On the influence of spatial hole-burning on mode selection and mode synchronization in a CW laser

The influence of spatial saturation modulation of both the amplifier and absorber in a laser with a Fabry-Perot resonator, and of the associated Bragg-reflection, on the frequency-dependent losses necessary for single-frequency operation, as well as on the frequency synchronization of three modes, has been examined. Conditions for mode selection and mode synchronization are given. To achieve single-frequency operation in a laser with a Fabry-Perot resonator the loss difference between the mode to be selected and modes to be quenched has to be proportional to the Bragg losses of the selected mode suffered by its self-induced grating. The proportionality factor depends on the length and position of the amplifier in the cavity. In the case of a short amplifier and a short absorber inside the resonator, the discussion of conditions for synchronization of three modes shows that the synchronization regime is also determined by the position of the absorber.     

Stable single frequency Er-doped all-fiber ring laser with fiber Bragg grating Fabry-Perot filter

Using a fiber Bragg grating (FBG) and a Fabry-Perot cavity composed of two fiber Bragg gratings (FBGFP) as its frequency-selective components, a type of single frequency all-fiber ring laser permits oscillation only on one longitudinal mode of the main cavity without modehopping while the cavity length can be up to tens of meters. The salient feature is due to the single narrowband resonance of the FBGFP filter. Such a fiber ring laser is achieved experimentally, and the laser mode is limited inside the single resonance of the FBGFP.     

光纤光栅F-P标准具选模单频环形腔光纤激光器

 讨论了光纤光栅法布里-珀罗(F-P)标准具选模光纤激光器的单频运转原理，并研制了全光纤结构单频掺Er³⁺光纤环形激光器。实验中采用两个976 nm激光二极管双向泵浦作为泵浦源，高掺杂浓度掺Er³⁺光纤作为增益介质，以行波腔消除空间烧孔效应，利用光纤光栅F-P标准具窄带选模特性，当泵浦光功率为36 mW时，得到了稳定的单频激光输出。实验中使用了长5和3 m的掺杂光纤，在泵浦光功率为145 mW时输出功率分别为19和42 mW，光-光转换效率分别为13%和29%，斜率效率分别达到了16%和33%。输出谱线3 dB带宽0.01 nm，无跳模现象。     

Wavelength-switchable erbium-doped fiber ring laser combined with a Fabry-Perot laser diode and a sampled grating

We present a wavelength-switchable erbium-doped fiber ring laser consisting of an anti-reflection coated Fabry-Perot laser diode (FP-LD) and a sampled fiber Bragg grating (SFBG). The wavelength switching is obtained easily by the shift of FP mode with respect to the reflection peaks of the SFBG. The proposed laser apparatus not only simultaneously emits two wavelengths but also switches between the two wavelengths by adjusting the modes of FP-LD with a change of the injection current of the FP-LD as little as ∼1 mA. The proposed wavelength-switchable erbium-doped fiber ring laser exhibits high laser performance with side-mode suppression ratio (SMSR) ?50 dB.     

Discretely tunable and multiwavelength erbium doped fibre lasers with Fabry-Perot etalon

Modern WDM telecommunication systems require stable and calibrated laser sources. In this paper, we present examples of such lasers based on erbium doped fibres, i.e., discretely tunable and multiwavelength ring lasers. For the first case, tuning is possible due to a tunable Bragg grating overlapping spectrally the third telecommunication window (1550 nm). When a Fabry-Perot etalon is inserted into the cavity, discrete tunable operation with appropriate spacing between operating wavelengths is possible. Different free spectral ranges (FSR) of applied etalons ensured operation at a wavelength spacing 0.4 nm, 0.8 nm or 1.6 nm, respectively. When an acoustooptical Bragg frequency shifter was inserted together with the Fabry-Perot etalon, the stable simultaneous multiwavelength operation covering the erbium doped gain range was achieved.     

Single-longitudinal-mode fiber ring laser using fiber grating-based Fabry-Perot filters and variable saturable absorbers

A single-longitudinal-mode (SLM) fiber ring laser is demonstrated by incorporating a fiber Bragg grating (FBG)-based Fabry-Perot (F-P) filter and a variable saturable absorber in a fiber ring cavity. An ultra-narrow bandpass filter is established by the combined mode filtering of the FBG-based F-P filter and the variable saturable absorber. No accurate control for cavity length is required, and stable single longitudinal-mode operation without mode hopping is conveniently achieved.     

Design of waveguide-integrated semiconductor laser sources for optical frequency comb generation

A numerical study of threshold gain and modal dispersion in integrated semiconductor laser optical frequency comb sources is presented. We consider an example device where one of the cleaved facets of the laser is replaced by a short Bragg grating section and show that as many as 16 modes can be selected at the first harmonic of the underlying Fabry-Perot cavity. An intracavity approach to limiting the grating-induced dispersion that can be implemented directly through the grating profile is demonstrated.     

