Analysis and filtering of phase noise in an optical frequency comb at the quantum limit to improve timing measurements

Abstract

It is shown that the sensitivity of a highly sensitive homodyne timing measurement scheme with femtosecond (fs) lasers [1] is limited by carrier-envelope-phase (CEO) noise. We describe the use of a broadband passive cavity to analyze the phase noise of a Ti:Sapph oscillator relative to the standard quantum limit. This cavity also filters lowest levels of classical noise at sidebands above 100 kHz detection frequency. Leading to quantum limited CEO-phase noise at µs-timescales, it can improve the sensitivity of the homodyne pulse timing measurement by 2 orders of magnitude. c 2014 Optical Society of America OCIS codes: 140.0140, 270.0270Introduction. Femtosecond optical frequency combs have revolutionized optical metrology [2] given their intrinsic frequency comb structure and the ability to measure and lock the phases of these frequencies. Recent experiments have demonstrated that they can be extended to massively parallel optical spectroscopy, using a dual comb configuration and an optical cavity to enhance the recorded signal [3]. Their dual time and frequency structure also make them ideal candidate for ranging or clock synchronization [4]. This measurement has been proven to be optimal when used in a homodyne configuration with a pulse shaped reference beam [1]. The underlying concept of projection on the temporal mode carrying the information can be extended to general parameter estimation [5].In this context of very high sensitivity metrology, measurements are limited by the low-level intrinsic noise of the laser, being of classical or quantum nature. Although the noise of Ti:Sapph oscillators has been characterized extensively relative to the carrier [6], no data are available relative to the quantum limit. We demonstrate here that it is possible to efficiently measure and even filter noise at such low levels. To this aim we combine shot noise resolving intensity noise detection and the filtering properties of a passive cavity.The paper is organized in the following way: The carrier-envelope-offset (CEO) phase noise, and for completeness the amplitude fluctuations of a commercial Ti:Sapph laser are determined relative to their common quantum limit. A broadband passive optical cavity is then proposed to filter the remaining fluctuations of the CEO phase. Besides filtering, it allows the detection of phase noise down to the quantum limit. The resulting realistic sensitivities for the homodyne timing measurement scheme [1] are discussed at the end of this paper.Theoretical concept. We consider a train of f s pulses generated by a commercial mode-locked Ti:Sapph oscillator (Fig.1). It can be described as a superposition of equally spaced monochromatic modes [7] of frequen-

Modal analysis for noise characterization and propagation in a femtosecond oscillator

“…As both fields originate from the same OFC source, a high finesse ( 1200) Fabry-Perot cavity is introduced in the reference arm. It acts on both optical quadratures as a lowpass filter with a cut-off frequency of f c 125 kHz [19]. This effectively decouples both arms of the interferometer, such that the noise measured at the output predominately originates from the signal beam for analysis frequencies higher than 2 f c [16].…”

Spectral Noise Correlations of an Ultrafast Frequency Comb

“…The cavity has a free spectral range equal to that of the pulse train repetition rate (i.e., 76MHz), and its length is locked with a Pound-Drever-Hall scheme. Sideband fluctuations of the reference field transmitted by the cavity are strongly attenuated for frequencies higher than the cutoff frequency of ∼ 90kHz [19], whereas the initial fluctuations persist in the signal arm of the interferometer [28]. These two fields are recombined on a 50:50 beamsplitter and then detected with a pair of balanced silicon photodiodes.…”