Eur. Phys. J. D (2008) DOI: 10.1140/epjd/e2008-00012-8
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Diode-pumped Yb:KYW femtosecond laser frequency comb with stabilized carrier-envelope offset frequency S.A. Meyer1,2 , J.A. Squier3 , and S.A. Diddams2,a 1 2 3
Department of Physics, University of Colorado, 2000 Colorado Ave., Boulder, CO 80309, USA National Institute of Standards and Technology, 325 Broadway MS 847, Boulder, CO 80305, USA The Center for Microintegrated Optics for Advanced Bioimaging and Control, Colorado School of Mines, 1532 Illinois St., Golden, CO 80401, USA Received 13 September 2007 / Received in final form 20 November 2007 c EDP Sciences, Societ` Published online 23 January 2008 – a Italiana di Fisica, Springer-Verlag 2008 Abstract. We describe the detection and stabilization of the carrier envelope offset (CEO) frequency of a diode-pumped Yb:KYW (ytterbium-doped potassium yttrium tungstate) femtosecond oscillator that is spectrally centered at 1033 nm. The system consists of a diode-pumped, passively mode-locked femtosecond laser that produces 290 fs pulses at a repetition rate of 160 MHz. These pulses are first amplified, spectrally broadened and temporally compressed to 80 fs, and then launched into microstructured fiber to produce an octave-spanning spectrum. An f − 2f nonlinear interferometer is employed with the broadened spectrum to detect and stabilize the CEO frequency through feedback to the pump laser current. These results demonstrate that such a Yb-doped tungstate laser can provide an efficient, compact, high-repetition-rate optical frequency comb with coverage from 650–1450 nm. PACS. 42.65.Re Ultrafast processes; optical pulse generation and pulse compression – 42.55.Xi Diodepumped lasers
1 Introduction There is little doubt of the significant impact that femtosecond laser optical frequency combs have already made in the areas of precision optical metrology, optical clocks, and precision spectroscopy [1,2]. However, the pursuit of the best laboratory implementation is a continuous one, and will likely be application dependent. Interestingly, among the hundreds of femtosecond lasers that have been described in the literature, only a handful have been self-referenced and applied to optical frequency metrology experiments. From a broader perspective, this apparent under-sampling of the available resources would suggest there is likely something to be gained by exploring new femtosecond laser systems for use in frequency metrologyand spectroscopy-related experiments. From a more focused perspective, there are certain advantages and drawbacks to existing frequency comb systems that one would like to optimize for each particular application (e.g. size, cost, low repetition rate, excess noise, transportability, etc.). In this article, we describe a promising frequency comb system based on an ytterbium-doped potassium yttrium tungstate femtosecond laser (Yb:KY(WO4 )2 or more coma
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monly Yb:KYW). The frequency comb consists of a directdiode-pumped femtosecond oscillator, a low-power fiber amplifier, continuum generation in microstructured fiber, and subsequent f −2f self-referencing leading to control of the carrier-envelope offset frequency [3,4]. As will be discussed in the following, this system has the particular advantage of being compact, low cost, robust, low power, and power efficient, which was part of our motivation for developing it. Our initial results show relatively low levels of frequency noise on the generated frequency comb, indicating that with reasonable improvements to the servo control the frequency noise properties of a Yb:KYW frequency comb should be reducible to levels already achieved with other systems. Two of the most widely used femtosecond laser frequency combs are based on Ti:sapphire and Er-doped fiber laser oscillators. For frequency metrology, Ti:sapphire frequency combs have the principle advantages of high repetition rates (up to 6 GHz [5]), very low residual frequency noise [6–9], and octave-spanning spectra directly from the laser [10–12]. The main disadvantages of the Ti:sapphire frequency comb are its bulk-optic design, requiring careful alignment and clean operating conditions, and the relatively large, expensive, and inefficient pump source at 532 nm. These are not fundamental limitations, but rather practical design constraints that may (or may
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not) be an issue in various applications. For example, a Ti:sapphire based comb can also operate with a low-power (1 W), small-footprint pump source [13]. Frequency combs based on Er-doped fiber lasers offer an attractive alternative to Ti:sapphire [14–17]. Such femtosecond fiber lasers are pumped directly with diodes at 980 nm or 1490 nm, making them efficient and compact. Emission at 1550 nm makes them well suited for fiber optic transmission, and the use of integrated fiber components minimizes alignment issues while taking advantage of the wide variety of robust and low-cost components designed for telecommunications systems. Moreover, improved active stabilization techniques have reduced the close-to-carrier frequency noise to similar levels achieved with Ti:sapphire lasers [18,19]. However, the main drawbacks of fiber frequency combs are the need for external amplification, relatively low repetition rate (which has thus far been limited to a few hundred megahertz [20]), and the reduced spectral overlap with several promising optical frequency standards. For some applications, the predicted higher noisefloor [21], associated with the lower cavity quality factor (Q) and amplified spontaneous emission in the laser and external fiber amplifier, could also be detrimental. While there has been extensive development of femtosecond lasers based on Yb-doped crystals, fibers and glasses, to our knowledge the application and properties of these systems as frequency comb sources has been investigated in only a few cases [22,23]. Here we focus our attention on Yb:KYW, but one could expect that similar arguments would apply to many of the other Yb-doped hosts. As a femtosecond laser source, Yb-doped hosts have much to offer [24]. Importantly, they can be directly pumped by common high-brightness laser diodes. The quantum defect is low, which leads to higher efficiencies and lower thermal loads on the crystal. The low lying states of the Yb ion have a relatively simple energy structure that avoids problems such as concentration quenching and excited state absorption, thus permitting high doping concentrations in glasses and crystalline hosts. Passive mode-locking via Kerr Lens Modelocking (KLM) [25,26] and/or a semiconductor saturable absorber mirror (SESAM) [27–29] is now routine in such Yb lasers, making for robust operation. Though high power versions have been demonstrated [30], our goals of compactness, efficiency and high repetition rate led us to develop a system pumped with single-mode fiber coupled, low-power (
