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A narrow linewidth diode laser system at 689 nm is realized by phase-locking an extended cavity diode laser to one tooth of a narrow linewidth optical frequency comb. The optical frequency comb is phase-locked to a narrow linewidth laser at 1064 nm, which is frequency stabilized to a high-finesse optical cavity. We demonstrate the magneto-optical trapping of Sr using an intercombination transition with the developed laser system.
©2012 Optical Society of America
1. Introduction
The physics of ultracold alkaline-earth-metal atoms have been extensively studied over the last decade. Recently, the quantum degenerate state was observed for 40Ca [1] and 84,86,87,88Sr [2–6]. The two valence electrons result in two term systems: a singlet and a triplet system. The spin-orbit coupling of these systems induces a weak intercombination transition. The intercombination transition has many interesting applications, including optical frequency standards [7–9], quantum computations [10], and low-loss optical Feshbach resonances [11]. An intercombination transition, even narrower than the single photon recoil shift also allows us to approach the photon recoil temperature using only the Doppler cooling technique [12–15]. To achieve the cooling limit, the cooling laser linewidth must be narrower than the transition linewidth. Most experiments employ a high-finesse optical cavity to reduce the laser linewidth [16]. However, highly reflective mirrors for the cavity may be difficult to prepare for every wavelength.
The recently developed optical frequency comb based on mode-locked erbium-doped fiber lasers (fiber combs) can be used as easily as a turnkey device and provides robust long-term operation [17–19]. A fiber comb is an excellent tool for controlling the laser frequency instead of a stable optical cavity and atomic or molecular transitions. For example; the frequency stability of the clock laser for Yb+ was transferred to another wavelength at 1.5 μm for frequency comparison [20]. A fiber comb was used to stabilize the laser frequency for the intercombination line magneto-optical trapping (MOT) of ytterbium atoms [21]. The linewidth of the 1S0-3P1 intercombination line of ytterbium is relatively large (182 kHz) due to its large spin-orbit coupling. The cooling laser linewidth of tens of kHz was sufficient to reach the cooling limit for the transition. A cooling laser with a much narrower linewidth is necessary for cooling atoms with a much narrower transition, such as Ca or Sr. Recently, the clock transition of Sr was observed and measured by using a light source at 698 nm with stability transfer from 729 nm [22]. The system, however, could not transfer the linewidth of the 729-nm laser to 698 nm due to the limited feedback bandwidth of the titanium sapphire laser of the comb. Therefore, a high finesse cavity was used to pre-stabilize the slave ECDL at 698 nm, besides a high finesse cavity for 729 nm.
In this paper, we demonstrate a narrow linewidth laser system by employing a “linewidth transfer method” using an optical frequency comb. One of the comb modes is phase-locked to an ultranarrow linewidth laser (master laser), and the carrier envelope offset frequency of the comb is stabilized by phase-locking the beat frequency to a stable microwave reference [23]. Consequently, the linewidths of all of the comb modes are, for practical purposes, reduced to that of the master laser. A slave laser at a desirable wavelength is phase-locked to the comb. In this system, the characteristics of the master laser such as linewidth and frequency stability are transferred to the slave laser. We employ the system to cool strontium atoms on the 1S0-3P1 intercombination line (7.6 kHz) and successfully achieve the near cooling limit temperature [12] for this transition. The system is directly applicable both for cooling a variety of atoms with a narrow transition [24] and for the easy preparation of a clock laser for almost all frequencies.
2. Experimental setup
Figure 1 is a schematic diagram of our experimental setup. We developed a Littrow configuration extended cavity diode laser (ECDL) with an anti-reflection-coated diode laser emitting at 689 nm. The free-running linewidth of the ECDL is approximately 300 kHz. The ECDL is temperature stabilized at 23.8 °C and generats 40 mW of output light with an injection current of 84 mA. A fraction of the laser power (1 mW) is used to measure the heterodyne beat (fcomb-ECDL) between the laser and a fiber comb. The remaining power is used for the magneto-optical trapping (MOT) of strontium using the 1S0-3P1 intercombination transition at 689 nm.
Fig. 1 Schematic diagram of experimental setup. ECLD, extended cavity laser diode; PPLN, periodically poled lithium niobate; S.A., spectrum analyzer; ULE cavity, ultra-low-expansion cavity; PDH lock, Pound-Drever-Hall lock; MOT, magneto-optical trap; EOM, electro-optic module. Local oscillators (LO) are microwave sources.
