# High-Power Broadband Frequency Chirped Intensity-Modulated Single-Frequency 1064-nm Laser
## 5 Experimental Results and Discussions
### Spectra of the Modulation  
We fixed the modulation frequency added to the EOM at 0.1, 0.8, 1.6, and 2 GHz. A fast InGaAs detector (CONQUER: KG-PR-12G-B, Beijing, China) and a spectrum analyzer (Agilent:N1996A, Palo Alto, CA, USA) were used to obtain the spectra of the modulation of the light. Figure 5a shows the spectra of modulation before YDFA and Figure 5b shows the spectra of modulation after YDFA. When the modulation frequency was low, we can see many high harmonics; for instance, there were three high harmonics at 200, 300, and 400 MHz, respectively. When the modulation frequency was set to 100 MHz, this was the problem from the signal source. When the modulation frequency increased, less higher order harmonics could be seen, e.g., at 800 MHz, we could see two higher order harmonics at 1.6 and 2.4 GHz, but we could not see high-order harmonics at the fundamental modulation frequency of 1.6 and 2.0 GHz.  
In the application of FDR, the high harmonic noise should be suppressed, and a specially designed filter will need to be added before the frequency sweeping signal is added to the EOM. After the modulated light was amplified by the YDFA, we saw less high-order harmonics in the spectra. When the modulated laser was transmitted in optical fiber, the absorption and scattering caused part of the loss. The scattering had gain characteristics and reduced the conversion efficiency. The higher the modulation frequency, the stronger the scattering intensity. Therefore, the high-order harmonic of the modulated laser was suppressed because of the smaller gain coefficien  
Figure 4: Power spectrum for the amplifier output power.  
Figure 5: Spectrum of the modulation (**a**) before and (**b**) after amplification.  
scattering. The amplification process suppressed some high-order harmonic noise to a certain extent and the signal to noise ratio was improved.  
The stability of the modulation frequency before and after amplification were measured with a frequency counter (Agilent: 53220A, Palo Alto, CA, USA) at 350 MHz. Figure 6 shows the results. The stabilities of the modulation frequency before and after the amplification within 20 min were 1.2 kHz and 400 Hz, respectively. There is no significant difference in stability at different modulation frequencies. Relaxation oscillation noise and environmental noise were suppressed by the feedback system of the YDFA, so the stability of the modulation frequency was effectively improved after amplification. In addition, the suppression of higher harmonics also improved the stability of the modulation frequency.  
We measured the frequency sweeping linearities of the modulation. The measurement was limited to a frequency range from 10 to 350 MHz by the bandwidth of the frequency counter. The modulation frequency added to the EOM was linearly swept from 10 to 350 MHz with a step of 100 kHz, and the dwell time of each point was 10 ms. The modulated light beam was detected by a photo diode detector with a 12-GHz bandwidth and the frequency of the modulation was counted by the frequency meter. In total, 3400 frequency points were recorded in 34 s. The frequency sweeping was repeated within 3.5 min of the measuring time. Figure 7a and b show the time-dependent curves of the modulation frequency before and after YDFA, respectively. The solid lines are linear fits of the measured points. The average linearities of the frequency sweeping with respect to time in six measuring periods were 0.9989 and 0.9966 before and after amplification, respectively. One can see that the amplification did not induce obvious degeneration of the linearity of the frequency sweeping. The maximum dwell point of LFM signal generated by the signal source was 65,535, so the maximum frequency resolution at the 2-GHz modulation bandwidth was 30.5 kHz, corresponding to an unambiguous range of 2.46 km and ranging resolution of 3.75 cm in the air.