Cw oscillation at 1.05 and 1.32 μm of LiNd(PO3)4 lasers in external resonator and in resonator with directly applied mirrors

The room temperature cw oscillation at 1.05 and 1.32 μm of lithium neodymium tetraphosphate [LiNd(PO₃)₄, LNP] lasers in an external resonator and, for the first time, also in a Fabry-Perot resonator with directly applied mirrors is reported. The growth and fluorescence properties of the LNP crystals are briefly described. Laser parameters such as threshold pump power, output power, differential efficiency, optical gain, emission cross section and laser modes are both measured and calculated. Laser experiments show the threshold pump power to be lower for 1.05 μm than for 1.32 μm emission because of the smaller emission cross section; the pump power density is lower for the external resonator than for the Fabry-Perot resonator because of its higher mode losses.     

Photonic generation of microwave signals using dual-wavelength single-longitudinal-mode fiber lasers

Photonic generation of microwave signals is demonstrated with a dual-wavelength single-longitudinal-mode erbium-doped fiber ring laser. Microwave signal with a frequency of 21.07 GHz is obtained by aligning the reflection band of a uniform fiber Bragg grating to the two transmission peaks within a fiber Bragg grating-based Fabry-Perot filter. Stable dual-wavelength single-longitudinal-mode operation is guaranteed by means of the combined filtering effect of a saturable absorber and the Fabry-Perot filter. This approach provides an effective solution to the photonic generation of high-frequency microwave signals.     

Synchronous tunable wavelength spacing dual-wavelength SOA fiber ring laser using Fiber Bragg grating pair in a hybrid tuning package

A Dual-Wavelength Semiconductor Optical Amplifier (DW-SOA) based fiber ring laser with synchronous wavelength tunability is proposed and experimentally demonstrated. The SOA gain medium strongly suppresses mode competition, thus allowing stable dual-wavelength laser oscillation. The wavelength spacing of the two lasers can be tuned synchronously using a modified hybrid-tuning package incorporating a pair of Fiber Bragg Gratings (FBGs). The DW-SOA demonstrates a laser output with a wavelength spacing of between 0.10 and 8.30 nm (wavelength shift inequality of 0.08 to 0.75 nm). The relationship between the applied strain and wavelength shift of the two tuning modes is also analyzed.     

Spatial modal control of two-dimensional photonic crystal Bragg lasers

We investigate the modal losses and field distributions of different order transverse modes supported by the photonic crystal Bragg structure using a transfer matrix method. We find that only the fundamental transverse mode has a single-lobed near field and far field and there exists a trade-off between ensuring lasing in the fundamental transverse mode and reducing the threshold. Employing these design principles, we experimentally demonstrate a large-area, edge-emitting, and single-mode semiconductor photonic crystal Bragg laser with a single-lobed, diffraction-limited far field under continuous wave condition.     

Long-distance fiber Bragg grating sensor system with a high optical signal-to-noise ratio based on a tunable fiber ring laser configuration

A novel tunable fiber ring laser configuration with a combination of bidirectional Raman amplification and dual erbium-doped fiber (EDF) amplification is proposed for realizing high optical signal-to-noise ratio (SNR), long-distance, quasi-distributed fiber Bragg grating (FBG) sensing systems with large capacities and low cost. The hybrid Raman-EDF amplification configuration arranged in the ring laser can enhance the optical SNR of FBG sensor signals significantly owing to the good combination of the high gain of the erbium-doped fiber amplifier (EDFA) and the low noise of the Raman amplification. Such a sensing system can support a large number of FBG sensors because of the use of a tunable fiber Fabry-Perot filter located within the ring laser and spatial division multiplexing for expansion of sensor channels. Experimental results show that an excellent optical SNR of approximately 60 dB has been achieved for a 50 km transmission distance with a low Raman pump power of approximately 170 mW at a wavelength of 1455 nm and a low EDFA pump power of approximately 40 mW at a wavelength of 980 nm, which is the highest optical SNR achieved so far for a 50 km long FBG sensor system, to our knowledge.     

Synchronized Chaos in a Modulated Bidirectional Ring Laser

Nonlinear dynamics of a pump-modulated bidirectional experimentally. Theoretical investigations are based on the pumped bidirectional ring laser that supports single mode in solid-state ring laser are studied theoretically and semiclassical equations of motion for an optically each direction. These equations include coupling of modes via their competition for gain as well as their coupling due to backscattering. Approximate analytical and numerical methods are used to study the dynamical behavior of the system when pump is square-wave modulated. It is found that depending on the depth and frequency of modulation, mode intensities exhibit a rich array of nonlinear dynamical behavior including synchronous and asynchronous chaos, and antiphase periodic and quasi-periodic oscillations.     

