Bidirectional Dual-Wavelength Raman Fiber Ring Laser

Narrow linewidth fiber lasers are intensively studied during these years, for their wide use in coherent optical communication, optical fiber sensing, high-precision spectroscopy and many other industrial and military applications. A usual technique to suppress linewidth is narrow band filtering in the resonating structure of the laser. However, it is hard in fabrication for a tradition optical filter to achieve an ultra narrow pass band such as kilo-Hertz. In this paper, the parallel feedback structure is proposed and experimentally studied. A fiber laser with ultra narrow linewidth (15Hz detected) is achieved in laboratory, basing on the parallel feedback mechanism in a fiber ring cavity. In multimode fiber, each transverse mode has a different propagation constant. Equivalently, when a light beam propagates form single mode fiber to multimode fiber, it will split into a few parallel light paths with different propagation constants. The parallel feedback structure of a fiber ring laser is carried out by introducing one or more pieces of multi-mode fiber into its resonant cavity. Lasing light in the cavity must fit the restrictions of all light paths, thus linewidth of the laser output is suppressed. Ultra narrow linewidth can be achieved by carefully adjusting the amount and length of multi-mode fiber pieces. The narrowest linewidth achieved in laboratory is 15Hz detected by a delayed self-heterodyne interferometer with a 100km fiber delay line. In this work, we focus on pressuring linewidth, and mechanism of selecting and stabilizing mode isn't employed. Therefore, the narrow linewidth fiber laser isn't single-longitudinal-mode (SLM). A similar ring laser structure with bidirectional output is also experimental implemented, achieving a bandwidth of the same order. The bidirectional ring laser is the basic component of a laser gyroscope, and its linewidth is one key limiting factor of the gyroscope accuracy. This narrow linewidth bidirectional ring laser is a good candidate for accurate fiber laser gyroscope. Theory analysis and experiment shows the fiber ring laser based on parallel feedback structure is robust against environmental disturbance, achieving high stability and ultra narrow linewidth. The parallel feedback mechanism in optical circuits can also used in other applications demanding narrow linewidth.

We present a technique for the absolute measurement of very low-level scattering. The method is absolute in that it relies on fundamental physics and does not require calibration against standards maintained by the National Institute of Standards. In a bi-directional mode-locked ring laser, a difference in longitudinal mode frequency between the two senses of circulation of the intracavity pulses can be measured as a beat note between the corresponding outputs. A very small amount of light backscattered by a sample (located at the pulse crossing) from one sense of circulation into the other, causes the beat note to vanish. The threshold in mode frequency difference that causes locking is a measure of the scattering amplitude.

Dispersion compensating fiber (DCF) has seen considerable progress over the last decade and due to its well-controlled dispersion profile, high reliability and passive operation it has ecome the preferred method for compensating chromatic dispersion in today's optical communication systems. Raman amplification in fibers has found increased use in especially long-haul, high-capacity systems, mainly due to its low noise figure and versatile gain bandwidth. In this paper, we describe how Raman amplification can be used in DCF to realize discrete dispersion-compensating amplifiers, the so-called dispersion compensating Raman amplifier (DCRA). The main focus is on system applications and demonstrated results using DCRAs. These application include terminal compensation using DCRAs, hybrid amplifiers where the DCRA can be used to extend the dynamic range of existing erbium-doped fiber amplifiers (EDFAs), as well as hybrid EDFA/DCRAs with ultra wide gain bandwidth.

Bidirectional single longitudinal mode fiber ring laser have advantages and potential in optical sensing, especially angular velocity sensing. The current methods for implementing bidirectional single longitudinal mode fiber ring laser cannot ensure complete reciprocity in the optical path, and the side mode suppression ratio needs to be increased, while the laser linewidth is relatively wide. This paper proposes a combined filtering structure using a bandpass filter and a cascaded add-drop type fiber ring filter. When realizing bidirectional single longitudinal mode output, the optical path structure is fully reciprocal including polarization state reciprocity, while the side mode suppression ratio is relatively high. The linewidth of the bidirectional laser is around 1.1 kHz, far lower than the previous report of tens of kHz. The fully reciprocal combined filter structure proposed in this paper does not require special devices, and its theoretical system is mature, which is conducive to the subsequent application and improvement of bidirectional single longitudinal mode fiber ring laser.

