From Chains to Short Pulses: Absolute Frequency Measurements Across - PowerPoint PPT Presentation

From Chains to Short Pulses: Absolute Frequency Measurements Across the Optical Spectrum A.A. Madej , J.E Bernard, A. Czajkowski, L. Marmet, P. Dubé, and K.J. Siemsen Work Partially March 13, 2003 Supported by : International Comb Workshop Canadian Institute for Photonic Innovations BIPM, Sèvres, France (CIPI)

Counting Optical Frequencies

Measurement of Optical Frequencies via Chains Measurement of Optical Frequencies via Chains * Frequency Chain Philosophy: • By non-linear stages link laser oscillators at different portions of the electromagnetic spectrum. • Cascade these intermediate oscillator stages to higher absolute frequencies until one reaches the frequency to be measured.

NRC Frequency Chain to the Visible A frequency chain was devised to span the factor of 50,000 ratio in frequency between the visible light probing the ion and the Cs reference standard at 9.1 GHz. The chain consisted of 5 CO 2 lasers, 2 solid state lasers, 2 microwave oscillators and 8 phase locked servo loop systems.

NRC Single Strontium Ion Trap Artist Impression of Trap and Excitation Beams View of Chamber and Photomultiplier

Single, Laser Cooled Sr + Optical Frequency Standard • Single Sr + Ion held in 1-mm Paul Trap and cooled to 5 -10 mK. • Probing on S-D quadrupole transition (0.4 Hz) using ULE cavity stabilized diode laser.

Single, Laser Cooled Sr + Optical Frequency Standard • Cs based frequency chain measurements on four different experimental days spanning over a year have yielded a hundred fold improvement on the previous value of the ion frequency. • Based on these results, the centre frequency of the ion transition is determined to be : 444 779 044 095. 4 ± 0.2 kHz (1 )

Extending Ion Accuracy across the Spectrum

Using the Sr + Standard to Measure the Frequency of the HeNe/I 2 Standard at 474 THz (633 nm)

Determination of 474 THz Standard Frequency On three different experimental days, determinations of the NRC INMS 3 standard frequency stabilized to component "f" gave 473 612 353 595 ± 1 kHz. The CIPM recommended value for this transition is: 473 612 353 597 ± 12 kHz. From April 10 -14, 2000 intercomparisons were performed with lasers from the BIPM and JILA/NIST.

A Miracle of Nature:Femtosecond Lasers as Frequency Combs • In the Time Picture, the mode-locked laser consists of a series of pulses of length � t separated by a pulse repetition of � = 1/ f rep . • In Frequency, this creates a comb of individual frequencies For Femtosecond lasers, the effective separated by ��� = 1/ � and bandwidth can be extremely large. where the spread is ( � t = 10 fs, �� = 160 THz) inversely related to the pulse duration. Locking the pulse repetition rate f rep stabilizes the comb to high accuracy.

Extending the spectrum of the comb elements

Measuring the Absolute Frequency of Each Comb Element Measuring the Absolute Frequency of Each Comb Element

NRC Femtosecond Comb for Measurement of Optical Frequencies

Characteristics of NRC Frequency Comb Comb Spectrum Over an octave spectral Fibre length = 25 cm coverage has been 1 Total power = 170 mW Pow er (arbritary units) obtained with the NRC frequency comb 0.8 extending from 510 nm 0.6 to beyond 1060nm. The generated broad 0.4 spectrum is coherent yet 0.2 possesses structure that depends of the input 0 launch polarization, 400 500 600 700 800 900 1000 1100 1200 intensity and coupling into the microstructure Wavelength (nm) May1602_Spec1.xls fibre.

