1. Introduction

High power, diffraction-limited and continuous-wave (CW) single frequency 780 nm lasers have received significant attention since they play an increasingly important role in atomic physics and quantum optics. Such 780 nm lasers are applied to cooling and manipulating rubidium atoms [1], frequency standard [2], etc. The 780 nm lasers with high output power, narrow linewidth are highly demanded.

Many efforts have been made to develop high power, CW 780 nm laser sources with narrow linewidth and good beam quality. GaAs diode lasers with tapered amplifiers and Ti: sapphire laser are commonly used to provide narrow linewidth, frequency stable laser at 780 nm [3,4]. The available tapered diode lasers and Ti: sapphire laser can provide suitable output power at 780 nm. However, tapered diode lasers have poor output beam quality and Ti: sapphire based system is often costly and has high maintenance cost. Efficient external-cavity single-pass frequency doubling of 1560 nm EDFA via PPLN crystals is another way to generate 780 nm laser, which is considered to be simple, compact, and robust laser systems for laser cooling and trapping. Compared with the 780 nm GaAs diode laser, the frequency doubling of a high power 1560 nm fiber amplifier has much better beam quality and power scaling potential. In addition, the EDFA frequency doubling scheme is more compact and cost-effective compared with Ti: sapphire laser.

Mugnier et al. achieved an up to 1.8 W fiber laser source at 780 nm with single pass second harmonic generation (SHG) efficiency of 15% [5]. Generally speaking, the single pass frequency doubling has a poor SHG conversion efficiency at low fundamental light power. In order to enhance the conversion efficiency, some researchers have adopted the cascade-crystal configuration or cavity-enhancement configuration. Thompson et al. have achieved the generation of over 900 mW 780 nm laser by single pass frequency doubling in a cascade of two PPLN crystals, with an efficiencies of 4.6 mW/W²-cm for crystal one and two separately, and 5.6 mW/W²-cm for the two crystal cascade [6]. Feng et al. realized a 670 mW CW 780 nm laser system by using external cavity-enhanced SHG of an EDFA with a maximum second-harmonic conversion efficiency of 58% [7]. Ge et al. have achieved a CW 780 nm laser with an output power of 1.5 W by ring-cavity-enhanced frequency doubling of an EDFA with a doubling efficiency of 73% [8]. Although cavity-enhanced frequency doubling can greatly increase conversion efficiency, precise cavity length control and special cavity matching made the laser rather complicated. Another way to enhance the SHG conversion efficiency is by pumping the crystal at high fundamental light power. Sané et al. have presented an over 11 W 780 nm laser based on single-pass frequency doubling of 30 W 1560 nm fiber laser with 36% conversion efficiency [9]. Chiow et al. have demonstrated a peak power of 43 W 780 nm light by two cascading PPLN crystals with an efficiency of 66% [10]. However, the laser didn’t work in the CW condition.

For high power 1560 nm EDFA, cladding pumped erbium-ytterbium co-doped fiber is the most common configuration. In this scheme, Yang et al. have obtained a one-stage all-fiber 1560 nm single-frequency master-oscillator power amplifier (MOPA) laser with the output power of 10.9 W [11]. However, the large quantum defect and troublesome parasitic lasing at the ytterbium transition hamper the power scaling. Compared with the 976 nm cladding pumped EDFA, the one that is core and tandem pumped by 1480 nm laser source can get a much higher amplifier efficiency and power scalability [12]. Cascaded Raman fiber laser or Raman fiber amplifier can provide adequate 1480 nm output [13–17].

Here, we report a 21.2 W CW single frequency 780 nm laser by utilizing single-pass frequency doubling of a 1560 nm fiber amplifier in an MgO: PPLN crystal. The laser source has no need of cascade crystals or cavity-enhancement configuration, which makes the laser system robust, simple, and compact. The 780 nm laser with a maximum output power of 21.2 W is obtained under the incident 1560 nm power of 49.8 W, which gives the conversion efficiency of up to 42.6%. We employ an in-band pumping scheme for the high power 1560 nm EDFA by a 1480 nm cascaded Raman fiber laser. The 1560 nm laser with maximum output power of 49.8 W is obtained at an incident 1480 nm laser of 60.6 W, corresponding to an efficiency of 79.7%. To the best of our knowledge, this is the highest reported CW single frequency 780 nm laser system.

