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A 1.03 W optical signal-to-noise ratio (OSNR) of > 70 dB single-frequency polarization-maintained master-oscillator power amplifier (PM-MOPA) laser at 1083 nm was demonstrated. The seed laser of this laser system was a distributed Bragg reflector short cavity Yb-doped phosphate fiber oscillator. A one-stage core-pumped amplification configuration was employed, in which the typical gain is 9.7 dB and the optical-to-optical conversion efficiency is 68.7%. The estimated laser linewidth is less than 3.5 kHz, the measured polarization-extinction ratio is greater than 25 dB, and the obtained relative intensity noise of fiber laser for frequencies of over 2 MHz is less than −130 dB/Hz.
© 2014 Optical Society of America
1. Introduction
Narrow-linewidth single-frequency lasers are attractive for some potential applications, such as coherent communications, optical fiber sensors, and high-resolution spectroscopy. In particular, 1083 nm laser has important applications in nonlinear frequency conversion, atomic and molecular spectroscopy efficiently [1–5]. For example, such laser has been used to study the multiplet of the helium (He) atom to improve measurement precision for the fine structure constant [6, 7]; In addition, using this naturally occurring metastable state of He, the upper neutral atmosphere from 300 to 1000 km can be actively probed with a resonance fluorescence Doppler wind/temperature lidar operating at 1083 nm [8, 9]. Thus, the narrow-linewidth, low-noise, watt-level and polarization-maintained (PM) lasers of radiation at 1083 nm are required for many of these applications.
There are several ways to supply a single-frequency 1083 nm laser source: Flash-pumped LNA solid state laser can deliver several watts with a 2 GHz bandwidth [10]; Distributed Bragg reflector (DBR) diode laser offers several milliwatts output with a few hundred kHz linewidth [11, 12]; Yb-doped fiber laser operating at 1083 nm with a ring-cavity utilizing a fiber saturable absorber and some polarization-dependent components, but the linewidth obtained was about several MHz [13], and the linear cavity configuration by introducing a loop mirror filter and a polarization controller, which can generate a few milliwatts output with a several kHz linewidth [14]. Usually, in order to further increase the output power of single-frequency laser, a master-oscillator power amplifier (MOPA) configuration must be used. Though the output powers of diode laser or fiber oscillator could be boosted to a few hundred milliwatts, even further be amplified to tens of watts based on several Yb-doped fiber amplifiers, which produced a low optical signal-to-noise ratio (OSNR) due to the amplified spontaneous emission (ASE) noise, a wide linewidth, an unstable operation at a certain longitudinal mode [9, 11–14], and a strong ASE around 1030 nm may even lead to parasitic lasing or laser diodes damage. Furthermore, the emission cross-section of the Yb-doped fiber exhibits a broad maximum centered on 1027 nm and drops to roughly one third of the peak (1027 nm) value at 1083 nm. The operating wavelength of 1083 nm is almost approaching the edge of the gain spectrum and the available gain is usually limited to below 14 dB [12, 15], thus it was difficult to amplify a 1083 nm seed laser if no schemes with ASE filter or suppression used.
Recently, we have developed a 1083 nm single-frequency single-polarization DBR laser oscillator based on a 1.8 cm long Yb-doped phosphate fiber. Low noise, linearly polarized, and more than 100 mW output with a linewidth < 2 kHz and RIN < −150 dB/Hz was achieved [16]. In this letter, we present a high OSNR watt-level single-frequency PM-MOPA fiber laser, which amplified the output of a 1083 nm laser oscillator using a single-mode core-pumped Yb-doped PM fiber amplifier.
2. Experimental setup
The PM-MOPA fiber laser configuration is shown in Fig. 1. The fiber laser is composed by a 1083 nm single-frequency linearly-polarized seed oscillator and a one-stage single-clad PM fiber amplifier. The seed oscillator was similar to our previously described [16, 17], which employed a DBR short cavity with a 1.8 cm long non-PM Yb-doped phosphate fiber (YPF), a PM narrowband fiber Bragg grating (PM-FBG), and a high-reflection (> 99.5%) wideband FBG (HR-FBG). The YPF was fabricated using a fiber-drawing tower based on the rod-in-tube technique, and more details of the fiber properties and the single-frequency laser oscillator can be found in our previous works [16–19]. The seed oscillator was backward pumped by a 976 nm fiber-coupled laser diode (LD) with a maximum pump power of approximately 350 mW and emitted as much as 110 mW. The measured laser linewidth is less than 3 kHz, the polarization-extinction ratio (PER) is 30 dB, and the relative intensity noise for frequencies of over 2 MHz is less than −140 dB/Hz. The single-frequency output of the seed oscillator was confirmed that only one longitudinal laser mode oscillated with a scanning Fabry–Perot interferometer.
