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• W1605311186 abstract "CBP-811 A FEMTOSECOND-LEVEL FIBER-OPTICS TIMING DISTRIBUTION SYSTEM USING FREQUENCY-OFFSET INTERFEROMETRY ∗ J. W. Staples, J. Byrd, L. Doolittle, G. Huang and R. Wilcox, LBNL, Berkeley, California, USA Abstract An optical fiber-based frequency and timing distribution system based on the principle of heterodyne interferome- try has been in development at LBNL for several years. The fiber drift corrector has evolved from an RF-based to an optical-based system, from mechanical correctors (piezo and optical trombone) to fully electronic, and the electron- ics from analog to fully digital, all using inexpensive off- the-shelf commodity fiber components. Short-term optical phase jitter and long-term phase drift are both in the fem- tosecond range over distribution paths of 2 km or more. Future Accelerator Timing Requirements The next generation of accelerators, spread over an area measured in kilometers, will require femtosecond-level synchronization of RF cavities, lasers, photoinjectors and diagnostic devices. Phase-stabilized optical fiber is well- suited for this, along with its immunity to electrical inter- ference, gigaHertz bandwidth, low loss and easy installa- tion in wireways. Commodity-level single-mode glass fiber, such as Corn- ing SMF-28, optimized for 1300-1550 nm wavelength, ex- hibits about the same phase velocity dependence on tem- perature as copper, although the mechanism is temperature dependence of the glass dielectric constant. All fiber com- ponents used are inexpensive off-the-shelf devices devel- oped for the telecommunications industry. The stabilization system developed at LBNL uses the technique of frequency-offset interferometry. As shown in Figure 1, the optical output of a 1550 nm CW laser is split between the short arm of a Michaelson interferometer and the long fiber to a remote receiver. The short arm of the interferometer is temperature-controlled to a variation of less than 0.01 C. At the end of the long fiber arm, an acousto-optical modulator (AOM) excited at 50 MHz up-shifts the 195 THz laser frequency by 50 MHz, where it is then reflected by a 50% Faraday rotator mirror. The shift of the laser frequency is phase coherent with the 50 MHz RF drive of the AOM. The reflected laser signal is again upshifted by 50 MHz by its return passage through the AOM, resulting in a 100 MHz total frequency shift where it returns along the long fiber, and combines with a sample of the original laser frequency from the interfer- ometer short arm in the splitter. The variation of the phase length of the fiber is phase coherent to the phase variation of the 100 MHz upshifted return signal. The original laser frequency, from the short ∗ This work is supported by the Director, Office of Science, U.S. Dept. of Energy under Contract No. DE-AC02-05CH11231 arm and the upshifted laser frequency are transmitted along a second fiber, the error signal fiber, to a photodiode at the stabilizer where they produce a 100 MHz beat note which is compared to the 100 MHz reference oscillator. Any change in the phase length of the long fiber is reflected in a phase shift of the 50 MHz signal to the AOM, derived from the 100 MHz reference oscillator, which adds or subtracts the same number of optical cycles in the AOM. The result- ing error signal is integrated, the integral representing the change of phase length of the fiber, which shifts the phase of the 50 MHz drive signal to the AOM. All these func- tion are combined in a single chip field-programmable gate array (FPGA) controller. Stabilizer/Receiver Sender Splitter 1550 nm Laser Faraday Rotator Mirror Stabilized Fiber AOM Reference Oscillator Error Fiber 50% Faraday Rotator Mirror divide by 2 Mixer Integrator Phase Shifter Figure 1: Frequency-Offset Stabilizer Configuration The fiber that carries the error signal to the stabilizer needs no stabilization itself, as it is providing optical phase information down-converted to 100 MHz. The 100 MHz beat note is phase-coherent with changes in optical phase in the stabilized fiber, but the frequency ratio of 195 THz (1550 nm wavelength) and 100 MHz is 2 × 10 6 , so a 1 nanosecond change in the error signal fiber produces an er- ror of only 0.5 femtosecond to the correction. In previous implementations of the stabilizer, the AOM phase was fixed, and mechanical phase shifters (piezo and motor-driven optical trombone) were placed in series with the stabilized fiber. These suffered from a finite range of correction and the usual problems with devices using mov- ing parts. The transition to an all-electronic system sig- nificantly simplified the system with a smaller parts count, essentially unlimited range of correction, and increased re- liability. The frequency reference for the system is the 195 THz laser frequency itself, which must be stabilized to 1 part in 10 9 for the system to provide 1 femtosecond stability with with variations of the long fiber of 1 nanosecond. The CW laser is stabilized by taking a sample of the laser, doubling its frequency, and locking it to a saturated absorption line in a Rubidium cell using a Pound-Drever-Hall (PDH) [2] stabilizer configuration. To verify its operation, two inde- pendent PDH-stabilized 1550 nm lasers were beat against each other with measured drifts in the 200 kHz range." @default.
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• W1605311186 date "2008-11-26" @default.
• W1605311186 modified "2023-09-26" @default.
• W1605311186 title "A FEMTOSECOND-LEVEL FIBER-OPTICS TIMING DISTRIBUTION SYSTEM USING FREQUENCY-OFFSET INTERFEROMETRY" @default.
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