Coherent beam combination of fiber laser array is an important technology of realize high-power, high-radiance fiber laser system. In this paper, Master Oscillator-Power Amplifier scheme is used to realize phase controlling of three ytterbium fiber amplifiers, the experiment results of both two and three fiber amplifiers are given and compared. Far-field patterns with different fill factor are studied experimentally. We perform optical phase-noise measurements of a commercial 1-W ytterbium fiber amplifier using our phase control electronics, the dominant phase noises of the 1-W fiber amplifier are at frequencies below one kilohertz.
©2008 Optical Society of America
Laser beam combining is of current interest for scaling lasers to high average output power while maintaining near-diffraction-limited beam quality. Various beam combining techniques are brought forward and investigated under low power condition[1,2,3]. Coherent combining and wavelength (spectral) combining are the two main approaches being studied. For wavelength combining, multiple single mode lasers at different wavelengths are combined into a single beam without mutual spatial interference, and the phases of the lasers are unimportant. Coherent beam combining, all of the array elements operate with the same spectrum and the relative phases of the elements are controlled such that there is constructive interference. We have been concentrating on coherent beam combining with fiber lasers and previously, we reported the experimental demonstration of coherent beam combination of two fiber amplifiers by use of “climbing hill” phase controlling . In this paper, we investigate Master oscillator-multiple amplifier (MOPA) configuration based on the technology of heterodyne detection phase control . First, we inspect the phase noise that is introduced by the commercial polarization maintaining ytterbium fiber amplifier. Second, the phase locking of two amplifiers is achieved by a feedback control loop. Finally, the experiment of such a 3-element fiber amplifier array is also demonstrated in the same way.
2. The Basic Principle of MOPA
Master oscillator-multiple amplifier (MOPA) configuration is shown in Fig. 1. It is based on the technology of heterodyne detection principle. All the fibers used in the configuration are polarization maintaining. A master laser provides a linearly polarized, narrow linewidth signal that is split into four paths, one reference arm and three signal arms. Light in the reference arm is frequency shifted by an acoustic-optic modulator. Light in the signal arms is individually amplified using polarization maintaining fiber power amplifiers. The light in the fibers is sent to free space via 3 collimators, which give Gaussian beams. A sample of each signal beam is interfered with the reference beam to generate a heterodyne beat waveform that is used to measure the phase of that arm relative to that of the reference arm. Photodiodes or detectors detect the heterodyne signal. Each amplifier uses one detector whose output is fed back to feed back controlling system that powers the phase modulator for the corresponding amplifier to close the feedback control loop. In the far field, the three beams are focused by a lens. The focal plane is imaged onto a CCD, which is not given in Fig. 1.
In the MOPA configuration, each amplifier is independently locked to the reference beam, so increasing the number of fiber amplifiers is feasible.
3. Experimental Results
Experiments are carried out in the laboratory. Following the Fig. 1 configuration, we measure the phase noise that is introduced by a commercial 1-W Yb fiber amplifier, and with this information we accomplish 2-element and 3-element fiber amplifier array MOPA experiment.
The master oscillator is a distributed feedback (DFB) polarization maintaining Yb-doped fiber laser whose operating wavelength is 1083 nm and output power is between 0~100 mW. The bandwidth of the oscillator is less than 1MHz. acousto-optic (AO) modulator shifts the optical frequency by 40 MHz. The phase modulator is a LiNO3 phase modulator with 500MHz 3dB bandwidth. The operation mode of polarization maintaining Ytterbium fiber amplifier is cw, and the central wavelength is 1083 nm. The output power is not less than 1W.
3.1 Phase Noise Measurements of Ytterbium Fiber Amplifiers
This phase noise of fiber amplifiers can be attributed mainly to temperature-induced changes in the index of refraction of the fused-silica fiber. Phase changes caused by thermal expansion of the fiber also contribute, but the second contribution amounts to less than 2% of the first one . In addition, this phase noise can be also caused by mechanical resonances, acoustic noise, seismic noise, cooling fans, etc. In the field of fiber laser coherent combining, phase control of ytterbium fiber amplifiers is the most difficult technology in the MOPA scheme.
