We present the demonstration of a compact linearly polarized low noise narrow-linewidth single-frequency fiber laser at 1014 nm. The compact fiber laser is based on a 5-mm-long homemade Yb3+-doped phosphate fiber. Over 164 mW stable continuous-wave single transverse and longitudinal mode lasing at 1014 nm has been achieved. The measured relative intensity noise is less than −135 dB/Hz at frequencies of over 2.5 MHz. The signal-to-noise ratio of the laser is larger than 70 dB, and the linewidth is less than 7 kHz, while the obtained linear polarization extinction ratio is higher than 30 dB.
© 2013 OSA
High-resolution spectroscopy of ultra-narrow inter-combination transitions, 1S0↔3P0 and 1S0↔3P2, in two-electron alkaline-earth metal like ytterbium, have attracted much attention for its application potential in optical lattice clocks (OLCs) [1–3], deep space detection , fundamental physics  and quantum computation .
OLCs based on the 1S0↔3P0 transition of Yb atoms have been pursued to realize ultra precision time and frequency metrology with a fractional uncertainty of less than 10−16, which exceed microwave atomic clocks in accuracy [1–3]. Benefitting from a radioactive lifetime (~15s) comparable to that of the 3P0 state (~20s) and a lower blackbody radiation (BBR) Stark shift, the ultra-narrow magnetic quadrupole 1S0↔3P2 transition has great potential . Moreover, it is concluded that the stability of the OLCs is limited by the external perturbation and the linewidth of the clock laser, rather than the natural linewidth of the transition . The magnetic field insensitive 1S0↔3P2 (m = 0) transition could be a prime candidate of the next-generation time and frequency standards for its robustness against external perturbation.
To excite the Yb atoms from ground state to the meta-stable 3P2 state, a narrow linewidth 507 nm clock laser must be used. In previous works, laser emission around 507 nm is achieved by means of, e.g., a frequency doubled Yb:YAG disk laser , frequency doubled single-frequency diode lasers [10,11]. Among these demonstrations, the narrowest linewidth is about 100 kHz . To further suppress the infrared laser linewidth, exhausted efforts including the Pound–Drever–Hall technique had been made to reduce the linewidth of the 1014 nm laser-diode based system to below 1 kHz . Due to the lack of suitable narrow linewidth laser source, only a few experiments on the 1S0↔3P2 transition were demonstrated [6,11,12], hence it would be rash to draw a careless conclusion about the linewidth requirement of the experiments. However, similar experiments on the 1S0↔3P0 transition of Yb atoms suggested that infrared sources with a laser linewidth of ~10 kHz will facilitate further frequency stabilization of the yellow (578 nm) laser [13,14]. Thus, it is desirable to develop a new type of narrow linewidth 1014 nm laser sources.
In this paper, we present a 1014 nm compact linearly polarized low-noise narrow-linewidth single-frequency fiber laser based on a 5-mm-long Yb3+-doped phosphate fiber. Over 164 mW laser output with an intrinsic laser linewidth of < 7 kHz and a signal-to-noise ratio (SNR) of >70 dB is achieved. A stable linearly polarized laser output with a linear polarization extinction ratio (LPER) of >30 dB is obtained.
2. Experimental setup
Figure 1 shows the experimental setup of the compact low-noise narrow-linewidth fiber laser at 1014 nm. The laser cavity is constructed on a polarization-maintaining fiber Bragg grating (PM-FBG) and a wideband fiber Bragg grating (WB-FBG) that are fusion spliced respectively to the end facets of a 5-mm-long Yb3+-doped phosphate gain fiber. The PM-FBG is written in a single mode PM fiber with a 3 dB bandwidth of 0.08 nm and a reflectivity of 60% at 1014.75 nm. Due to the birefringence of the PM fiber, the reflecting spectrum of the PM-FBG splits into two reflection peaks with central wavelengths corresponding to the fast and slow axes of the PM fiber. The WB-FBG is irradiated with a 3 dB bandwidth of 0.4 nm and a reflectivity of >99.5% at signal wavelength. This pair of FBGs is specially chosen so that only the reflection peak corresponds to the slow axis of the PM-FBG would fall into the center of the WB-FBG. So the state of polarization (SOP) of the lasing signal could be locked at the slow axis of the PM-FBG and the laser output with stable linear polarization could be achieved. The resonator cavity was assembled into a copper tube, which was temperature-controlled by a cooling system with a resolution of 0.02°C. The Yb3+-doped phosphate glass fiber is drawn using a fiber drawing tower through a phosphate glass perform fabricated by the rod-in-tube technique . 15.2 wt% Yb3+ ions were doped uniformly in the core region. The characteristics of the Yb3+-doped phosphate glass fiber can be found in .