By optimizing the length of the highly Yb-doped PM single clad fiber (YSF, CorActive YB 501-PM), a 2.5 m long fiber was employed as the laser gain medium. 2.1 wt% Yb3+ ions were doped uniformly in the core region and the core absorption was 140 dB/m at 915 nm. The YSF had a 6.0 μm mode-field diameter at 1064 nm, a 0.14 numerical aperture (NA) and a 125 μm cladding. The amplifier was bidirectionally pumped by two FBG-stabilized single-mode fiber-coupled LDs centered at 976 nm. Both fiber ends of the YSF and the pigtailed fibers of the LDs were fused with two 980/1083 nm PM wavelength division multiplexers (PM WDM 1 and PM WDM 2), respectively. The LD 1 with a maximum output power of 750 mW was employed as the forward pump, and another LD 2 with a maximum output power of 750 mW was also employed as the backward pump. The seed and pump lasers are coupled through two PM WDMs. Two all-fiber in-line PM band-pass filters (PM BPFs) with an operating wavelength of 1083 ± 3 nm are exerted to filter out the 1 μm-band ASE during the laser system. To minimize the optical reflection due to Fresnel reflections, the output port of the amplifier is connected with square connector with angled physical contact (SC/APC)-type optical connector.
3. Results and analysis
The primary limitation on the output power or the gain achievable at 1083 nm from Yb-doped fiber amplifier is the generation of ASE around 1030 nm, which can extract significant pump power. It was necessary to suppress ASE in the amplifier. A reasonable way is to increase the input power or the OSNR of the seed oscillator, also another alternative way is to filter ASE directly and reserve the signal light in amplifier stage.
First of all, the seed oscillator with a PM BPF installed to strip the ASE, the OSNR was significantly improved from 60 to 75 dB. The output spectra of the seed oscillator at a maximal output power (110 mW) are recorded with a spectrum resolution of 0.1 nm by an optical spectrum analyzer (OSA), as plotted in the inset of Fig. 2(a). Subsequently, the input power was increased from 30 to 110 mW to suppress the ASE. For different input powers of the seed oscillator, the output spectra of the PM-MOPA laser are recorded by an OSA when the pump power was 1300 mW, as shown in Fig. 2(a). The laser spectra were normalized to the same peak power to compare the OSNRs. It is observed that the signal light around 1083 nm is clearly exhibited in the spectral regime, where the ASE builds up evidently in the 1.0 μm-band wavelength regions, and there was no sign of parasitic lasing. When the input power was increased from 30 to 110 mW, the whole ASE power level has been suppressed successfully and the peak could be suppressed by more than 15 dB to the lowest value.
Fig. 2 (a). Output spectra of the fiber laser with different input powers of seed oscillator. Inset: Output spectra of the seed oscillator with PM BPF and no PM BPF used. (b). Output spectra of the PM-MOPA laser with PM BPF and no PM BPF used.
At higher input power level of 110 mW and the maximal pump power of 1500 mW, the typical output spectra of the PM-MOPA fiber laser were also recorded by an OSA and are depicted in Fig. 2(b). It can be seen that the laser spectrum is centered at 1083.1 nm, and the OSNR is about 60 dB due to the residual ASE, mainly between 1030 and 1070 nm. While the PM BPF was used, the OSNR was indeedly increased by 10 dB, to a higher level of 70 dB. However, compared with that of the seed oscillator, the OSNR of the fiber laser was slightly deteriorated due to the ASE noise.
Figure 3(a) shows the output power of the PM-MOPA laser versus the launched pump power for different input powers of the seed oscillator. The pump power of the LD 1 was fixed at 750mW, and that of the LD 2 was gradually tuned from 0 to 750 mW. With the increment of the pump power, the output power of the fiber laser was observed to increase linearly. For the input power of 30 mW, when increasing the pump power to approximately 1200 mW there occurs slight gain saturation at 1083 nm, thereafter pump power is converted into ASE. When the input power increased to 110 mW continuously, obviously the given input power was also enough to saturate the gain and slight output power saturation phenomenon was observed, indicating that higher output power could not be generated even if more pump power was available. Hence, the maximal input power (110 mW) and pump power (1500 mW) were provided in our experiment, the seed oscillator could be amplified up to the highest value of 1.03 W with the typical gain of 9.7 dB, and the highest optical-to-optical conversion efficiency (the output power versus the launched pump power) was about 68.7%. To our knowledge, this is the highest output power, among all the single-mode core-pumped MOPA lasers at 1083 nm. With the laser output power of 780 mW, the long-term stability was measured over more than 8 hours; the data are shown in the inset of Fig. 3(a). If the ambient temperature is keep constant to about 0.5 °C, the output power instability that was less than 1.5% of the average power was observed during the entire period.