2. Experimental setup

The experimental setup of the 780 nm laser is shown in Fig. 1, which contains two functional units. One is an EDFA pumped by a high power 1480 nm Raman fiber laser and the other is a frequency doubling unit. The 1480 nm cascaded Raman fiber laser is pumped by a 1064 nm fiber laser. The high power 1064 nm fiber laser has a common master oscillator power amplifier (MOPA) configuration. The 1064 nm oscillator emits 5 W randomly polarized light. A piece of 6-meter-long ytterbium-doped fiber (YDF) is adopted as the gain medium in the MOPA. The YDF has a 4.8 dB/m cladding absorption at 976 nm with fiber core diameter of 10 µm, cladding diameter of 125 µm, and core/cladding NA (numerical aperture) of 0.075/0.46. High power laser diodes (LDs) with center wavelength at 976 nm are used as the pump source. A cladding power stripper (CPS) is used to remove the surplus pump laser in fiber cladding. After amplification, the 1064 nm fiber laser emits light with power as high as 170 W.

 

Fig. 1. Schematic diagram of the high power 780 nm laser system.

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1238 nm and 1480 nm laser are generated in sequence in compact Raman fiber oscillators. The Raman gain media are phosphorous-doped single-mode fibers (PDF) with a core NA of 0.18, a 5 µm core diameter and a 125 µm cladding diameter. 50 m 100 m long PDF are adopted in the 1238 and 1480 nm Raman fiber oscillators, respectively. Fiber Bragg grating (FBG) 1 is highly reflective with a reflectivity of more than 99%, which has a full width at half-maximum (FWHM) of 2.2 nm. The corresponding OC (output coupler) FBG (FBG 2) has a narrow FWHM of 0.18 nm with a reflectivity of 15%. Such FBG pair design (broad HR-FBG and narrow OC-FBG) can minimize the power leaking into backward direction [18]. FBG 3 has a reflective of 99% and FWHM of 1.88 nm. FBG 4 is an OC FBG with a reflective of 15% and FWHM of 0.19 nm. All the FBGs in Raman fiber oscillator are written in SMF-28 fiber. One 1064/1238 nm wavelength division multiplexer (WDM) and one 1064/1480 nm WDM are inserted between the 1238 nm oscillator and the 1064 nm amplifier to prevent the backward 1238 and 1480 nm laser destroying the 1064 nm amplifier.

The 1560 nm laser also has a MOPA configuration. The master oscillator is a DFB fiber laser (NKT Photonics), which has a power of 40 mW and a linewidth < 0.1 kHz. It is amplified to 1.5 W before injecting to the boost amplifier. The 1480 nm fiber laser and 1560 nm laser are coupled into the boost amplifier via a 1480/1560 nm WDM. The gain fiber is a piece of polarization-maintaining (PM) 2.5-meter-long Er-Yb co-doped fiber with core/cladding diameter of 12/125 µm and core/cladding NA of 0.19/0.46. The amplified 1560 nm laser is collimated and injected into the frequency doubling unit.

The frequency-doubling crystal is a 40-mm-long, 1-mm-thick, and 2-mm-wide MgO: PPLN crystal with a period of 19.5 µm, whose end faces have a low reflectivity of less than 0.2% at both 780 nm and 1560 nm. The crystal is housed in a home-made oven and is maintained at the optimum phase-matching temperature of 74.5°C with temperature stability of better than ± 0.01°C, which guarantees a steady 780 nm output. A dichroic mirror, coated for high transmission (T > 99%) at 780 nm and high reflectivity (R > 99%) at 1560 nm, is used to extract the frequency doubled light from the fundamental light. The lens L1 and L2 have a focal length of 50 mm and 60 mm, respectively. The optimum beam waist radius $\omega $ in the MgO: PPLN can be calculated according to [19].

3. Results and discussion

Figure 2(a) illustrates the output power of the 1480 nm oscillator with incident 1064 nm power up to 170 W. At the maximum pump, the 1480 nm Raman fiber laser can output more than 60 W with an optical conversion efficiency of 35.2% from 1064 nm. The efficiency is low compared to previous reports, mainly due to the excess splicing loss between the PDF and SMF28. The power of the 1480 nm laser is calculated by the contribution of different lines in the output spectra and the total power. Figure 2(b) shows the spectrum at maximum output power, which is measured by an optical spectral analyzer Yokogawa AQ6370D with a resolution of 0.2 nm. The PDF has a strong Raman gain peak shifted from the pump wave frequency by 1330 cm⁻¹ [20]. At the same time, PDF has a Stokes shift of 440cm⁻¹ relative to the pump wave and corresponds to the silica Stokes band. In our previous work on random Raman fiber laser, we observed Raman Stokes light corresponds to 440 cm⁻¹ in PDF [21]. In this work, Raman Stokes waves corresponding to silica Stokes band are not observed due to the appropriate 1238 and 1480 nm OC FBG pair design, which provide enough Raman gain competition and restrain the Stokes wave at 440 cm⁻¹. A thorough wavelength conversion with the most of power in 1480 nm band is realized. A spectral purity of 85.2% for 1480 nm is achieved with small residual 1238 nm laser during the cascaded Raman conversion process.

