Thesis or Dissertation Generation of phase-locked high harmonics by implementing an optical frequency divider


pp.1 - 86 , 2016-03-25 , The University of Electro-Communications
We have successfully generated a new broadband coherent light source in the continuous wave (CW) regime which is an ensemble of multi-harmonic radiations including the fundamental frequency by implementing a frequency dividing technology. The frequency dividing technology which divides an optical frequency to an integer ratio has been studied with the aim of coherently linking independent lasers as part of a historical trend toward the establishment of an optical frequency standard. Such lasers are attractive not only for establishing a frequency standard but also as a coherent light source itself for various practical uses. The laser radiations produced with such technology has provided us five phase-locked harmonics with extremely wide frequency spacing and bandwidth that extends from 124.7 to 623.7 THz, f1: 2403 nm (36 mW), f2: 1201 nm (371 mW), f3: 801 nm (26 mW), f4: 600 nm (28 mW), f5: 480 nm (4 mW). Each harmonic shows a good beam quality, practical spectral power and fine phase coherence among them both in time and space. The system is uniquely designed that all harmonics are generated and propagate coaxially which has not been demonstrated by anyone yet. This design gives the advantage of robustly maintaining the phase coherence among the harmonics against disturbances. The potential applications for this light source can be diverse. In the frequency domain, each of the harmonics can be used as a single-frequency laser source. Moreover, by locking the system to a frequency standard, they can be used for ultra-high precision technology such as high resolution spectroscopy and high precision frequency metrology. In the time domain, the broadband spectrum can be used to synthesize ultrashort pulses train which can be very useful in the study of ultrafast science and technology. The highlight is that this light source has a high potential for the arbitrary optical waveform (AOW) synthesis in the continuous wave (CW) regime. The arbitrary-shaped optical field wave in the time scale of sub-femtosecond can be synthesized by precise control of the spectral phase and amplitude of a broadband harmonics. Such waves can be used to control the motion of electron through atoms and molecules which can be very useful for atto-science study. This technology has not been performed yet due to the limitation of the existing CW light source. In this thesis, we demonstrated that the frequency of 801-nm ( f3 ) single-frequency diode laser is exactly divided by three by generating the difference frequency with another single-frequency diode laser at wavelength of 1201nm (f2~2/3f3), producing a radiation at wavelength of 2403 nm (f1=f3?f2). Then, we generated the second harmonic of f1, (f2′=2f1) and compared it with the frequency f2 of 1201-nm laser. Based on such frequency comparison, we controlled the 1201-nm laser so that the two frequencies, f2 and f2′, coincide exactly. When this frequency control is achieved with a phase precision, the optical-frequency divide-by-three process is completed, resulting in generation of three phase-locked harmonics with an exact integer-frequency ratio of f1 : f2 : f3 = 1 : 2 : 3. Then, based on the generated three phase-locked harmonics, we further generated the 4th ( f4: 600 nm) and the 5th (f5: 480 nm) phase-locked harmonics by generating sum frequencies among the three phase-locked radiations, as f4 = f1 + f3 and f5 = f2 + f3. The total of five harmonics was evaluated in terms of phase coherence stability, spectral power and beam quality.

Number of accesses :  

Other information