Single frequency 1083nm ytterbium fiber master oscillator power amplifier system was demonstrated. The oscillator was a linear fiber cavity with loop mirror filter and polarization controller. The loop mirror with unpumped ytterbium fiber as a narrow bandwidth filter discriminated and selected laser longitudinal modes efficiently. Spatial hole burning effect was restrained partially by adjusting polarization controller appropriately in the linear cavity. The amplifier was 5 m ytterbium doped fiber pumped by 976nm pigtail coupled laser diode. The linewidth of the single frequency laser was about 2 KHz. Output power up to 177 mW was produced under the launched pump power of 332 mW.
©2005 Optical Society of America
Narrow linewidth single frequency lasers at 1083nm have important applications in atomic and molecular spectroscopy. For example, such laser sources have been used to study the multiplet of the helium atom  with the aim of improving measurement precision for the fine structure constant, pushing the uncertainty down to the level of a few parts in 108. Usually, there are several ways to supply narrow linewidth 1083 nm laser source: Flash-pumped LNA solid state lasers can deliver several watts in a continuous wave (cw) regime with a 2-GHz bandwidth ; Semiconductor lasers around 1083nm offer larger tuning ranges and operation on a single frequency with a linewidth of 100 KHz ; Ytterbium doped fiber laser operated at 1083nm, but the linewidth envelope was about 1-3GHz . Refer to narrow linewidth fiber laser generation, there is an efficient method to use one section of unpumped gain fiber as the saturable absorber (SA), act as very narrow filter [4–8]. In detailed, the researchers used one section unpumped fiber as the SA, in which counterpropagating waves can form very narrow dynamic absorption grating, such that to select single longitudinal mode. If the SA was placed in the fiber loop mirror, the total device was called loop mirror filter (LMF) .
In our experiment, in order to produce high power, narrow linewidth single frequency laser, we used a fiber master oscillator power amplifier (MOPA) configuration. The laser oscillator was linear cavity with LMF, we adopted ytterbium (Yb) doped fiber as the gain material and as the SA in the LMF to generate single frequency 1083 nm laser . From this fiber laser oscillator, stable single frequency 1083 nm laser was generated , the linewidth was about 2 KHz. Output from the laser oscillator, the single frequency signal was coupled into 5 m Yb fiber amplifier, which was pumped by pigtail coupled laser diode at 976nm. From the MOPA fiber laser system, 177mW single frequency laser was produced.
2. The operation of the single frequency 1083nm laser oscillator
The fiber MOPA configuration is shown in Fig. 1, the above section is the laser oscillator, which is a linear cavity with LMF. The unpumped ytterbium fiber was used as the SA in the LMF, in which the two counterpropagating waves formed an interference patterns. This patterns formed Bragg grating. This grating is a dynamic absorption Bragg grating, more efficient than the normal fiber Bragg grating for discriminating and filtering laser longitudinal modes, because the narrower bandwidth cover sub-MHz to GHz range.
As we know, for linear laser cavity it is easy to produce spatial hole burning (SHB) effect in the gain material, which can arouse multilongitudinal mode oscillation, and reduce the laser coherence . The SHB effect is produced by the nonlinear wave mixing of the two counterpropagating waves in laser gain material. If we destroy the interference of the two waves, we can restrain SHB effects. In our experiment, we used simple PC (as shown PC2 in Fig. 1) to adjust the polarization state of the counterpropagating waves, and to destroy the interference of them to some extent.
In addition, according to reference , even if the SHB effect in the amplifying section can not be eliminated completely, if we adopt appropriate fiber with large pump cross sections and small signal cross sections, the destabilizing effect of SHB in the pumped section can be significantly weaker than the stabilizing effect in the absorbing region (LMF). In our experiment, in order to generate single frequency 1083nm laser, we used highly doped Yb fiber pumped by 975nm laser. This fiber has large pump cross section = =2625×10-27 m 2, and small signal cross section = 268×10-27 m 2 , = 2×10-27 m 2 . The concentration of the fiber is 23900 ppm/wt%. In the experiment, we optimized the length of the gain fiber and SA to be 25 cm and 16 m, respectively. The cavity was restricted by the LMF and the FBG with reflectivity of 90%, bandwidth of 0.4 nm (equal to 102 GHz at center wavelength of 1083nm), the overall linear cavity length was about 1 m. PC1 in the LMF was used to control the polarization state of the laser waves in order to optimize the reflection of the LMF. PC2 was used in the laser gain section to adjust the wave polarization. WDM1 was used to input 976 nm LD pump laser, and WDM2 to output the residual pump power ensuring the Yb fiber in LMF is not pumped. We measured and confirmed the single frequency oscillation with a scanning Fabry-Perot interferometer (Newport SuperCavity SR-150), which had a free spectral range (FSR) of 6 GHz and a resolution of 150KHz.
