A multiple frequency-swept source based on a recirculating frequency shifter loop (RFSL) is established. Three distributed feedback (DFB) lasers are used as a seed source and sweep in the RFSL synchronously. The swept spectra of the separate DFB lasers are precisely controlled and stitch together without overlap. The key significance of this technique is that the swept range increases as the number of the multiple seed wavelengths increase. Experimentally, the swept range of our system can broaden to 4.5 nm/3.9 nm with an 8.6 GHz/7.5 GHz swept step. The swept rate is 200 kHz. The output of the source is sent into a Mach-Zehnder interferometer and the interference signal is detected to measure the length difference of interferometer arms.
Nyquist pulses, which are defined as responses of a Nyquist filter, can be used in time-division multiplexing transmission which can simultaneously achieve ultrahigh data rate and spectral efficiency. Generally, the methods of Nyquist pulse generation are based on optical Nyquist filters, optical parametric amplifier effect and electro-optical (EO) modulation. In this paper, we focus on the method of EO modulation. Traditionally the limitation of this method is the complex structure and driven signal synchronization between multiple EO modulators when cascaded EO modulators or special modulator structures are using to generate Nyquist pulses. To address this issue, we proposed a novel setup in which only one EO intensity modulator and an electrical arbitrary waveform generator (AWG) are employed. With this method, it is required less on devices. Furthermore, duty cycles of the ideal Nyquist pulses generated by this new method can be changed by using different tones number to drive the EO modulator. The duty cycles of Nyquist pulses we generated can set at 21%, 16% and 12.5% at the repetition of 2.5 GHz by programming the tones number at 2, 3 and 4 on the AWG. The narrowest pulse full width at half maximum is 50.2 ps, which the measured bandwidth is 22.5 GHz by the optical spectrum analyzer, are generated using only one EO intensity modulator with lower bandwidth down to 10 GHz. This method has a potential benefit to reduce the duty cycle further if we use a modulator with bandwidth more than 10 GHz.
We demonstrated experimentally a new method for generation of linearly chirped light waves with almost perfect linearity over a broad range of about 800 GHz. The external modulation method that we adopt can maintain frequency jitters at a very low level by avoiding relaxation oscillation effects which are an intrinsic property in intra-cavity modulation methods. The linearly chirped light could provide an excellent time-frequency mapping tool for wide-range applications.
Proc. SPIE. 10103, Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications X
KEYWORDS: Optical filters, Continuous wave operation, Modulation, Databases, Data transmission, Modulators, Frequency combs, Free space optics, Time division multiplexing, Signal generators, Bragg cells, Data communications, Orthogonal frequency division multiplexing, Nonlinear filtering, Nyquist pulse
Nyquist pulses, which are defined as responses of Nyquist filter, can be used in time-division multiplexing transmission which can simultaneously achieve ultrahigh data rate and spectral efficiency (SE). Generally, the methods for Nyquist pulse generation are based on optical Nyquist filters, nonlinear effects in fiber and phase-locked frequency comb. In this paper, we focus on the third method of phase-locked frequency comb. However, this method has a problem which the large duty cycle of generated Nyquist pulses limits their applications. To address this issue, we proposed a new setup in which one optical intensity modulator and an electrical arbitrary function generator (AFG) are employed. The various duty cycles of ideal Nyquist pulses are generated using one optical intensity modulator so that the phase-locking between the different RF signals is no need any more. And the ideal Nyquist pulses in microwave domain are generated successfully. The duty cycles ranging from 21% to 11% are obtained by programming the number of frequency comb lines in the RF signal which is generated by the AFG. The method has a potential to generate ideal Nyquist pulses in radio frequency domain if a high bandwidth AFG is used to replace the low bandwidth AFG used in this paper.
An external modulation technique for the generation of optical linearly chirped frequency continuous waves is proposed and experimentally demonstrated. The output from a distributed feedback (DFB) laser is modulated by a single sideband (SSB) modulator which is driven by a linearly chirped electrical signal generated by an arbitrary waveform generator (AWG). A high precise linear optical frequency sweep with a tuning range of 2.5GHz is successfully achieved. Due to the good modulation quality of SSB and high stability of AWG, the chirp of the electrical signal can be perfectly modulated onto the optical signal. The linearity of the optical signal is almost consistent with the linearity of the electrical signal. The best linearity of 0.9996 and the lowest jitter rate of 4.76 % are obtained from the experimental measurements which are reported in this paper.