Stable single-longitudinal-mode fiber laser using PM FBG F-P etalon and PM fiber saturable absorber

we present a stable and high efficiency single-longitudinal-mode (SLM) fiber ring laser by using polarization-maintaining fiber Bragg grating Fabry-Perot (PM FBG F-P) etalon combining PM fiber saturable absorber. The PM FBG F-P etalon, which is directly fabricated in hydrogen-loaded PM fiber, is used as reflector and mode-selecting component to reduce the longitudinal-mode density greatly. Meanwhile, a segment un-pumped PM Er³⁺-doped fiber (EDF) serves as saturable absorber to suppress mode hopping and ensure the stable SLM operation. Stable SLM laser output at 1550.4 nm with linewidth of 2.0 kHz is observed. Laser output power is 43.7 mW for a pump power of 148 mW, optical-optical efficiency is 29.5% and signal-to-noise ratio (SNR) is larger than 55 dB.     

Multiwavelength fiber ring lasing by use of a semiconductor optical amplifier

A multiwavelength fiber ring laser obtained by use of a semiconductor optical amplifier (SOA) with a simple laser cavity configuration is reported. A Fabry-Perot filter was used in the fiber laser ring cavity to achieve more than 50 simultaneous wavelength lasing oscillations with a frequency separation of 50 GHz. The resulting stable broadband multiwavelength lasing operation was attributed to broadband and flat gain of the SOA, which has a gain flatness of 0.8 dB for more than 20 nm. The laser has a total output power of -3 dBm and a signal-to-spontaneous-noise ratio of 30 dB.     

Experimental characterization of an Yb-doped fiber ring laser with frequency-shifted feedback

We present experimental characteristics of an Yb³⁺-doped fiber ring laser operating with frequency-shifted feedback (FSF) through an acousto-optic modulator (AOM) and seeded by both a stationary continuous-wave (CW) laser and spontaneous emission. We show the spectrum and output characteristics for operations with several effective gain bandwidths, as established by Fabry-Perot etalons inside the cavity. Observation using a high finesse Fabry-Perot interferometer shows that, as expected from earlier work, although the spectrum of the FSF laser without seeding is continuous, when seeded by a CW-laser the spectrum consists of a comb of discrete modes, each offset from the seed by an integer number of AOM frequency shifts. The experimental results are in excellent quantitative agreement with the theory developed earlier [L. Yatsenko, B.W. Shore, K. Bergmann, Opt. Commun. 236 (2004) 183].     

Mode decoupling in solid state ring laser based on stimulated Raman effect in polar crystals

In this paper we study the gain saturation induced mode-coupling control in solid state ring laser devices based on the stimulated Raman effect of the polar crystals in order to realize solid state ring laser gyroscopes.We theoretically investigate the mode coupling induced by gain saturation between clockwise (CW) and counterclockwise (CCW) propagating laser modes.Because the CW and CCW running waves are pumped with counter-propagating lasers respectively,the independent coexistence can be ensured.     

Disappearance of relaxation oscillation frequencies in a multimode solid-state laser

We study analytically the Tang, Statz and deMars rate equations describing a solid-state Fabry-Perot laser. When the modes have equal gains, there is a critical number of lasing modes, above which the low-frequency relaxation oscillations responsible for antiphase dynamics disappear. These results are generalized to include unequal modal gains resulting from a Lorentzian gain profile.     

Switchable dual-wavelength Erbium-doped fiber ring laser with cascaded fiber Bragg gratings

We have experimentally shown wavelength mode switching in a dual-wavelength Erbium-doped single cavity fiber laser where the initial two wavelengths of 1 nm spacing are determined by the cascaded reflection type short-period fiber Bragg gratings having two different centre wavelengths of 1550.5 and 1551.5 nm. The lasing mode depends on the polarization in the ring cavity to migrate from one wavelength to another or operates in both modes in a polarization beam splitter output. To effectively control the polarization in the ring cavity, the polarization controllers were positioned before and after the polarization beam splitter. This method of wavelength switching provides a simple way of mode tuning in dual-wavelength fiber lasers.     

Light guidance in a photonic bandgap slab waveguide consisting of two different Bragg reflectors

By means of a zigzag-wave model, we perform a comprehensive analysis of the guided modes of a photonic bandgap slab waveguide that consists of a low-index guiding layer placed between two different Bragg reflectors. We show that the guided modes of the waveguide operate inside the overlapped bandgaps of the two Bragg reflectors. We label each mode with three mode orders, which reflect the features of the mode-field distribution in the guiding layer and the two Bragg reflectors, respectively. We discuss in detail the transmission characteristics of the guided modes, including the dispersion curves, the cutoff conditions, and the confinement factors, and highlight the mode-selection characteristics of the waveguide. We also study the effects of truncating the Bragg reflectors by deriving an analytical expression to show the dependence of the leakage losses of the modes on the numbers of the periods in the truncated Bragg reflectors.