In the experiment process, firstly, we generated 1083nm laser by increasing pump power to appropriate value. Then rotated PC1 to maximize the loop mirror reflection and laser output power. Next rotated PC2 to change the polarization state of the counterpropagating waves. During this process, it is easy to construct 2 mode or 3 mode laser oscillation. Especially, the 2 mode laser oscillation was very stable. Then we rotated PC1 and PC2 slightly and carefully, when rotated to certain position, single mode laser was generated. This single frequency oscillation was very sensitive to the polarization state of the laser wave. Rotated any PC slightly, single frequency oscillation would be destroyed, 2 mode or 3 mode laser started to oscillate. We slowly changed the state of PC1 and PC2, looked for the optimum state for single frequency oscillation. Once single frequency oscillation was constructed, it was very stable under the optimum state, there was no mode hopping during one hour’s observation. Figure 2 (a) shows a scan over one FSR and confirms that only one longitudinal laser mode oscillated. The maximum laser power was up to 14 mW when the pump power increased to 100 mW, the corresponding optical-optical conversion efficiency was about 14%. When increasing the pump power more than 100 mW, multilongitudinal modes appeared, and the frequency became unstable. This is because when the pump power was increased more, the laser power in the cavity became higher, which induced strong SHB effect in amplifying section, the PC2 and LMF could not suppress this strong effect in this condition.
The linewidth of the laser was measured using the delayed self-heterodyne method. Firstly, the laser beam was splitted into two beams by 3dB coupler, then one beam was delayed by 25 km single mode fiber, the other beam was through an acoustooptic modulator (AOM) with a carrier frequency of 80 MHz. After that, the two beams was combined at a beam splitter and analyzed by an RF spectrum analyzer (Anritsu MS 2661C), whose frequency resolution was set at 1 KHz and 10 times averaging. Figure 2 (b) shows the result of the measurement. From the heterodyne signal, we take 3 dB down from the maximum value to estimate its bandwidth, which is about 4 KHz. The laser linewidth is equal to the half-width of the heterodyne signal, which is about 2 KHz
3. The ytterbium doped single mode fiber amplifier
In Fig. 1, the lower section is the fiber amplifier. A pump power of up to 332 mW at 976nm was coupled into the amplifier by single mode fiber WDM coupler. The gain material is Yb doped single mode fiber with 5 m length. Inline isolator is used between the laser oscillator and amplifier to prevent feedback into the oscillator, which can affect the performance of the oscillator.
From the laser oscillator, stable single frequency 1083nm laser was generated. Through the fiber isolator, the net power of 4.25mW was coupled into the amplifier. We increased the pump power of the amplifier, we found ASE at 1030nm emission was very weak during this total process. As shown in Fig. 3, when the pump power was up to the maximum value of 332 mW, the intensity difference between 1083nm laser and 1030nm emission is larger than 25 dB.
Figure 4 shows the output signal power as a function of pump power for given input signal power of 4.25 mW. When the pump power is the maximum, the amplified signal power is about 177 mW, and the maximum gain is about 16 dB. Obviously the available input pump power was not enough to saturate the gain. Therefore, more signal output power can be obtained under the given input signal power if more pump power were provided.
Single frequency 1083 nm single mode fiber master oscillator power amplifier laser system was constructed. Loop mirror filter was used in the linear laser cavity to discriminate and filter the laser longitudinal modes efficiently. The SHB effect was restrained partially by controlling the light polarization in the linear cavity. The linewidth of the single frequency laser was about 2 KHz. After the Yb fiber amplifier, the maximum power of 177 mW was produced.
This work is partly supported by the 21st Century COE program of Ministry of Education, Science and Culture of Japan.
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