Short Gauge Block Interferometer, which is a primary level measurement system. With the interferometric measurement method, short gauge blocks (0,3-300 mm) can be measured with a resolution of 10-9 (nanometer).
By the end of the project, the staff of the TÜBİTAK UME Wavelength” Laboratory will have utilized their experience, skill and knowledge to produce a Short Gauge Block Interferometer that can be made available commercially.
- Gauge Block Length Measurement with Mechanical Method
The main aim of this project is to design and construct a Long Gauge Block Comparator, which is a secondary level measurement system.
a) Long Gauge Block Comparator
In this system, utilization of the comparison method with a reference gauge block is the main principle. Long gauge blocks (125-1000 mm) having square and rectangular cross sections can be measured.
When the project is completed, it will be possible for UME to produce Long Gauge Block Comparator as a commercial product for use by secondary laboratories and metrology institutes in countries that are in the process of developing their metrology infrastructure.
- Sub-nanometer Displacement Measurements
TUBITAK UME together with five more European Measurement Institutes (NMIs) under the coordinatorship of INRIM (Italy) has participated within the European Metrology Research Programme (EMRP) funded project NANOTRACE. The aim of the Project was to develop the next generation of optical interferometers having a target uncertainty of 10 pm. TUBITAK UME has designed and developed temperature and vacuum controlled interferometers within this project. Differential Fabry - Perot Interferometers have also been developed and have been compared with the NPL x-ray interferometer generating traceable reference displacements. The developed differential Fabry-Perot interferometer system make use of frequency stabilized tunable External Cavity Diode Lasers (ECDLs) and Beat Frequency measurement techniques. The half and one fringe displacements of the x-ray interferometer have been measured with an accuracy of less than 5 pm with the developed system. This activity is continued under new EMRP Project “Traceability of sub-nm length measurements”.