The oscillator of our fiber comb is an erbium-doped fiber based mode-locked laser with an intra-cavity electro-optic modulator (EOM) [23]. The repetition rate (frep) is approximately 43.4 MHz. The circulating power is partially coupled out of the cavity and distributed to several branches for different purposes, including one branch for detecting the carrier-envelope offset frequency (fCEO) of the comb. Each branch includes an erbium-doped fiber to amplify the power. One of the branches is used to detect a heterodyne beat with a master laser (1064-nm Nd:YAG laser). The master laser is frequency-stabilized to a high-finesse, ultra-low-expansion (ULE) cavity by using the Pound-Drever-Hall method. The linewidth of the master laser is approximately 2 Hz. The master laser frequency drift due to creep in the optical cavity spacer is about 20 kHz per day [25]. The heterodyne beat between the master laser and the m-th mode of the comb (fcomb-YAG) is phase-locked to a stable microwave reference by using the intra-cavity EOM. The frequency of the n-th mode of the comb is given by
where νYAG is the frequency of the Nd:YAG laser. As n/m is of the order of 1, the frequency characteristics of the master laser can be transferred to other frequency modes of the comb by stabilizing fCEO and fcomb-YAG. In addition, the relative linewidth of the comb is less than 30 mHz [23], which ensures high-quality linewidth transfer.The comb light of another branch is sent to a remote optical table, where the ECDL at 689 nm is located, to phase-lock the ECDL to the narrow-linewidth optical comb. The spectrum is broadened with a highly nonlinear fiber and covers 1368 nm. The comb modes around 1368 nm are frequency-doubled by using a periodically poled lithium niobate (PPLN) crystal. The output from the PPLN and the fraction of the ECDL output are mixed in a beam splitter, and a heterodyne beat note (fcomb-ECDL) is detected with a photo detector. The ECDL is phase-locked to one appropriate mode of the narrow-linewidth optical comb by employing direct feedback control of the injection current of the diode laser [26]. The beat note frequency fcomb-ECDL is monitored with a spectrum analyzer and measured with a dead-time-free frequency counter.
3. Experimental results
Figure 2(a) and (b) show the beat note (fcomb-ECDL) between the ECDL and the fiber comb modes observed with a spectrum analyzer when the ECDL was phase-locked to the fiber comb. The achieved servo bandwidth was evaluated to be about 1.3 MHz from the observed servo bumps (Fig. 2(a)). The observed linewidth of the beat signal was limited by the resolution bandwidth of the spectrum analyzer of 10 Hz, and the signal to noise ratio of the coherent δ-function peak was 65-70 dB, which indicates the tight phase-locking of the ECDL to the fiber laser (Fig. 2(b)).
Fig. 2 The beat spectra between the ECDL and the frequency-doubled fiber comb mode at 689 nm. The resolution bandwidths, and video bandwidths were (a) 30 and 1 kHz and (b) 10 and 10 Hz, respectively.
Figure 3(a) shows the variation in fcomb-ECDL measured with a dead-time-free frequency counter. The counter averaging time was 1 s. The Allan deviation was calculated from the measured beat frequency (Fig. 3(b)). Beat measurement without a cycle slip for 1800 s and the obtained 1/τ characteristic of the Allan deviation indicate successful phase locking between the ECDL and the fiber comb. In the present experiment, optical fibers were used to send the laser light between different optical tables. These fibers induce phase noise in light transmissions and increase the linewidth of the phase-locked ECDL to several 100 Hz. In future applications that require much less laser linewidth, fiber noise cancellation will be introduced and the Hz-level linewidth of the master laser will be completely transferred to the slave laser.
Fig. 3 (a) Variation of the measured beat frequency between the ECDL and the frequency doubled fiber comb mode. The data are the frequency deviation from the average value. (b) The Allan deviation calculated from the measured beat frequency.
The frequency drift of the master laser due to creep in the optical cavity spacer causes a small drift in the slave laser. This drift is compensated by slightly changing the locking frequency of the fcomb-ECDL in the experiment when it is necesarry.
