1. Introduction

High power laser sources with very narrow linewidth and low noise have found applications in cold atom physics, high resolution metrology and spectroscopy, high precision long-distance detection, and high fidelity intersatellite optical communication [1–3]. Master-oscillator power-amplifier (MOPA) laser systems using a single-frequency laser as the seed laser have been extensively employed to achieve high power narrow-linewidth lasers for a variety of applications [4]. However, the signal-to-noise ratio (SNR) of MOPA lasers always degrades due to amplified spontaneous emission (ASE) noise, making them not suitable for the aforementioned applications. There is a great demand for single-frequency laser oscillators with watt-level or higher output power.

Single-frequency laser oscillation has been achieved with almost all current solid-state and semiconductor laser technologies in configurations including traditional free-space ring cavity, microchip, non-planar ring oscillator (NPRO), distributed Bragg reflector (DBR) cavity and distributed feedback (DFB) cavity. Most recently, a single-frequency all-solid-state laser capable of producing > 100 W output power at 1064 nm has been demonstrated with two Nd:YVO₄ rods using the mode self-reproduction technique [5]. However, single-frequency operation of a solid-state laser is always obtained with complicated cavity design and careful alignment. Thus, it is susceptible to vibration and environmental changes and, most importantly, its size, weight, and power (SWaP) do not meet the requirements for practical applications in harsh and airborne environments. A microchip laser with dielectric cavity mirrors deposited directly onto the polished faces of a thin gain medium facilitates single-longitudinal-mode laser operation with compact size and high integration [6]. However, the output power of a single-frequency microchip laser is generally constrained to hundreds of milliwatts due to small thickness of the gain material and subwavelength control of cavity length is required to achieve high-efficiency single-frequency laser. Single-frequency NPRO is a compact and monolithic platform, capable of producing watt-level output with very high intensity and frequency stabilities [7]. However, NPRO usually has complex cavity design and fabrication and requires a magnetic field to achieve unidirectional propagation of the laser beam inside the ring cavity for highly stable single-frequency operation. DBR and DFB semiconductor lasers have the highest efficiency and most compact size among single-frequency laser sources [8,9]. However, they are usually very sensitive to environmental changes. Moreover, the MHz spectral linewidth, relatively high noise, and frequent mode hopping constrain their use in some specific applications where ultra-narrow linewidth lasers with ultra-low noise are required. Due to the limited output power of a single emitter, it is still very challenging to achieve watt-level, stable, single-frequency output with semiconductor lasers.

Compared to solid-state lasers and semiconductor lasers, fiber lasers have the advantages of outstanding thermal dissipation capability and excellent beam quality [10], which allow us to develop power scalable, single-frequency fiber lasers with ultra-high stability and ultra-low noise. Single-frequency fiber lasers have been developed in ring cavity, DFB and DBR configurations [11]. Ring-cavity fiber lasers have been extensively used to achieve single-frequency high-power output with low noise and wide wavelength tunable range because of the long gain fiber; Faraday isolator and narrow-band filters also can be used in a ring cavity. A watt-level, single-frequency Er³⁺-doped phosphate fiber ring laser, cladding pumped by a multimode diode laser, was demonstrated by using a pair of fiber Bragg gratings (FBGs) as ultra-narrow bandwidth filters inside the cavity [12]. However, single-frequency ring-cavity fiber lasers generally have a tendency to mode hop due to the very small free-spectral range and the high sensitivity of the narrow-band filters to environmental changes. A DFB fiber laser constructed by inscribing a phase-shift Bragg grating in the gain fiber core exhibits excellent integration and robust single-frequency operation. However, it is challenging to achieve watt-level DFB fiber laser because the laser stability degrades under high pump power due to thermally induced refractive index changes and dephasing of the phase-shift FBG [13]. DBR fiber laser is an excellent platform for robust and compact watt-level, single-frequency laser with superior stability and low noise because the short-length, all-fiber laser cavity is constructed by splicing a rare-earth doped fiber with very high unit gain to a pair of FBGs inscribed in two passive fibers, which suffer from much lower thermal effects than a DFB fiber laser. In this paper, we report the first demonstration of single-mode-diode-pumped watt-level DBR Yb³⁺-doped phosphate fiber laser.