Stabilizing the Frequency Movement of the Comb Ensemble * By comparing the fo Beat with the Comb locked second harmonic of the lower frequency 0 end of the comb -10 with the upper end located at same -20 frequency, a -30 heterodyne beat is Power (dBm) -40 obtained which is equal to the offset -50 of the comb as a -60 whole when extrapolated to zero -70 frequency. Phase -80 Resolution bandwidth = 100 kHz locked stabilization -90 of the beat is 290 291 292 293 294 295 296 297 298 299 300 utilized. Frequency (MHz)

Measurement of I 2 /HeNe Standard using Comb Measurement of I 2 /HeNe Standard using Comb

Recent Results from NRC Comb : 474 THz I 2 /HeNe Standard 15000 Absolute Frequency [ f -473 612 353 595 000 + X] [Hz] 10000 10000 * Full Phase-Locked operation of the Allan Deviation [Hz] 5000 repetition rate and comb position for the NRC 0 Frequency comb was 1000 achieved. The system -5000 was used to measure the INMS 3 Iodine stabilized -10000 HeNe Laser at 474 THz. 100 -15000 1 10 100 0 100 200 300 400 500 600 Averaging Time [s] Elapsed Time [s]

Recent Results from NRC Comb : 474 THz I 2 /HeNe Standard (cont.) • Comparing the Comb based Observed Absolute Frequency of NRC INMS 3 Laser on Component "f" Observed Absolute Frequency of NRC INMS 3 Laser on Component "f" measurements with results Absolute Laser Frequency [ 473 612 353 595 000 +X] [Hz] Absolute Laser Frequency [ 473 612 353 595 000 +X] [Hz] obtained in April,2000 leads 1400 1400 April 12, 2000 to a remarkable agreement Ion Based Chain 1200 between methods at the 1200 200Hz level. 1000 April 12, 2000 1000 Ion Based Chain • The April 2000 work used 800 the single Sr + ion standard 800 as a reference and 600 connected the 474 THz light 600 400 by a 28 THz CO 2 laser May 21, 2002 Comb Measurements measured by the old NRC 400 May 21, 2002 Comb Measurements 200 frequency Chain through non-linear mixing methods. 200 0 0 2 4 6 8 0 2 4 6 8 Run Identifier Run Identifier

Measurement of Single Ion Standard using Comb Measurement of Single Ion Standard using Comb S-D Frequency [ 444 779 044 095 400 ] [Hz] 2000 1500 1000 500 0 -500 -1000 -1500 76400 76600 76800 77000 77200 77400 Time of Day [s]

Recent Results from NRC Comb : 445 THz Single Ion Frequency • Absolute measurements S-D Frequency [ f - 444 779 044 095 400] [Hz] of the 88 Sr + 5s 2 S 1/2 - 4d 300 S-D Frequency [ f - 444 779 044 095 000] [Hz] 2 D 5/2 transition were 2000 performed with the Comb 200 applied to the NRC Single 1500 Ion Standard. 100 • The preliminary total 1000 result of the 4 days of NPL 0 study spanning a period of 500 Comb 4 months yielded an NRC (2002) Comb uncertainty of ±50 Hz -100 (2002) 0 whose value agrees with the NRC Chain f SD = 444 779 044 095 494 ± 50 Hz -200 -500 NRC measurement and recent f SD = 444 779 044 095 494 ± 50 Hz Chain comb measurements by (1999) -300 -1000 the NPL group. -2 -1 0 1 2 3 4 5 0 5 10 15 20 25 30

Summary Comparison of Chain and Comb Technologies Summary Comparison of Chain and Comb Technologies Chain Technology Comb Technology • Laser Technology necessary • One Modelocked Femtosecond across the spectrum laser (Ti:Sapphire or ...) • Many Phase Locked Loops • 1-2 PLLs only necessary to (PLL) necessary to reach up to stabilize comb spacing and visible region (phase noise possibly comb offset (phase noise multiplication, Complex) much lower, Simpler) • Built to measure one specific • Can in principle measure frequency region. frequencies across one spectral octave. • Need power from laser to be measured ( > 100 µW- 1mW). • Need power from laser to be measured (> 100 µW). • Multi-year projects that existed in a few laboratories. • Widely available technology with short startup times and flexibility.

Applications of Optical Combs