The power stability of a fiber amplifier based on a multifunction card and PID control program

The power stability of a fiber amplifier was significantly improved by means of simultaneously controlling the current of a fiber amplifier and the diffraction efficiency of an acousto-optical modulator. The real-time fluctuation of laser power was recorded by a multifunction card and processed by a proportional–integral–derivative (PID) control program. The feedback loop voltage was introduced to the fiber laser amplifier and acoustic-optic modulator through the analog output of the multifunction card. The control method based on a multifunction card and PID program has good scalability, flexibility and reliability for the complex system on the condition in which the frequency and power of the laser need to be precisely stabilized.


Introduction
A fiber amplifier can magnify the light power of a fiber laser to as low as several milliwatts input, to as much as a few watts output. The fiber amplifier has many advantages, such as good beam quality, high power and compatibility [1]. In the research of ultracold atomic and molecular physics, it has been widely used to generate the optical trap or optical lattice for confin ing atoms or molecules [2][3][4][5][6][7]. There are two important para meters of an optical lattice: the well depth and the periodicity. In experiments, the well depth of an optical trap or lattice could be influenced because of the laser power drift and could affect the loading rate of atoms. However, some known parameters, such as the polarization of input light and the fluctuation of amplifier current [8], would affect the efficiency of the ampli fier, which leads to negative influence on the applications mentioned above. So we need tuned in real time by modulat ing the light power and keeping the well depth stabilization. Generally, the work current of the laser or amplifier can control the output power. However, the work current cannot precisely adjust the power, especially for the amplifier. Moreover, the laser frequency would vary with the current. It is a drawback in the application of optical trapping. An acoustooptical modula tor (AOM) is used to accomplish precise control of the power and frequency of the 1st order diffraction light in the experi ments [9,10]. But it can not provide a large adjustable range since it is limited by reasonable diffraction efficiency.
In this paper, we present the solution to precisely stabi lize the light power of a fiber amplifier based on the dual path feedback loops of both the AOM and fiber amplifier. The error signal is processed by a proportional-integral-derivative (PID) control program. The control program will generate the suitable voltage and output to the AOM and the amplifier by the analogue output of a multifunction card. The light power is significantly stabilized and has 0.03% relative standard devia tions (RSD) of error signals.

Laser Physics
The power stability of a fiber amplifier based on a multifunction card and PID control program Linjie Zhang, Wenguang Yang, Hao Zhang, JianMing Zhao and Suotang Jia The power stability of a fiber amplifier was significantly improved by means of simultaneously controlling the current of a fiber amplifier and the diffraction efficiency of an acoustooptical modulator. The realtime fluctuation of laser power was recorded by a multifunction card and processed by a proportional-integral-derivative (PID) control program. The feedback loop voltage was introduced to the fiber laser amplifier and acousticoptic modulator through the analog output of the multifunction card. The control method based on a multifunction card and PID program has good scalability, flexibility and reliability for the complex system on the condition in which the frequency and power of the laser need to be precisely stabilized.
Keywords: fiber amplifier, power stability, PID control (Some figures may appear in colour only in the online journal)

Astro Ltd
Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Figure 1 is a schematic figure of the power stability system. In the experiment, the output of the fiber laser (RFLM25 11064.461S0 NP Photonics) with fiber amplifier (NUA 1064PC0010B2, Nufern) was controlled. The maximum power of the fiber laser is 25 mW and the wavelength is 1064 nm. The output beam of the fiber amplifier is linearly polarized. The maximum output power is about 10 W. The frequency and amplitude of the laser beam from the fiber amplifier was modulated by an AOM (MT200B100A0, 51064, AA Opto Electronic). The firstorder diffraction beam was divided into the reference beam and experimental beam with a glass with reflectivity 5%. The reference beam was detected by a fast photodetector (FDS100, Thorlabs) and acquired by a highspeed data acquisition card (PCI 1742U, Advantech). The sampling rate is about 1 KHz. The data was processed by the PID control program [11]. The feedback voltage was introduced to the AOM and the fiber amplifier through the DAQ output port.