Yb³⁺-doped materials have been extensively used to develop high power laser sources in the 1 µm wavelength region due to their simple energy level structure as well as in-band pumping and emission, resulting in the highest efficiency among all solid-state lasers [10]. To develop a watt-level DBR single-frequency fiber laser, we used an 8 wt.% Yb³⁺-doped phosphate fiber as the gain fiber. Due to the extremely high solubility of phosphate glass, highly Yb³⁺-doped phosphate fibers capable of absorbing 976 nm pump light in centimeter-order lengths have been fabricated and used to develop single-frequency DBR fiber lasers. The first single-frequency DBR Yb³⁺-doped phosphate fiber laser was demonstrated in 2004 [14]. More than 200 mW output power was obtained with a 15-mm-long Yb³⁺-doped phosphate fiber pumped at 660 mW. In 2011, S. Xu et al. reported a 400-mW single-frequency Yb³⁺-doped phosphate fiber laser at 1064 nm [15]. However, this laser suffered from severe thermal issues at high pump power, resulting in high noise and output power clamping, because the cavity was comprised of a narrowband FBG and a dielectric mirror, which butt-coupled to the gain fiber. Moreover, the output of this laser is not linearly polarized because this cavity design cannot force the laser operate in single linear polarization state. In this research, a compact and robust single-frequency laser with watt-level linearly polarized output was developed in an all-fiber DBR configuration. 1.15 W single-frequency output with an optical signal-to-noise ratio (OSNR) > 71.5 dB, polarization extinction ratio (PER) of 29.6 dB, and spectral linewidth of 9.6 kHz was obtained.

2. Experimental setup and results

The experimental setup of the watt-level, linearly polarized single-frequency DBR fiber laser at 1050 nm is shown in Fig. 1. The DBR laser cavity was developed by splicing a piece of 18-mm-long 8 wt.% Yb³⁺-doped phosphate fiber to a pair of FBGs using an asymmetric fusion splicing technique [16]. The splice loss of the short fiber cavity chain was measured at 1310 nm to be 0.67 dB. The Yb³⁺-doped phosphate fiber has core and cladding diameters of 6 µm and 125 µm, respectively, and core numerical aperture of 0.14, which are compatible with commercial HI1060 fiber. The high reflection FBG (HR FBG) written in HI1060 fiber has a reflectivity > 99.9% and 3-dB spectral bandwidth of 0.29 nm. A partial reflection FBG (PR FBG) was written in a polarization maintaining (PM) fiber (Thorlabs PM980), which has a reflectivity of 18.5% and 3-dB spectral bandwidth of 0.02 nm. The PR FBG has two reflection peaks with a wavelength spacing of 0.26 nm due to the birefringence of the PM980 fiber. The DBR fiber laser can be forced to operate at one linear polarization state as either reflection peak of the PR FBG overlaps with the reflection band of HR FBG. The grating lengths of HR FBG and PR FBG are 9 mm and 19 mm, respectively. The effective cavity length was calculated to be ∼26.9 mm [17], corresponding to a longitudinal mode spacing of 3.8 GHz. Two 1-W wavelength-stabilized single-mode laser diodes (Thorlabs BL976-PAG900) at 976 nm were power combined through a polarization beam combiner (PBC) to serve as the pump source. A maximum pump power of 1.78 W was obtained after the PBC. The pump laser was launched into the laser cavity by a 980/1030 filter-type wavelength division multiplexer (FWDM). Another 980/1030 PM FWDM was spliced to the other end of the laser cavity to separate the residual pump and the output laser. The laser cavity was placed on an aluminum plate and covered with thermal paste so that the fiber cavity temperature can be quickly stabilized.

 

Fig. 1. Experimental setup of the watt-level linearly polarized single-frequency DBR fiber laser at 1050 nm.

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The output power as a function of the pump power of the single-frequency Yb³⁺-doped phosphate fiber laser was measured with a power meter (Thorlabs SC310C) and is shown in Fig. 2. The laser had a pump threshold of 35 mW, indicating the low loss of the all-fiber cavity and extremely high gain of the 8 wt.% Yb³⁺-doped phosphate fiber. A maximum output power of 1.15 W was achieved at the maximum available pump power of 1.78 W. The slope efficiency with respect to the launched pump power is about 66%. No power roll-off or saturation was observed even at the maximum pump power. The PER of the laser output was measured with a calcite polarizer (Karl Lambrecht MGT3B8, PER=43 dB) and a power meter (Newport 918D-IR-0D3). By rotating the calcite polarizer, the maximum and minimum transmitted optical power were recorded and the PER was calculated by their ratio. When the temperature of the fiber laser chain was stabilized, a stable PER of 29.6 dB was obtained at the maximum output power of 1.15 W. The high stability of the linearly polarized output at 1.15 W was confirmed by measuring the minimum transmitted optical power for 5 minutes.

 

Fig. 2. Output power as a function of pump power of the 1050 nm single-frequency Yb³⁺-doped phosphate fiber laser.