We used the 689-nm laser system we developed for the intercombination line MOT (second stage MOT) of Sr. First, the atoms were magneto-optically trapped using the 1S0-1P1 transition at 461 nm (first stage MOT) [27]. About 106 atoms were pre-cooled to the mK level. After the first stage MOT, the 461-nm light was turned off. The 689-nm light was then turned on to transfer the atoms to the second stage MOT. To increase the velocity capture range, the cooling laser at 689 nm was frequency modulated using a double pass acousto-optical modulator to broaden the laser spectrum to several MHz. The laser detuning was 1.5 MHz at the broadband frequency cooling stage. After applying the broadband frequency cooling for 85 ms, we turned off the frequency modulation and decreased the detuning of the cooling laser (single frequency cooling). After 80 ms of single frequency cooling, the laser was turned off to free the atoms from MOT for measuring their temperature. We shone the 461-nm light to probe the expanding atomic cloud 7, 10, and 20 ms after releasing the atoms (Fig. 4(a) , 4(b) and 4(c), respectively). From the expansion speed of the atomic cloud, we evaluated the temperature of the atoms (solid circles in Fig. 4(d)). The dependence of the temperature on the cooling laser intensity (I) at the single frequency cooling stage is shown in Fig. 4(d) when the detuning of the laser frequency at the single frequency cooling stage was 260 kHz. The atom temperature follows the traditional Doppler limit of (solid line in Fig. 4(d)). is the power-broadened linewidth, where Γ, I, and Is are the natural linewidth of the atom, the cooling laser intensity, and the saturation intensity for the cooling transition. We observed a temperature as low as 325 nK. The observed temperature was near the quantum mechanically predicted value of half of the photon recoil temperature (broken line in Fig. 4(d)) [28]. The successful laser cooling in the intercombination line MOT confirms that the linewidth of the cooling laser is less than that of the intercombination line (7.6 kHz).
Fig. 4 CCD images of freely expanded atomic cloud after (a) 7 ms, (b) 20 ms, and (c) 30 ms. (d) Measured atom temperature as a function of the cooling laser intensity. The detuning was −100 kHz at the single frequency cooling stage. The solid and dashed lines show the traditional Doppler limit temperature and half of the recoil limit temperature, respectively.
4. Discussion and conclusion
The repetition rate of a frequency comb can be stabilized to either an optical reference or a microwave frequency source. Commercial microwave frequency sources are usually inexpensive, but have more frequency noise than an optical reference. To study the feasibility of the microwave reference, we employed a commercial microwave source instead of an ultranarrow linewidth laser as a frequency reference for the comb. We phase-locked the 689-nm ECDL to the comb and employed it for the second stage MOT. We observed that both the temperature and the number of obtained cooled atoms varied greatly in each shot at small cooling intensity (I/Is < 500). Furthermore, a momentum-space crystal was observed with a small cooling intensity, which indicates the cooling laser was blue-detuned in the measurement case [29]. The frequency jitter of the ECDL was several tens of kHz when the microwave reference comb was used. The frequency jitter originated from that of the microwave frequency source. With the weaker cooling light, the atoms are trapped between hard wall boundaries, where the Zeeman shift and the detuning of the cooling laser are in balance. Therefore, the cooling laser can be blue detuned momentarily by the frequency jitter.
In contrast with ref [22], in the present experiment, there was no need to use an optical cavity for making a slave laser since we used the fiber comb with an intracavity EOM, which has a sufficiently large feedback bandwidth. All the frequency characteristics of the master laser can be transferred to the fiber comb and from the comb to the slave laser by tight phase locking. Furthermore, we can realize multiple narrow linewidth lasers at arbitrary frequencies covered by the fiber comb and its harmonics with only a single highly stable cavity rather than multiple cavities.
In our future work, we will utilize the system described in this paper not only for the cooling laser for Sr, but also the clock lasers for Sr and Yb [30, 31] optical lattice clocks. We do not see any technical difficulties to realize this scheme using our multi-branch configuration fiber comb [23]. We plan to measure the frequency ratio of the two lattice clocks to investigate the constancy of fundamental constants and local position invariance [32]. The frequency noise of the clock lasers can be eliminated by synchronous measurement [33] when both lasers are locked to the same ultranarrow linewidth comb. With this method a frequency instability of 10−17 can be realized within seconds.
In conclusion, we have developed a narrow linewidth light source at 689 nm by phase-locking an ECDL to a narrow linewidth optical frequency comb. With this system, we have demonstrated the intercombination line magneto-optical trapping of Sr and observed a temperature as low as 325 nK. The light source operates continuously for more than 18000 s.
Acknowledgments
This research receives support from the JSPS through its FIRST Program.
References and links
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