Experimental approach
A flow chart of the PID controlling program is shown in figure 2. The reference light was detected by a fast photo detector. The conversion voltage is fed into the DAQ card and compared with the 'setpoint' in the PID control program. The 'setpoint' is a relative value of the voltage corresponding to the light power that we expected. The deviation between the 'setpoint' and realtime measured value was calculated firstly in the controlling program. According to the adjustable range of the AOM, a threshold value will be generated in the program. If the calculated deviation is greater than the thresh old value, the control program will execute the PID program branch I for the current control of laser amplifier. Otherwise, it will execute the branch II for the modulation control of the AOM. The feedback signals were produced by the processing of the PID control program and translated into the suitable voltage value, which was then introduced into the modin port of the AOM and the modulation port of current of the fiber amplifier.
The control curve of fiber amplifier output is shown in figure 3(a). Taking account of the large control range and moder ate output power of the fiber amplifier, the control volt age range of the fiber amplifier that we choose is from 0.5 V to 3.5 V. The nonlinear control curve is processed in the program. The control of the fiber amplifier would feedback the fast and widerange variation of power from several milliwatts to a few watts output. The diffraction efficiency of the AOM is measured with different modulation voltage, as shown in figure 3(b). The AOM would bring the precise and finite range feedback. In order to obtain high diffraction efficiency and intended control range, the range of the AOM control voltage is from 2.5 to 5 V in the program. The control accuracy of the AOM is up to 0.05 mW mV −1 com pared with the 0.3 mW mV −1 of the fiber amplifier.

Results and discussion
To check this method, we theoretically simulate the dualpath feedback loop control of both the AOM and fiber amplifier through the program, and the derived formulas of the control program are as follows: (1) where the P amp and P aom correspond to the output power of the fiber amplifier and the AOM, respectively. K P , K I , ′ K P and ′ K I are PID parameters. α(t n ) corresponds to manmade noise. C 1 , C 2 , C 3 and C 4 are constants. SP 1 and SP 2 are the value of power we expect.    The numerical simulation was applied to evaluate the dualpath feedback loop control of both the AOM and fiber amplifier. The stochastic noise of 1% was modulated into the controlled value in the program. We use the Allan variance to analyze the results of simulation. As shown in figure 4, the Allan variance of laser power decreases significantly under the feedback loop control.
We experimentally perform three steps mentioned above using the dualpath stability system: (1) we detect the power variation with the PID control program of the AOM turning on while a constant voltage was sent to the fiber amplifier.
(2) Then we detect the power variation with the PID control program of the active fiber amplifier while a fixed voltage was sent to the modin port of the AOM. (3) Finally, PID control programs of both the AOM and amplifier are used to observe the power dithering. Meanwhile, the three results are com pared with the power changes when the fiber laser was run ning freely, as shown in figure 4. When only the PID control program of the amplifier or AOM is active, the RSD is 0.84% and 0.05%, respectively. But the RSD rises up to 1.04% when the fiber laser is running freely. The RSD of the error signals reaches about 0.03% with the PID control programs of the fiber amplifier and AOM simultaneously turning on. The sta bility of laser power was improved significantly when the PID control programs of both the fiber amplifier and AOM were turned on.
Moreover, the longterm stability of the fiber amplifier output was evaluated through analyzing the Allan variance when the fiber amplifier operated under the PID control of the dualpath feedback loops. The results were compared with those of when one of the PID control programs of the fiber amplifier or AOM was turned on. The results in figure 6 shows that the Allan variance significantly decreased when the fiber laser was controlled by a PID program in the period of experimental measurement. In comparison, an obvious increment of the Allan variance appeared when the control loop was switched off.

Conclusion
By means of a dualpath feedback loop accomplished by the PID program and a multifunction card, both the AOM and the fiber laser amplifier can be controlled by a computer pro gram. The power stability of the fiber amplifier is significantly improved. The stability system is robust and highly scalable. It can be extended easily to control more devices for the com plex systems.