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3-5 Development of an Ultra-Narrow Line-Width Clock Laser

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3-5 Development of an Ultra-Narrow Line-Width Clock Laser 3-5 Development of an Ultra-Narrow Line-Width Clock Laser LI Ying, NAGANO Shigeo, MATSUBARA Kensuke, KOJIMA Reiko, KUMAGAI Motohiro, ITO Hiroyuki, KOYAMA Yasuhiro, and HOSOKAWA Mizuhiko An optical lattice clock and a single ion optical clock are being developed in National Institute of Information and Communications Technology (NICT). Diode lasers are used for the develop- ment of extremely narrow linewidth clock lasers for optical frequency standards. Using the Pound- Drever-Hall technique, the required reduction of linewidth was achieved by locking the laser to an ultrahigh-fi nesse ultralow-expansion glass (ULE) reference cavity, which is set in the high vacuum chamber with a constant temperature and isolated against environmental noise and vibration. As a result, the laser linewidth is decreased down to several Hz. The Allan deviation is less than 4×10−15 at an averaging time over 100 s. A vibration-insensitive optical cavity has been designed, aiming the linewidth below 1 Hz. In this chapter, we report the present status of development of the clock lasers at NICT. Keywords Laser frequency stabilization, Diode laser, Optical reference cavity, Optical clock, Optical frequency standard 1 Introduction an uncertainty of 10−17 or less is divided into two types. The fi rst is the “optical lattice The second, the current time and frequency clock” [4] [5], neutral atoms are trap in the opti- standard unit, is defi ned according to the hy- cal lattice “magic wave length” and the second perfi ne structure transition of cesium atoms by is the “single ion clock” [6], single ion is con- the General Conference of Weights and Mea- fi ned to the Lamb-Dick regime. The extremely sures in 1967 and its accuracy is reached to 4 × narrow spectrum optical transitions are used in −16 10 recently [1]. However, it has become diffi - both optical clocks as clock transitions. Nation- cult to achieve a higher accuracy since the al Institute of Information and Communica- measuring time is long in order to reach this tions Technology (NICT) is currently develop- accuracy. Since the development of the coher- ing “a strontium (Sr) optical lattice clock” and + ent frequency comb generators using mode- “a single calcium (Ca ) ion optical clock” [7] [8]. 1 3 locked femtosecond lasers [2] [3], novel frequen- A S0– P0 spin forbidden transition (698 nm cy standards based on narrow optical transition wavelength, 10 mHz natural linewidth) and A 2 2 frequencies are being developed rapidly as a S1/2– D5/2 electric quadrupole transition (729 result of high optical oscillation frequency be- nm wavelength, 0.2 Hz natural linewidth) are ing capable of being measured at 4 – 5 digits or used as clock transitions of Sr atoms and a sin- more than the oscillation frequency of the mi- gle Ca+ ion respectively. Extremely narrow cro-wave range. linewidth and ultrastable clock lasers play a The development of an optical clock with signifi cant role in recent development of opti- LI Ying et al. 175 cal frequency standards, since the laser line- width often limits the accuracy of frequency measurements. The clock laser of the Sr optical lattice clock utilizes a diode laser. For spectroscopy of 2 2 + S1/2 – D5/2 electric quadrupole transition in Ca ions, a Ti:sapphire laser is a representative light source [9] until now. With the progress in quan- tum-well semi-conductor lasers in recent years, a diode laser of 730 nm can be manufactured. Since the saturated absorption power density is only 5 × 10−7 mW/cm2 for Ca+ ion, a low ener- gy consumption, compact size and robust struc- ture diode laser is used. Research and development on the “Sr opti- cal lattice clock” clock laser is detailed in a Fig.1 Experimental confi guration of clock laser system separate paper [10]. However, here we will ex- + The paths of the laser beams and the electric plain the development of the “single Ca ion signals are denoted by thick lines and thin lines, optical clock” in detail. respectively. Acronyms are photodiode (PD), double-balanced mixer (DBM), polarization- 2 Clock laser stabilization beam splitter (PBS), electro-optic modulator (EOM), half wave plate (λ/2), quarter wave 2.1 The composition and control plate (λ/4), acousto-optic modulator (AOM), Polarizer (Pol.), Faraday Rotator (FR), and half system for the clock laser Mirror (HM). Figure 1 shows a schematic of the laser fre- quency stabilization. Up until now, we have as- sembled a Littman-Metcalf external cavity di- ode laser (ECDL) utilizing an antirefl ective MHz, and it is coupled to TEM00 mode of the (AR) coated laser diode (LD) (Toptical, 730 ultrahigh fi nesse ultralow expansion (ULE) nm, 