Gaoting Chen

Response characteristics of thin-film-heated tunable fiber Bragg gratings

We investigate the thermal response of a tunable fiber Bragg grating (FBG) with a metal coating on bare fiber surface as a heater both theoretically and experimentally. By solving a differential equation of temperature as a function of time, together with some reasonable approximation, we obtained an explicit description of the thermal response characteristics with the structures heat capacity and a time constant as its parameters. Using a squared wave modulation current and a tunable laser, we measured the temporal response of the tunable FBG. The obtained response curves match our solution. A time constant in the range of subseconds was deduced. Discussions on the tuned FBG spectra are also given at the end to explain the linewidth broadening effect at higher temperature.

Characteristics of metal-coated long-period fiber gratings

Long-period fiber gratings (LPFGs) are usually used as the spectrally selective loss filter in optical fiber communication networks and sensor systems. In this paper, we investigate the characteristics of a tunable filter based on metal-coated long-period fiber grating. By applying different current to the metal coat, the resonance wavelengths can be tuned, with the spectrum shape changing little. The experimental results and theoretical analysis indicate that the metal-coated LPFGs may find interesting applications such as tunable and adjustable filters.

Noises and Stability of Semiconductor Lasers

This chapter is engaged in description and explanation of noises and stability of semiconductor lasers. Section 3.1 introduces the characteristics and mathematical relations of laser noises, which are regarded as stochastic variables. Section 3.2 discusses the linewidth of semiconductor laser, determined by laser’s phase and frequency noises due to the spontaneous emission and photon-carrier coupling. Compared with other kind lasers, the linewidth of semiconductor laser is enlarged by a linewidth enhancement factor, which comes from the high rate of refractive index variation with carrier concentration. The linewidth measurement methods of delayed self-homodyne and self-heterodyne are described. Section 3.3 discusses the characteristics of frequency and intensity noises in the low frequency band, and introduces the measurement methods. Section 3.4 introduces the frequency stabilities and its characterization. The last section briefly describes some other effects and appearances of laser noises, such as chaos, jitter, and mode noises.

Fundamentals of Semiconductor Lasers

The principles and main characteristics of semiconductor laser are introduced in this chapter. The operation of semiconductor laser obeys the basic theory of laser physics; however, it has its own features, different from solid-state laser, gas laser, and other lasers. The laser transition occurs between energy bands, instead of levels; the pump is simply by current injection, the same as semiconductor electronic devices, not by optical pumps or electric discharges. As results the semiconductor laser has attractive characteristics. In this chapter, the condition of population inversion and pump methods are explained in Sect. 1 and Sect. 2. The cavity structure and rate equations are introduced in Sect. 3 and Sect. 4. The longitudinal mode properties and modulation characteristics are discussed in Sect. 5 and Sect. 6. In the last section its thermal performances are described.

Monolithically Integrated Semiconductor Lasers

The ordinary LD with an F-P cavity composed of cleaved facets works often in multiple longitudinal modes; and the mode frequency is susceptible to pump levels and environmental conditions. To make it working in a single longitudinal mode, especially in case of high frequency modulations, it is necessary to insert a wavelength selective element in the cavity. Three monolithically integrated semiconductor lasers with excellent characteristics of single longitudinal mode operation are introduced in this Chapter: the distributed feedback (DFB) laser, the distributed Bragg reflector (DBR) laser, and the vertical cavity surface emitting laser (VCSEL) in three sections, respectively. All of them are based on the principle of Bragg refraction, but have different characteristics.

Optical Phase Locked Loop and Frequency Transfer

The optical phase locked loop (OPLL) technology is used to lock not only the central frequency but also the optical phase of a laser onto a highly stable low noise laser. It is used to transfer laser frequency arbitrarily not only to another statically operated stable laser, but also to a dynamically frequency modulated laser, which is an important function in laser technology. Sect. 8.1 gives an explicit introduction to OPLL and its related devices. Sect. 8.2 discusses main applications of OPLL, including coherent optical communications, transportation of time and frequency standards, microwave photonics, and physical researches. Sect. 8.3 gives a brief introduction to the frequency comb, which is an important progress in optics and a powerful tool in laser frequency transfer and laser spectroscopy. Sect 8.4 describes some of related applications of the optical frequency comb.

Applications of Single-Frequency Semiconductor Lasers

Semiconductor lasers have been widely used in variety of applications, becoming one of the most widely used members of the laser family. The single-frequency semiconductor laser plays more and more important roles in the fields of advanced sciences and technologies in recent years. This chapter introduces four of the typical applications. Section 9.1 introduces the atomic physics and related technologies, especially the laser cooling of atoms and the atomic clock. Section 9.2 is engaged in the development of optical communications, including brief reviews of ordinary fiber communications, coherent optical communications, the microwave photonics, and the free-space communications. Section 9.3 is devoted to the metrology and sensing, including interferometry, optical fiber sensors, and laser spectroscopy. The optical radar, i.e., lidar, and the synthetic aperture lidar are briefly introduced in Sect. 9.4. Each of the topics involves its own detailed theories and technologies; the introductions here give just qualitative descriptions and simplified explanations.

Frequency Sweeping of Semiconductor Lasers

Frequency-swept lasers are used in many important applications, such as the high-precision optical spectrum analyzer, the high-resolution laser spectroscopy, the frequency-modulated continuous wave (FMCW) lidar, the optical frequency-domain imaging (OFDI), the optical time-domain reflectometry (OTDR) and the optical frequency-domain reflectometry (OFDR), the optical communications and microwave photonics, and fundamental scientific researches, as introduced in Sect. 7.1. The method of direct current modulation of LD is introduced in Sect. 7.2; the sweeping linearity and its characterization are discussed. Section 7.3 describes methods by using intra-cavity modulators and beam deflectors inside the ECDL cavity and the related electro-optic materials and devices. Section 7.4 introduces methods by using external modulations placed in the path out of laser cavity, and the related acousto-optic modulators and electro-optic modulators, especially the single side band modulator.

Narrow linewidth hybrid integrated external cavity diode laser for precision applications

A butterfly-packaged narrow-linewidth hybrid integrated external cavity diode laser module based on the polarization maintaining fiber Bragg grating is reported. The module emits at the wavelength of 1550 nm and provides 21 GHz of continuous tunability. It produces ≥ 20 mW of polarization maintaining fiber-coupled output power with intrinsic Lorentz linewidth ≤ 3 kHz and RIN ≤140 dB/√Hz@100 kHz. To qualify the reliability of the laser module under harsh environmental conditions, random vibration test and high-low temperature cycling test are carried out, and no degradation of the power current characteristic is observed.

Q-switched erbium-doped fiber laser based on MZI and ECL

In this paper, we report, for the first time to our knowledge, a new decreasing Q-switched pulse width method. We can reduce the pulse width of the MZI Q-switched by moving the pulse of ECL (whose pulse width is narrower than the MZI Q-switched pulse) into the MZI Q-switched pulse. And in our experiment, the pulse width of the MZI Q-switched is 4? s, the ultimate pulse width is O.8? s. So the ultimate pulse width is one fifth of the MZI Q-switched.