 

Fig. 2. (a) Output power of the 1480 nm oscillator as a function of the 1064 nm pump laser power. (b) Output spectrum of the 1480 nm oscillator at maximum output power level.

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Figure 3(a) shows the pure 1560 nm output versus 1480 nm pump power. The 1560 nm laser with a maximum output power of 49.8 W and polarization extinction ratio of ∼18 dB is obtained at an incident 1480 nm laser of 60.6 W, corresponding to an amplifier efficiency of 79.7%. No sign of stimulated Brillouin scattering is observed. We believe that the output power is limited by the available pump power. From the Fig. 3(b), one can deduce that the spectral purity is over 89.5% for the 1560 nm component. The 1560 nm spectrum shows an ultra-narrow band single frequency character with amplified spontaneous emission (ASE) suppression of more than 45 dB. The percentage of integrated ASE at 1.5 µm is below 0.2%.

 

Fig. 3. (a) The output power of 1560 nm laser versus pump power. (b) The output spectrum of 1560 nm at maximum output power level.

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The output power and conversion efficiency of 780 nm are plotted in Figs. 4(a) and 4(b), respectively. The 780 nm laser with a maximum output power of 21.2 W is obtained. Considering the incident 1560 nm power of 49.8 W, the conversion efficiency reaches up to 42.6%. At low fundamental power, the 780 nm power increases nonlinearly with the 1560 nm power, which is fitted to a quadratic dependence to the 1560 nm power, ${P_{SHG}} = {P_0}{ = _{SHG}}P_0^2$. η_SHG and η are referred as the nonlinear conversion efficiency and power conversion efficiency or conversion efficiency, respectively. The experimental results fit with relation quite well. We get a nonlinear conversion efficiency of η_SHG = 1.262%/W. The formula is not valid at high power for the depletion of the fundamental laser and thermal effects in the nonlinear crystal. The conversion efficiency tends to saturate when the fundamental power increases to over about 30 W. Then the output power increases linearly with respect to the incident fundamental wave. Based on previous studies [22,23], the phenomenon of saturation in SHG efficiency based on PPLN crystal is likely due to thermal de-phasing.

 

Fig. 4. The output power (a) and conversion efficiency (b) of the 780 nm versus 1560 nm power.

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A full spectrum of the 780 nm laser is plotted in Fig. 5(a). Laser lines at all intermediate conversion processes are seen due to the ultrahigh dynamic range of the measurement. But all these lines are negligible in power as it can be found by integrating the spectrum. More than 99% of the output is at 780 nm. Figure 5(b) shows a fine spectrum of the single frequency 780 nm laser measured by the optical spectrum analyzer with a resolution of 0.02 nm, which is resolution limited. We expect the exact linewidth is in the order of 0.1 kHz, because the seed laser has a linewidth <0.1 kHz, the linewidth broadening is negligible in fiber amplifier [11], and the frequency doubling process usually just doubles the linewidth. Resolving such narrow linewidth is not an easy task. Unfortunately, by the time of the experiment we don’t has the conditions to do the exact measurement. But as discussed, it is for sure the linewidth is less than 1 kHz and suitable for Rb laser cooling.

 

Fig. 5. The (a) output spectrum and (b) fine spectrum of 780 nm

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4. Summary

In summary, we have realized a stable high-power single frequency 780 nm laser by simple and robust single pass frequency doubling configuration based on 1560 nm EDFA and MgO: PPLN crystal. The 1560 nm fundamental laser adopts an MOPA configuration and the boost amplifier is in-band core-pumped by a cascaded 1480 nm fiber Raman laser. The highest output power is 49.8 W, and the boost amplifier has an amplification efficiency as high as 79.7%. The 780 nm frequency doubled laser is achieved up to 21.2 W, corresponding to an optical efficiency of 42.6% from the fundamental laser. It is the first report of a more than 20 W CW single-frequency, single-mode 780 nm laser system. Such high power 780 nm laser is urgently pursued for advanced quantum technology applications like large scale atom interferometers. Further power scaling of the single frequency 1560 nm erbium amplifier is foreseeable, based on the 1480 nm inband core-pumping technique. The further power scaling of the 780 nm laser is limited by the damage threshold of the PPLN crystal.