Fig. 3 (a). Measured laser output power versus the launched pump power for different input powers of seed oscillator. Inset: Power stability of the fiber laser in 8 hours. (b). Net gain versus the launched pump power for different input powers of seed oscillator.
Figure 3(b) shows the gain of the PM-MOPA laser versus the launched pump power for different input powers of the seed oscillator. It can be seen that the gain increases with increasing pump power or the decrement of input power, and the gain saturation phenomenon was observed subsequently. At lower input power level of 30 mW and the maximal pump power of 1500 mW, though the gain could reach the maximum value of 14.3 dB, the output power and the corresponding optical-to-optical conversion efficiency were only 801 mW and 53.4%, respectively. However, when the input power was increased to 110 mW unceasingly, the whole gain level dropped off rapidly due to the strong signal significantly depletes the inversion and the pump is not able to replenish it. On the other hand, the highest output power and the maximum optical-to-optical conversion efficiency could be achieved in our experiment.
The single-frequency nature of the PM-MOPA fiber laser was verified that only one longitudinal-mode oscillated by using a scanning Fabry–Perot interferometer with a resolution of 7.5 MHz and a free spectral range (FSR) of 1.5 GHz, as shown in the inset of Fig. 4(a). With the proper temperature control, the laser operated stably in a single-frequency without mode hopping and mode competition. To further investigate the spectral characteristics of the fiber laser, the laser linewidth was measured by the self-heterodyne method using a 20 km fiber delay, as shown in Fig. 4(a). The heterodyne signal was fit to a Lorentzian profile to estimate the spectral linewidth from the data. The fitting lineshape indicates that the laser linewidth is measured to be less than 3.5 kHz FWHM (full width at half maximum), which was found to be the same as that of the seed oscillator. Thanks to the effective ASE suppression approaches and low relative intensity noises, there was no obvious linewidth broadening observed.
Fig. 4 (a). Measured self-heterodyne signal using a 20 km fiber delay. Inset: The longitudinal modes operation of the fiber laser. (b). Measured RIN of the fiber laser. Inset: DOP of the fiber laser (red dot) represented by a Poincaré sphere.
With the maximum output power, the relative intensity noises (RIN) of the PM-MOPA fiber laser was measured using an electrical spectrum analyzer (ESA) with a bandwidth resolution of 1 kHz, as shown in Fig. 4(b). In the low-frequency region below 500 kHz, the RIN decreases from −90 to −105 dB/Hz with increasing the frequency and is stabilized at around −105 dB/Hz. An obvious relaxation oscillating frequency strong peak of −95 dB/Hz was observed at the frequencies of 720 kHz, depending on the laser cavity layout and the pump current. For frequencies above 2 MHz, the RIN is less than −130 dB/Hz, slightly higher than that of the seed oscillator 10 dB due to the power fluctuations of LDs, the ambient acoustics, and vibration.
The polarization state of the PM-MOPA laser with an output power of 780 mW was investigated using an optical polarization analyzer, as shown in the inset of Fig. 4(b). It can be found that the dot demonstrated a polarization is clearly located on the equator of Poincaré sphere, which indicates that the laser has good linear polarization characteristics. The degree of polarization (DOP) is measured to be 99.5%, corresponding to a PER is greater than 25 dB.
4. Conclusions
In conclusion, a 1.03 W single-frequency PM-MOPA fiber laser at 1083 nm with an OSNR of > 70 dB employing a DBR phosphate fiber oscillator and a core-pumped PM amplifying stage has been demonstrated. The measured optical-to-optical conversion efficiency of fiber laser is 68.7%, the estimated laser linewidth is less than 3.5 kHz, and the PER is greater than 25 dB. For frequencies of over 2 MHz, the obtained laser RIN is less than −130 dB/Hz. The results indicate that the fiber laser might be a promising candidate as an efficient narrow-linewidth single-frequency PM laser source for atomic and molecular spectroscopy.
Acknowledgments
Work presented in this paper was supported by the China State 863 Hi-tech Program (2011AA030203 and 2013AA031502), NSFC (11174085, 51132004, U0934001, and 60977060), Guangdong Province and Hong Kong Invite Public Bidding Program (TC10BH07-1), Fundamental Research Funds for the Central Universities (2011ZG0005, 2012ZZ0002, 2013ZP0003, and 2013ZP0013), The Fund of Guangdong Province Cooperation of Producing, Studying and Researching (2012B091100140), and Guangdong Natural Science Foundation (S2011030001349 and S20120011380).
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