Kevin W. Holman

Remote transfer of ultrastable frequency references via fiber networks

Three distinct techniques exist for distributing an ultrastable frequency reference over optical fibers. For the distribution of a microwave frequency reference, an amplitude-modulated continuous wave (cw) laser can be used. Over kilometer-scale lengths this approach provides an instability at 1 s of approximately 3 x 10(-14) without stabilization of the fiber-induced noise and approximately 1 x 10(-14) with active noise cancellation. An optical frequency reference can be transferred by directly transmitting a stabilized cw laser over fiber and then disseminated to other optical and microwave regions using an optical frequency comb. This provides an instability at 1 s of 2 x 10(-14) without active noise cancellation and 3 x 10(-15) with active noise cancellation [Recent results reduce the instability at 1 s to 6 x 10(-18).] Finally, microwave and optical frequency references can be simultaneously transmitted using an optical frequency comb, and we expect the optical transfer to be similar in performance to the cw optical frequency transfer. The instability at 1 s for transfer of a microwave frequency reference with the comb is approximately 3 x 10(-14) without active noise cancellation and <7 x 10(-15) with active stabilization. The comb can also distribute a microwave frequency reference with root-mean-square timing jitter below 16 fs integrated over the Nyquist bandwidth of the pulse train (approximately 50 MHz) when high-bandwidth active noise cancellation is employed, which is important for remote synchronization applications.

Delivery of high stability optical and microwave frequency standards over an optical fiber network

Optical and radio frequency standards located in JILA and National Institute of Standards and Technology (NIST) laboratories have been connected through a 3.45-km optical fiber link. An optical frequency standard based on an iodine-stabilized Nd:YAG laser at 1064 nm (with an instability of ∼4×10-14 at 1 s) has been transferred from JILA to NIST and simultaneously measured in both laboratories. In parallel, a hydrogen maser-based radio frequency standard (with an instability of ∼2.4×10-13 at 1 s) is transferred from NIST to JILA. Comparison between these frequency standards is made possible by the use of femtosecond frequency combs in both laboratories. The degradation of the optical and rf standards that are due to the instability in the transmission channel has been measured. Active noise cancellation is demonstrated to improve the transfer stability of the fiber link.

Mode-locked fiber laser frequency-controlled with an intracavity electro-optic modulator

We demonstrate a mode-locked, erbium-doped fiber laser with its repetition frequency synchronized to a second fiber laser via an intracavity electro-optic modulator (EOM). With servo control from the EOM (bandwidth approximately 230 kHz) and a slower speed intracavity piezoelectric transducer (resonance at approximately 20 kHz), we demonstrate stabilization of the repetition frequency with an in-loop rms timing jitter of 10 fs, integrated over a bandwidth from 1 Hz to 100 kHz. This represents what is to our knowledge the first time an EOM has been introduced inside a mode-locked laser cavity for fast servo action and the lowest timing jitter reported for a mode-locked fiber laser.

Remote transfer of a high-stability and ultralow-jitter timing signal

Transfer of a high-stability and ultralow-jitter timing signal through a fiber network via a mode-locked fiber laser is demonstrated. With active cancellation of the fiber-transmission noise, the fractional instability for transfer of a radio-frequency signal through a 6.9- (4.5-) km round-trip installed (laboratory-based) fiber network is below 9(7) x 10(-15) tau(-1/2) for an averaging time tau > or = 1 s, limited by the noise floor of the frequency-counting system. The noise cancellation reduces the rms timing jitter, integrated over a bandwidth from 1 Hz to 100 kHz, to 37 (20) fs for the installed (laboratory-based) fiber network, representing what is to our knowledge the lowest reported jitter for transfer of a timing signal over kilometer-scale distances using an installed (laboratory-based) optical-fiber network.

Precise frequency transfer through a fiber network by use of 1.5-µm mode-locked sources

We report the precise transfer of radio-frequency signals by use of the pulse repetition frequency of mode-locked laser sources at 1.5 microm transmitting through a fiber network. The passive transfer instability through a 6.9-km fiber is below 3 x 10(-14) at 1 s, which is comparable with the optical carrier-frequency transfer of a narrow-linewidth cw laser. The instability of the measurement system is below 7 x 10(-15) at 1 s. It is noted that the pulsed mode of operation offers almost an order-of-magnitude improvement in stability at 1 s over that with a sinusoidal amplitude modulation on an optical carrier.

Detailed studies and control of intensity-related dynamics of femtosecond frequency combs from mode-locked Ti:sapphire lasers

The authors have conducted detailed experimental investigations of the intensity-related dynamics of the pulse repetition and carrier-envelope offset frequencies of passively mode-locked Ti:sapphire lasers. Two different laser systems utilizing different intracavity dispersion compensation schemes are used in this study. Theoretical interpretations agree well with experimental data, indicating that intensity-related spectral shifts, coupled with the cavity group-delay dispersion, are important in understanding the dynamics of the frequency comb. Minimization of spectral shifts or the magnitude of group-delay dispersion leads to minimization of the intensity dependence of the femtosecond comb.

Precision spectroscopy and density-dependent frequency shifts in ultracold Sr.

By varying the density of an ultracold 88Sr sample from 10(9) to>10(12) cm(-3), we make the first definitive measurement of the density-related frequency shift and linewidth broadening of the 1S0-3P1 optical clock transition in an alkaline earth system. In addition, we report the most accurate measurement to date of the 88Sr 1S0-3P1 optical clock transition frequency. Including a detailed analysis of systematic errors, the frequency is [434 829 121 312 334+/-20(stat)+/-33(syst)] Hz.

Ultralow-jitter, 1550-nm mode-locked semiconductor laser synchronized to a visible optical frequency standard

Using high-bandwidth feedback, we have synchronized the pulse train from a mode-locked semiconductor laser to an external optical atomic clock signal and achieved what is to our knowledge the lowest timing jitter to date (22 fs, integrated from 1 Hz to 100 MHz) for such devices. The performance is limited by the intrinsic noise of the phase detector used for timing-jitter measurement. We expect such a highly stable device to play an important role in fiber-network-based precise time/frequency distribution.

Orthogonal control of the frequency comb dynamics of a mode-locked laser diode

We have performed detailed studies on the dynamics of a frequency comb produced by a mode-locked laser diode (MLLD). Orthogonal control of the pulse repetition rate and the pulse-to-pulse carrier-envelope phase slippage is achieved by appropriate combinations of the respective error signals to actuate the diode injection current and the saturable absorber bias voltage. Phase coherence is established between the MLLD at 1550 nm and a 775-nm mode-locked Ti:sapphire laser working as part of an optical atomic clock.

Optical phase coherent locking of a 1550-nm mode-locked source to an optical atomic clock's fs comb

We have obtained optical phase coherence between a 1550-nm mode-locked diode laser and a 800-nm femtosecond frequency comb used in an optical atomic clock.