As shown in Fig. 1, the effective length of the resonator includes the 5-mm-long phosphate fiber and half of the 15-mm-long PM-FBG. It is less than 20 mm, giving a longitudinal mode spacing of >5 GHz. The NB-FBG has a reflection bandwidth of less than 13.3 GHz. With a proper temperature control, the laser will operate in a single-frequency mode without mode hop and mode competition phenomena. The fiber laser is pumped with a 976 nm laser diode (LD) through a 980/1014 nm polarization-maintaining wavelength division multiplexer (PM-WDM).
3. Results and discussion
Laser emission at 1014.75 nm is recorded with a resolution bandwidth of 0.1 nm by an optical spectrum analyzer (OSA), as shown in Fig. 2(a) . A signal-to-noise ratio (SNR) of >70 dB is obtained. The lasing wavelength of the fiber laser can be thermally tuned by a thermoelectric temperature controller (TEC). Figure 2(a) shows the laser output power at 1014 nm versus the pump power. The lasing threshold is around 11 mW. When the pump power is above the threshold, the laser output power enhances linearly. An output power of 164.3 mW is achieved at pump power of 760 mW. No output power saturation is observed. The slope efficiency of the fiber laser is measured to be 21.9%. The low slope efficiency is a result of the low net gain coefficient of the gain fiber at 1014nm. The stabilities of the output power at 164 mW in 2 hours are investigated, as shown in Fig. 2(b). The power instability of < ± 1% of the average power is observed, which is mainly caused by the minute fluctuations in pump power and the small changes in ambient temperature.
The single frequency characteristic is confirmed by a scanning Fabry-Pérot interferometer (FSR = 1.5 GHz, finesse = 200), as shown in Fig. 3(a) . With a proper temperature control, the fiber laser operates stably in a single frequency without mode hop and mode competition. The SOP of the fiber laser is measured by a polarization analyzer and the result is visualized on a Poincaré sphere in Fig. 3(b). A stable linearly polarized laser output with a degree of polarization (DOP) of 0.998 is confirmed, from which an LPER of >30 dB is deduced.
The relative intensity noise (RIN) of the fiber laser is measured using a resolution bandwidth of 3.1 kHz, as shown in Fig. 4 . As shown by the inset of Fig. 4, at low frequencies, the RIN decreases from –120 dB/Hz to –130 dB/Hz. The rise of RIN at low frequencies is mainly caused by ambient acoustics and vibrations. A relaxation oscillation frequency peak of −120 dB/Hz is observed at around 2.1 MHz. The RIN stabilizes at −135 dB/Hz for frequencies above 2.5 MHz. No other noise components are observed for frequencies above 2.5 MHz.
To further investigate the phase noise characteristics of the fiber laser, the laser linewidth is measured by a self-heterodyne method using a 10 km fiber-delay. The sweep time of the measurement is about 0.12 s at a resolution bandwidth of 100 Hz. The heterodyne signal is stable. As shown in Fig. 5 , the oscillatory shape of the heterodyne signal is an evidence of the long coherence length of the fiber laser. It is 34 kHz with −20 dB from the peak, which indicates the measured laser linewidth is approximately 1.7 kHz FWHM. The linewidth resolution of the self-heterodyne measurement with a 10 km fiber delay is about 6.6 kHz, so the laser linewidth is affirmed to be less than 7 kHz.
In conclusion, a compact linearly polarized low noise narrow-linewidth single-frequency fiber laser at 1014 nm is demonstrated. 164.3 mW stable continuous-wave single transverse and longitudinal mode lasing at 1014nm has been achieved. A laser linewidth of less than 7 kHz and a SNR of higher than 70 dB is obtained. A stable linearly polarized laser output with an LPER of >30 dB is attained. Considering similar experiments on the 1S0↔3P0 transition of Yb atoms, a laser linewidth of <7 kHz would facilitate further frequency stabilization of the 507 nm laser. It is a prominent candidate for the second harmonic generation of 507 nm laser signal used in the experiments on the 1S0↔3P2 transition of Yb atoms.
The authors would like to acknowledge support from the China State 863 Hi-tech Program (2012AA041203, 2011AA030203), the National Natural Science Foundation of China (NSFC) (11174085, U0934001, and 60977060), the Guangdong Province and Hong Kong Invite Public Bidding Program (TC10BH07-1), the Science and Technology Project of Guangdong (2011B090400055), and the Fundamental Research Funds for the Central Universities (2012ZZ0002).
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