The optical frequency combs (OFCs) with widely and precisely tunable frequency spacing have several unique applications such as generation of microwave to terahertz signals, high-precision phase-coherent wavelength conversion, coherent wireless and wavelength division-multiplexed (WDM) communications. In recent years, a number of approaches have been proposed for OFCs generation (OFCG). Mode-locked lasers and microresonator can generate OFCs with large bandwidth and high stability but suffer from poor tunability because of their fixed resonator. An OFCG based on an optoelectronic oscillator (OEO) can generate OFCs with good tunability but has a complex configuration. Another typical type of OFCG is based on modulators. It is a potential and economic method due to its advantages of simplicity, stability and tunability. In this paper, a novel approach to generating optical frequency combs with widely and precisely tunable frequency spacing based on a double quadrature phase shift key (DQPSK) modulator and highly nonlinear optical fibers (HNLFs) is proposed and experimentally demonstrated. A DFB-LD seed laser at 1550nm is modulated by the DQPSK modulator which is driven by RF signals. 5-line OFCs are generated as the seed OFCs at the output of DQPSK modulator and then sent into a segment of HNLFs. In this scheme, the frequency spacing of OFCs is directly decided by the RF signals’ frequency, which can be widely and precisely tuned. Four-wave mixing (FWM) effect in HNLFs can effectively increase the number of comb lines and expand bandwidth of the seed OFCs without influence on frequency spacing. The configuration is relatively simple and adjustable. The frequency spacing can be precisely tuned from 10 MHz to 20 GHz in our experiments. The typical 25-line OFCs are experimentally generated with 432 GHz bandwidth at 16 GHz frequency spacing.
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.
Based on the spontaneous four wave mixing in micro/nano-fiber (MNF), we report the generation of quantum-correlated
photon pairs. The wavelengths of the signal and idler photons are in the 1310 nm and 851 nm bands, respectively. The
measured ratio between the coincidence and accidental coincidence rates of signal and idler photons is up to 530.
Moreover, we characterize the spectral property of the signal photons in the wavelength range of 1270-1610 nm. The
results reveal that the bandwidth of the photon pairs is much greater than the theoretically expected value due to the
inhomogeneity of the MNF; while the spectrum of Raman scattering in MNF is different from that in conventional
optical fibers and photonic crystal fibers, which may originate from the heating used for fabricating the MNF. Our
investigation shows that the MNF is a promising candidate for developing the sources of quantum light in micro- or
nanometer-scales, and the spectral property of photon pairs can be used to non-invasively test the diameter and
homogeneity of the MNF.