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A three-stage dual-frequency laser signal amplification system is presented. An output from a narrow-linewidth Nd:YAG nonplanar ring-oscillator (NPRO) is split into two parts, one of them is frequency shifted by an acoustooptic modulator (AOM) then coupled into a single mode optical fiber. The other part is coupled into another single mode fiber then combined with the frequency-shifted beam with a 2 to 1 single mode fiber coupler. The combined beam has a power of 20 mW containing two frequency components with frequency separation of 150 ± 25 MHz. The dual-frequency signal is amplified via a three-stage Yb3+-doped diode pumped fiber power amplifier. The maximum amplified power is 50.3 W corresponding to a slope efficiency of 73.72% of the last stage. The modulation depth and signal to noise ratio (SNR) of the beat signal are well maintained in the amplifying process. The dual-frequency laser fiber power amplifier provides robust optical carried RF signal with high power and low noise.
© 2016 Optical Society of America
1. Introduction
Hybrid lidar-radar uses microwave beat frequency of two different optical frequencies as lidar carrier, the target reflected signal is collected by an optical receiver and converted into electric signal by photo detector. It has the optical propagation characteristics of lidar which offers high spatial resolution, meanwhile, signal-processing techniques of radar are applied to extract the information of the target which can simplify the system. Moreover, the synthesized wavelength of the two optical frequencies is much larger than optical wavelength, so atmospheric turbulence or random scattering in the intervening medium have less degrading effects on the received signal. Lidar-Radar has broad applications in military and civilian areas, such as, precise range finding, imaging, and velocity measurements [1–7].
Optical Carried RF (OCRF) signal has been done in different ways, externally modulating the lasers [8,9], overlapping two laser beams with a frequency difference [10] or dual frequency lasers [11,12]. In order to apply the lidar-radar concept into practice, high power, high SNR OCRF signal has to be developed. Many efforts have been put to generate high purity OCRF signals. Marc Brunel and his associates from Rennes University in Fance use diode pumped microchip solid-state lasers to obtain two frequency output by inserting birefringent element inside the cavity to lift frequency degeneracy of two orthogonally polarized modes. Frequency difference of hundreds MHz to tens GHz have been achieved [13]. Y. S. Juan and F. Y. Lin proposed broadly tunable microwave signals generation schemes utilizing a dual-beam optically injected semiconductor lasers. By injecting a slave laser with two detuned master lasers at stable locking states, microwave signals with frequencies up to 120 GHz had been generated [14]. M. R. K. Soltanian’s group also fabricated a widely tunable dual-wavelength erbium-doped fiber laser that used a 10-cm photonic crystal fiber as a Mach–Zehnder interferometer to filter out the switchable dual-wavelength signals so as to generate tunable continuous-wave terahertz radiation [15]. Nevertheless, the above mentioned methods have a common limitation, the output power of the dual frequency laser generally are in the range of a few milliwatts, and the frequency stability of the beat note from a dual wavelength laser cavity is generally not good enough for practical application in lidar-radar system. On another hand, using an acoustooptic modulator to shift the frequency of a laser beam and recombine with zero order beam can generate a very stable OCRF signal since the RF frequency added to an AOM can be easily controlled as precise as sub Hertz, a level which is very difficult or expensive to reach by any laser technology.
In this paper, we present a OCRF signal and fiber power amplification system, an output from a stable single frequency NPRO is diffracted by an AOM, the frequency is shifted by 150 ± 25 MHz, the shifted beam is recombined with zero order beam with a 2 to 1 single mode fiber coupler. The power of this beam is amplified by a three-stage fiber power amplifier. We obtain 50.3 W OCRF signal, the SNR before and after amplification is measured to be 78 dB and 80 dB respectively, modulation depth is well maintained after the amplification.
2 Experimental setup
2.1 dual frequency laser seed
Figure 1 is the schematic of the dual frequency seed signal. Single frequency NPRO laser was made in our lab, the highest output power is 500 mW, linewidth is 1 kHz. The output beam is incident into an AOM at the Bragg angle, the diffraction efficiency of the first order beam is 50%, the frequency of the signal added to the RF driver can be varied between 125 MHz-175 MHz. The first order beam and zero order beam are coupled into single mode fibers respectively. Two polarization controllers are added into the beam paths. The two beams are combined with a 2 to 1 single mode fiber coupler, therefore, the output from the coupler contains two frequency components with frequency difference of the RF frequency added to the AOM. The modulation depth can be varied by adjusting the polarization controllers.
A photo diode is used to monitor the beat note. Figure 2 shows the beat note of the dual frequency seed laser. The RF frequency is set at 150 MHz. 3 dB bandwidth of the AOM in free space is 50 MHz, after single mode fibers, it drops to 17.5 MHz, between 142.0 MHz and 159.5 MHz.