The amplification effect on stimulated Brillouin scattering (SBS) and Rayleigh scattering in the backward pumped G652 fiber Raman amplifier (FRA) is studied. The pump source is a 1427.2-nm fiber Raman laser whose power is tunable between 0-1200 mW, and the signal source is a tunable narrow spectral bandwidth (l10 MHz) external cavity laser (ECL). The Rayleigh scattering lines are amplified by the FRA and Stokes SBS lines are amplified by the FRA and the fiber Brillouin amplifier. The total gain of SBS lines is the production of the gain of Raman amplifier and that of Brillouin amplifier. In experiment, the SBS gain is about 42 dB and the saturation gain of 25-km G652 backward FRA is about 25 dB, so the gain of fiber Brillouin amplifier is about 17 dB.

Chapter 5 - Optical amplifiers

We investigate the dynamical behavior of a class-B, bidirectional, solid-state ring laser with a square-wave modulated pump. Our treatment includes the coupling of oppositely directed traveling wave modes via backscattering in addition to their coupling via the gain medium. We find that depending on the pump ratio and the depth and frequency of modulation, the intensity waveforms of the two oppositely directed modes may exhibit periodic, quasi-periodic, and chaotic behavior. We also find that although the periodic waveforms of mode intensities are antisynchronized, chaotic waveforms may be synchronized or unsynchronized. A detailed map of different operating regimes as functions of frequency and depth of modulation is presented. Curves are presented to illustrate the behavior.

Room temperature multiwavelength erbium-doped fiber ring laser using a highly nonlinear photonic crystal fiber

DCF-based fiber Raman amplifiers with fiber grating reflectors for tailoring accumulated-dispersion spectra

Tunable multiwavelength Erbium-doped fiber laser based on in-fiber Fabry-Perot interferometer fiber Bragg gratings in linear and ring cavity configurations

We present the theory of Raman amplification in long multimode optical fibers, where strong random mode coupling within groups of quasi-degenerate modes is unavoidable. In such fibers, the signal components in modes that belong to the same strongly coupled group experience the same Raman amplification, where the differential gain is linearly dependent on the aggregate powers of the pump in each of the mode groups. The equations that we derive significantly facilitate the numerical and analytical study of Raman amplification in long multimode fibers.

Mechanisms of rogue waves (RWs) emergence have been extensively studied in fiber lasers with nonlinearly driven cavities [1], Raman fiber amplifiers and lasers [2], and fiber lasers via modulation of the pump [3]. Previously, it has been found that RWs can be emerged because of soliton-soliton interaction through the overlapping of their tails or soliton-dispersive wave interaction. The result of these interactions is a coupling enhancement that leads to chaotic pulse bunching in the form of soliton rain at the time scale of a round-trip time in a high pump power (800 mW) [4]. Such interaction can be controlled by the pump power modulation or/and by injecting a weak seeding signal and noise. Here, we report for the first time the observation of the soliton rain and soliton-soliton interaction that leads to the optical rogue wave emergence in a carbon nano-tube saturable absorber mode-locked fiber laser at low pump power (140mW) without pump modulation or injecting a weak signal and noise to the laser cavity. As we shown recently [5] that by tuning the interaction of two orthogonal-states of polarization (SOP) of the in-cavity birefringent and also the pump SOP whereas the polarization instability leading to the emergence of different optical RW events.

A multi-wavelength laser source is demonstrated with a semiconductor optical amplifier (SOA) as a gain medium. A multi-wavelength comb with equal spacing is achieved due to Fabry–Pérot modes of the SOA which oscillates in the ring cavity. A 100 m long photonics crystal fiber (PCF) is inserted in the ring cavity to provide a nonlinear gain by four-wave mixing (FWM) so that the output comb spectrum can be greatly broadened and flattened. The stability of the ring laser is also increased due to the efficient FWM phenomenon occurring in the PCF. The SOA-based laser can generate 35 lasing lines with equal spacing of 0.28 nm and extinction ratios of more than 30 dB at room temperature. The number of channels of the multi-wavelength laser can be controlled flexibly by changing the ratio of the coupler used in the ring cavity configuration as well as controlling the polarization state of the oscillating laser.

Generation of Q-switched-mode-locked operations in Er-doped fiber laser based on dispersion compensating fiber saturable absorber

Dispersion-compensating fibers (DCF) are the most widely used technology for dispersion compensation. The idea to additionally use the DCF as a Raman gain medium was originally proposed by Hansen et al. in 1998 [1]. This was quickly followed by Emori et al., [2], who demonstrated a broadband lossless DCF using multiple-wavelength Raman pumping. DCF is a good Raman gain medium, due to a relatively high germanium doping level and a small effective area. Normally, a discrete Raman amplifier will contain several kilometers of fiber, adding extra dispersion to the system that must be handled in the overall dispersion management. Dispersion-compensating Raman amplifiers integrate two key functions, dispersion compensation and discrete Raman amplification, into a single component.