5mW or 10mW) and developed a clock la- glass reference cavity in the high vacuum −7 ser [11]. However, in 2009, a single longitudinal chamber (10 Pa). The refl ected light from the mode laser diode (Opnext, HL7301MG) with a ULE reference cavity is detected by a Si PIN 730 nm band InGaAsP multi-quantum well photodiode (PD1). After demodulation of pho- structure, 75 mA low-current operation, 40 tocurrent in a double-balanced mixer, the low- mW output is manufactured. We antirefl ective frequency component is amplifi ed, integrated, (AR) coated it (by Koshin Kogaku Co., Ltd.) and fed back to the master laser via two chan- and assembled it as a 5 mW output ECDL. In nels of the electrical servo circuit: a slow feed- addition, as result of amplifying the output of back loop (~100 Hz) – driving the PZT of the the slave laser up to 40 mW by the injection ECDL mirror – adjusts the laser frequency to locked method and cleaning the spatial mode reference cavity resonance frequency. Fast fre- using polarization-maintaining signal mode quency fl uctuations are compensated by super- (PANDA) fi ber, we obtained a maximum 12 imposing the feedback current signal onto the mW TEM00 mode light at the exit end of the laser cathode. The total servo bandwidth reach- fi ber. Using the Pound-Drever-Hall (PDH) es to as large as 1 MHz. A portion of the stabi- method [12], we stabilize an extremely narrow lized laser beam is transmitted by a PANDA linewidth clock laser. A weak laser light of 100 fi ber of 10 m length to the optical comb that we μW is phase modulated by an electro-optic developed [13] for measuring and evaluating the modulator (EOM, Linos PM25) operating at 15 laser frequency. The remaining laser beam is 176 Journal of the National Institute of Information and Communications Technology Vol.57 Nos.3/4 2010 transmitted to a separate room by a PANDA fi - spheric pressure arises. In order to prevent this, ber of 40 m length, and is frequency shifted by it is necessary to insert the optical reference an acousto-optic modulator (AOM), then it was cavity into an ultrahigh vacuum to ensure that used to observe quantum jumps of the trapped fl uctuation of atmospheric pressure do not in- single Ca+ ion. fl uence the optical reference cavity. Next, changes in the cavity length caused 2.2 The impact of environmental by thermal expansion in the optical reference disturbance on the optical cavity must be considered. Changes in the cav- reference cavity and optical ity length due to temperature changes can be reference cavity control expressed by the Equation: Because the frequency of the clock laser is stabilized at the resonance frequency of the op- （2） tical reference cavity, the frequency stability of the optical reference cavity is the most impor- tant factor. where α is the thermal expansion coeffi - The Fabry-Perot optical reference cavity cient and ΔT is the temperature change. The consists of two mirrors that face each other thermal expansion coeffi cient α should be small with a spacer placed in between. When integral for the optical reference cavity materials. Sap- multiple of the beam’s half wavelength is equal phire is one of these materials. The temperature to cavity length, the frequency is the resonance of the sapphire is 3 – 4K and α is extremely frequency. Consequently, even a very small small at 10−11 K−1. However, it is not easy to change in the cavity length can change the res- cool the optical reference cavity to liquid heli- onance frequency. The cavity length and reso- um temperature. The ULE glass manufactured nance frequency changes are expressed in the by Corning and Zerodur manufactured by Equation: Schott are considered to be other alternatives. Although these α are 10−8 K−1, they have large differences that are the phenomena known as （1） creep or ageing drift. Creep refers to the very slow change in the glass crystal structure with where L is the optical path length, ΔL is the time. The ULE change is one digit smaller than change in the optical path length, ν0 is the reso- Zerodur. Furthermore, ULE smoothly changes nance frequency and Δν is the change in the but Zerodur repeats non-continuous changes to resonance frequency. If the wavelength is 729 change shape [14]‒[16]. ULE glass is often se- nm, the refraction index is 1 and the cavity lected for materials of optical cavities. Since length is 10 cm, the cavity frequency changes 4 the SiO2 base of ULE glass is doped with TiO2, Hz when the cavity length is changed by 1 fm. the polarity of thermal expansion coeffi cient In other words, in order to achieve a 1 Hz line- can be reversed at room temperature by the width, the optical reference cavity length must doping amount and homogenization. As a re- be controlled with an accuracy of less than 1 sult, if the zero-crossing temperature is found fm.
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