In the Fig. 1 configuration, we make use of one amplified signal beam and the reference beam to measure the phase noise of ytterbium fiber amplifiers. The output power of the fiber amplifier is 1.2W. In the heterodyne detection control system, the control-loop bandwidth is more than 10 kHz, control precision is better than λ/20. Fig. 2 shows the 40MHz heterodyne beat signal which results from the interference of the reference with the amplified signal. When the control loop is off, the beat signal varies with the phase difference changes of the signal beam and the reference beam. While the control loop is on, the phase difference of the signal beam and the reference beam is close to zero. The phase noises of fiber amplifier are properly controlled. The beat signal becomes a perfect 40 MHz sine signal.
Figuer 3(a)(b)(c)(d)(e) show the phase noises of fiber amplifier for the time regimes from turn-on to steady-state and the corresponding compensation signals outputted by the control electronics. The sampling rate of the oscilloscope is 5.0 kS/s. The channel 1 traces of the oscilloscope demonstrate the phase change in amplifier; the channel 2 traces of the oscilloscope demonstrate the corresponding compensation signals. When the fiber amplifier is just turned on, the phase changes quickly. After about five or six minutes, the amplifier has reached thermal equilibrium, and the phase change becomes slow, which is shown as in Fig. 3(e). During the experiment, the phase change of the fiber amplifier and its compensation signals is entirety reverse. This indicates that the phase control electronics can successfully correct the phase noise of the fiber amplifier.
Figure 4 shows the phase noise spectral density obtained by applying Fourier transforms to the steady-state phase noise. We compare the spectral density between operation of the amplifier at 200mW and at 1 W. for two different powers, the difference of the spectral density is small, and the dominant phase noises are both at frequencies below one kilohertz, which suggests a required servo bandwidth in the range of several kilohertz.
3.2 Coherent Combination Experiment of Two Fiber Amplifiers
We first realize the coherent beam combination of two fiber amplifiers in the Fig. 1 configuration. The collimated beam diameters are 0.5 mm of two fiber amplifiers. Fig. 5, Fig. 6 and Fig. 7 show the far field interference patterns of two fiber amplifiers when the spacing between the beams is 4 mm, 5 mm and 6 mm, and the fill factor is 12.5%, 10% and 8.33%. The far field interference fringes become thinner and denser with smaller fill factor. The energy contained in the central lobe is 11%, 9.8% and 5.3%, respectively.
3.3 Coherent Combination Experiment of Three Fiber Amplifiers
With coherent combination of two fiber amplifiers, we accomplished 3-element fiber amplifier array MOPA experiment as shown in Fig. 1 again. We add one path fiber amplifier and one path corresponding feedback control system. Each control system is independent. The fill factor of three collimated beams is 12.5%. Fig. 8 shows the far field interference pattern of three fiber amplifiers and its cross section.
Figure 9 shows the far field patterns of coherent beam combination of two and three fiber amplifiers. Compared with those of two fiber amplifiers, the far field interference fringes of three fiber amplifiers become thinner, and the peak intensity increases. In addition, increasing the number of MOPA chains from N = 2 to N = 3 only duplicates the hardware including optics and electronics components. Furthermore, this system can be readily extended to a large number of parallel fiber amplifier chains.
In summary, we have demonstrated coherent beam combining of both 2-element and 3-element fiber amplifier arrays in the MOPA configuration. It is power-scalable and can produce a near-diffraction-limited fiber laser array beam. It can be used not only in the beam coherent combination to obtain high power laser, but also in laser space communication. We also perform optical phase-noise measurements of a commercial 1-W ytterbium fiber amplifier using our phase control electronics. The dominant phase noises of the 1-W fiber amplifier are at frequencies below one kilohertz, which can be used to determine the bandwidth for a control system.
References and links
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