From the beat signal on the oscilloscope Fig. 2(b), the modulation depth can be calculated as:
where, and are the maximum and minimum values of the voltage, respectively. The modulation depth of the beat note is calculated as 72.66%.In order to measure the frequency stability of the beat notes, we use a 14 bit frequency meter to monitor the frequency of the beat notes, we set the RF frequency at several values, for each frequency, the frequency values shown on the meter varies on the sub Hz digit. Figure 3 shows the frequency values versus time.
The standard deviation of beat frequency values can be calculated as:
where,is the average of frequency values, N is the sample number of frequency values. From the data we calculated that the standard deviation of beat frequency before amplification is 0.4792 Hz.2.2 Dual frequency fiber power amplification
Since the two frequencies are very close to each other, both the gain and the loss of the two frequency components are the same in the active fiber,therefore we expect there is no competition between them, both signal can be amplified simultaneously. The two-frequency fiber amplification can be modeled by the equations [16]:
where and are the pump power and signal power, and are the power filling factors for the pump and signal, andare scattering loss of pump and signal, is the dopant concentration of the active fiber,andare the emission and absorption cross section of the pump, andare the emission and absorption cross sections of the signal. Supposed the pump and signal light are input into the fiber from the same end, therefore, the pump power and amplified signal power from the output end is calculated by Eqs. (3) and (4). The results are shown in Fig. 4. It can be seen that when the fiber length is 5 m, 95% of the pump power is absorbed and amplified signal reach its maximums. When the pump power is set at 70 W, optical to optical efficiency of more than 75% is expected. We choose 5 m fibers in the amplifier.In order to obtain low noise and high power dual frequency amplification, three-stage fiber power amplifier is built. Figure 5 shows the setup.
The first stage is a preamplifier, the dual frequency seed signal passes an optical isolator then input into a 5 m Yb3+-Doped Fiber (YDF) (Nufern,6/125 um,NA = 0.13), the pump light is a 600 mW fiber coupled laser diode at 976 nm. A WDM is used to couple the signal and pump light into the active fiber.
In the second stage, the amplified laser signal from the preamplifier passes an optical isolator, then input to a (1 + 1) × 1 fiber coupler. A 10 W laser diode is used to pump a 5 m long, double clad YDF (Nufern, 10/125 um, NA = 0.075/0.46), a mode field adapter (MFA) is inserted between the fiber coupler and the active fiber to adapt the different diameters of the fiber cores. An cladding power stripper is inserted on the output end of the amplifier to get rid of the residual pump light, ASE and signal leaked into the inner clad.
In the third stage, amplified signal from the second stage passes a high power optical isolator, then input into a 5 m long double cladding, large mode area YDF (Nufern,25/250 um, NA = 0.065/0.46) pumped with two 35 W diode lasers via a (2 + 1) × 1WDM,a 25/250 um double cladding non doping fiber is inserted between the WDM and the active fiber for better heat dissipation. A high power cladding power stripper is used on the output end of the amplifier to improve the beam quality of the amplified signal. A high power isolator is used to prevent Fresnel reflection from damaging the MOPA system on the output end.
3. Results and discussions
When the power of the dual frequency laser seed is 20 mW, the power is amplified to 100 mW by the first stage, 1 W by the second stage. The last stage can boom the dual frequency laser power to 50.3 W when 70 W of pump power is added. Corresponding to a slope efficiency of 74%, Fig. 6 shows the amplification power with respect to the pump power at the last stage.
Setting the amplified power at 20 W, RF frequency at 150 MHz, we test the beat note spectrum of the amplified signal. Figure 7 shows the output from the optical diode on the oscilloscope and spectral meter. One can see that there is no broadening of beat note spectrum and the modulation depth before and after amplification remain the same.
Figure 8 shows the frequency stability of the amplified signal measured with the frequency meter, one can see that the frequency stability is well maintained during the amplification. The standard deviation of beat frequency after amplification is calculated as 0.5571 Hz.
The phase noise is measured with a spectral meter as shown in Fig. 9. Figure 9(a) shows the phase noise spectrum of the dual frequency seed, Fig. 9(b) shows the spectrum of the amplified signal. There is a peak at 270 kHz from central frequency which corresponding to the relexation oscillation frequency of the NPRO. The signal to noise ratio before and after amplification are 78 dB and 80 dB respectively. With three stages amplification, ASE is well suppressed.
4. Conclusion
A low noise dual-frequency laser seed and three-stage fiber power amplifier are built and tested. A NPRO together with an AOM form the dual frequency laser seed. The narrow linewidth of NPRO and the stable RF signal added to the AO driver guarantee a very stable beat note with frequency stability at sub hertz level. A three-stage fiber amplification is designed to suppress the ASE in fiber amplifiers and maintain the high SNR of the beat note. More than 50 W dual-frequency laser power is achieved, corresponding to a slope efficiency of 74% with respect to the pump power of the last stage. The beat note stability, modulation depth as well as SNR are maintained well before and after amplification. This high power dual-frequency light source can be used in long distance raging and imaging.
Acknowledgments
This work is supported by National Natural Science Foundation of China (NSFC) under contract 61275053. Special thanks are addressed to Dr. Haiming Sun for fruitful discussions and assistance on fiber fusing.
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