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The optical spectrum of the 1050-nm single-frequency Yb³⁺-doped phosphate fiber laser at the maximum output power was measured with an optical spectrum analyzer (OSA, YOKOGAWA AQ6370B) and is shown in Fig. 3. The laser spectrum measured with an OSA resolution of 0.02 nm shows the central wavelength of the laser at 1050.25 nm and a high OSNR > 71.5 dB, which is limited by the close-in dynamic range (70 dB) of this OSA. The highly symmetric profile of the laser spectrum is a typical feature of a single-frequency laser. The inset of Fig. 3 presents the laser spectrum in a broad wavelength range from 950 nm to 1100 nm with an OSA measurement resolution of 0.2 nm. The residual pump has been separated from the signal through the FWDM.

 

Fig. 3. Optical spectrum of the single-frequency Yb³⁺-doped phosphate fiber laser at the maximum output power of 1.15 W measured with an OSA resolution of 0.02 nm in a wavelength range of 1049-1051.5 nm. Inset: Laser spectrum measured with an OSA resolution of 0.2 nm in a broad wavelength range of 950-1100 nm.

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Single-frequency laser operation of the 1050-nm Yb³⁺-doped phosphate fiber laser was further confirmed by checking the longitudinal mode characteristics with a scanning Fabry-Perot interferometer (Thorlabs SA210-8B), which has a free spectral range (FSR) of 10 GHz and finesse of 180. In our experiment, stable single-longitudinal-mode operation of this fiber laser was obtained at output power < 750 mW without the need of active temperature control. As the output power exceeded 750 mW, stable single-longitudinal-mode was obtained with active temperature control. The temperature control with a resolution of 0.02°C was needed for the stable single-longitudinal-mode operation. The scanning curve at the output power of 1.15 W is shown in Fig. 4. No other transmission peaks appeared between the two major transmission peaks within an observation time of 30 minutes, confirming robust single-longitudinal-mode operation of this DBR fiber laser. When the temperature of the whole fiber laser chain is stabilized, single-frequency operation of this laser is very robust without any mode hopping or multi-mode oscillation.

 

Fig. 4. Scanning curve of the 1.15-W 1050-nm DBR fiber laser obtained with a scanning Fabry-Perot interferometer.

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The laser linewidth of the watt-level Yb³⁺-doped phosphate fiber laser was measured with a delayed self-heterodyne method using a 50-km fiber delay line and a 100-MHz acousto-optical modulator (AOM), offering a linewidth measurement resolution of about 4 kHz. Figure 5 shows the heterodyne signal measured at the maximum output power of the laser, which is well-fit with a Lorentzian lineshape. A 20-dB linewidth of 192 kHz was obtained from the Lorentzian function fit, so that the laser linewidth of the watt-level single-frequency Yb³⁺-doped phosphate fiber laser is 9.6 kHz or less.

 

Fig. 5. Measured result of the heterodyne signal and the Lorentz fitting lineshape of the single-frequency Yb³⁺-doped phosphate fiber laser operating at an output power of 1.15 W.

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The relative intensity noise (RIN) of the 1050-nm single-frequency Yb³⁺-doped phosphate fiber laser was measured with a detector (Thorlabs DET01CFC) and electrical spectrum analyzer (Advantest R3131A) with a resolution of 10 kHz. Figure 6 shows the RIN measured in a frequency range of 0-15 MHz for the laser operating at different output laser power levels. At an output power of 10 mW, the RIN spectrum of the single-frequency laser is dominated by a peak with an intensity of −102 dB/Hz at the relaxation oscillation frequency (ROF) of 590 kHz and then decreases rapidly with increasing frequency. As the frequency goes beyond 10 MHz, the RIN approaches a level of −152 dB/Hz, which is the sensitivity limit of our current RIN measurement setup. It is clear that the ROF increases with increasing laser power as expected by theoretical analysis [18], and ROF goes to 3.88 MHz at the maximum output power of 1.15 W. The RIN peak at ROF also decreases slightly with the increasing output power and the RIN peak at ROF is −108 dB/Hz at the maximum output power of 1.15 W. Theoretically, RIN of a laser decreases with increasing output power [19]. However, as shown in Fig. 6, the RIN peak amplitude stops decreasing as the output power exceeds 350 mW, which is due to increased thermal noise at higher output power as this single-frequency fiber laser was not under optimum thermal management.

 

Fig. 6. Measured RIN spectra of the single-frequency Yb³⁺-doped phosphate fiber laser at different laser power.

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3. Conclusion

In conclusion, we have demonstrated a watt-level, linearly polarized single-frequency DBR fiber laser with 18-mm-long 8 wt.% Yb³⁺-doped phosphate fiber pumped with two 1-W beam combined laser diodes. An output power of 1.15 W was obtained at the maximum available pump of 1.78 W, corresponding to a slope efficiency of 66%. The PER and spectral linewidth of the 1.15 W single-frequency fiber laser were measured to be 29.6 dB and 9.6 kHz, respectively. Much higher output power can be achieved by using 976 nm Yb³⁺-doped fiber lasers as the pump source [20].