A transportable optical frequency comb based on a mode-locked fibre laser B. R. Walton, H. S. Margolis, V. Tsatourian and P. Gill National Physical Laboratory Joint meeting for Time and Frequency Club and Location and Timing KTN, 27 November 2007
Motivation Frequency combs have revolutionised optical metrology. So far frequency combs are too large to move between labs reduced flexibility. Therefore need to find a robust system with small footprint, low weight, to make transportable. 2
Outline An introduction to frequency combs A description of the transportable frequency comb system Long-term frequency measurements A comb comparison between the transportable comb and another frequency comb system 3
Optical femtosecond frequency combs Wednesday, 14 May 2008 Based on a mode-locked pulsed laser: generates a comb of phase-coherent optical modes, equally spaced in frequency. The mode frequencies can be locked to a stable frequency standard (e.g. a hydrogen maser). Can be used to measure the frequency of almost any stable optical laser source. Conversely, can be used to generate stable signals at almost any optical frequency. 4
Optical femtosecond frequency combs Wednesday, 14 May 2008 Modes are spaced by the repetition rate of the pulses (f rep 1 / L cavity ) Dispersive elements in the cavity lead to a carrier-envelope phase slip. This manifests as an offset of the mode spectrum by f 0. Frequency of nth mode is therefore f n = n f rep + f 0 5
Finding the offset frequency f 0 f rep is found from inter-mode RF-frequency beats on a photodetector. f 0 can be found if the comb spectrum covers at least one octave. This usually requires the comb spectrum to be broadened by passage through a non-linear microstructured fibre. f:2f interferometer 6
Microstructured fibre The beam is tightly guided in the core by a high refractive index contrast. This enhances optical non-linear effects which broaden the spectrum. Coherent non-linear effects preserve the comb structure (i.e. f rep, f 0 are unchanged). 7
Optical frequency measurement The frequency f laser of a stable laser may be measured by beating it with the nearest comb mode. Wednesday, 14 May 2008 More generally:s apples f laser =nf rep ±f 0 ±f beat The optical frequency f laser may therefore be related in a single step to countable RF frequency signals. These signals can be referenced to the signal from a caesium fountain or hydrogen maser, providing traceability to the SI second. 8
Combs at NPL A comb based on a Ti:sapphire laser is currently in use at NPL It can be used to measure frequencies between ~500 1000 nm. Prisms for dispersion compensation require long cavity large footprint (f rep ~ 90 MHz). Small cavity (f rep ~ 800 MHz) comb under development at NPL (compensation with chirped mirrors) reduced footprint. The high pump laser power (several Watts) makes Ti:sapphire lasers unsuitable for transportable combs. 9
Comb measurements at NPL The Ti:sapphire comb has been used to determine absolute frequencies of several optical frequency standards at NPL: Trapped single-ion standards: 88 Sr + quadrupole transition at 674 nm (with an uncertainty of 3.4 10-15 ) 171 Yb + octupole transition at 467 nm Experiments probing 2S ns,d transitions in the hydrogen atom to determine the Rydberg constant via 2-photon interactions 10
Comb measurements at NPL The Ti:sapphire comb has been used to determine absolute frequencies of several optical frequency standards at NPL: Acetylene gas-cell standard at (1.5 μm outside comb spectral range transfer lasers at 1542 and 771 nm were required), Calibration of iodine-stabilized helium neon lasers at 633, 594 and 543 nm. 11
Why a transportable comb? Gives laboratories without a frequency comb access to optical frequency metrology. Enables NPL to expand scope of comb applications, e.g. dimensional metrology, spectroscopy. 12
Transportable combs requirements Wednesday, 14 May 2008 Compact. Either high repetition-rate (small cavity small footprint) or fibre system (can coil fibre). Robust. Fibre systems have advantage - fibre coupling reduces alignment drift. Low pump power low power supply weight. Large spectral measurement range maximise usefulness and flexibility of the system. 13
MenloSystems FC1500 A diode-pumped erbium-doped fibre laser system operating at 1.5 μm, with f rep = 100 MHz. 14
GPS-disciplined reference signal The GPS network provides a timing signal that enables comb measurements to be traced to the SI second. Wednesday, 14 May 2008 The transportable comb uses a 10 MHz reference signal from a Rapco quartz oscillator which is locked to a rubidium oscillator which is in turn locked to the GPS signal. 15
Reference signal performance Normalized frequency deviation from 10 MHz over a 3 hour averaging period 4x10-12 2x10-12 0.0-2x10-12 -4x10-12 0 5 10 15 20 Time (days) Allan deviation 10-11 10-12 10-13 10-14 Quartz + Rubidium + GPS Rubidium + GPS H Maser 10 1 10 2 10 3 10 4 10 5 10 6 Time interval τ (s) 16
Mounting arrangement The system is mounted on a wheeled aluminium frame measuring 1.21 m (height), 0.95 m (width) and 1.72 m (length). Robust system has performed measurements with minimal readjustment after transportation over rough ground. Frequency doubler 2 EDFAs, IR broadening Laser & EDFA f:2f interferometer Visible broadening 17
Long-term frequency measurements Wednesday, 14 May 2008 Long-term measurements were undertaken to determine the stability and robustness of the system. The transportable frequency comb was used to measure the frequency of a high-finesse-cavity-stabilised diode laser at 674 nm. A tracking oscillator (TO) was locked to f beat to amplify the beat without giving additional noise. Measurements were referenced to a hydrogen maser signal. 18
Long-term frequency measurements Wednesday, 14 May 2008 A continuous measurement of the laser frequency was performed for more than 60 hours. Drift in frequency due to variation in highfinesse cavity length. 19
Long-term frequency measurements Wednesday, 14 May 2008 The frequency of a commercial stabilized 594 nm helium-neon laser was measured over an 18 hour period. A stable HeNe is usually calibrated against an iodine-stabilised HeNe, however a 594 nm I 2 -stabilized laser was not available. Frequency - 504618992 MHz (MHz) 25 20 15 10 5 0 0 5 10 15 20 Time (hours) 20
Comb comparison A concurrent measurement of the same frequency by a separate comb system is an important check of the comb s accuracy and stability. A measurement of the frequency of a 934 nm beam from a cavity-stabilised CW Ti:sapphire laser was performed. The measurement was performed simultaneously by the transportable comb and by the Ti:sapphire comb. 21
Stability Comb comparison results Transportable comb: σ (10 s) = 1.0 10-12 (GPS ref.) N.B. GPS-disciplined oscillator: σ (10 s) = 1 10-12 σ (10 s) = 2.4 10-13 (Maser ref.) Ti:Sapphire comb: σ (10 s) = 1.8 10-13 (Maser ref.) Accuracy Mean GPS-referenced comb measurement differed by +9.0 10-13 compared with Ti:sapphire comb. Accuracy of GPS-disciplined oscillator: ~4 10-12 over a few hours. Good enough for most applications. 22
Summary A new transportable frequency comb system is in use at NPL. The system has been used to perform continuous measurements over more than 60 hours. A comparison between this system and another comb has shown that the transportable comb has a GPSreference-limited stability of 1.0 10-12 at 10 seconds, and an accuracy of approximately 4 10-12 when averaged over a few hours. 23