Fiber distributed-feedback lasers used as acoustic sensors in air

Contributions to the acoustic signal sensitivity of fiber distributed-feedback (DFB) lasers in air are investigated both theoretically and experimentally. The theoretical results show that the dominant contribution to the laser frequency shift comes from adiabatic temperature shifts in the surrounding air at lower frequencies and from pressure at higher frequencies. The transition frequency was found to be between 5 and 20 kHz, depending on the elastic boundary conditions of the fiber laser. The acoustically induced frequency shifts of two fiber DFB lasers were measured, and the sensitivities varied from 0.61 MHz/Pa at a 100-Hz acoustic frequency to 0.34 kHz/Pa at a 15-kHz acoustic frequency.     

Characterization of phase noise in a single-mode fiber grating Fabry–Perot laser

A comprehensive study on the phase noise characteristics of a single-mode fiber grating Fabry–Perot (FGFP) laser was conducted numerically. Adding to previous studies, the effects of external optical feedback (OFB), external cavity length, temperature, injection current, cavity volume, nonlinear gain compression factor and fiber grating parameters on phase noise characteristics are presented. The temperature dependence (TD) of phase noise was calculated according to the TD of laser parameters and not by the well-known Parkove equation. The frequency spectra of FGFP laser phase noise were calculated by using a Fourier transform. Results show that the TD of the phase noise in FGFP lasers is smaller than that for distributed feedback lasers. The shortest external cavity length that provides the minimum phase noise is found to be around 3.1?cm. In addition, the relaxation oscillation frequency shifts towards more than 6?GHz, which provides larger flat frequency range. Furthermore, phase noise can be eliminated either by increasing the injection current or the OFB level.     

Analysis of distributed-feedback lasers with fractionally organized gratings

We analyze the performance of distributed-feedback lasers with special grating structures. These grating structures consist of subgratings with different lengths (that are integer multiples of a reference length) and/or different phase shifts (that are also integer multiples of a reference phase shift). These grating structures can provide transmission peaks with high quality factors, which may be useful for distributed-feedback lasers. To assess the usefulness of these devices, threshold and steady-state analysis are performed for a few selected examples. A given design example is shown to provide a low threshold without major variations in photon density along the device.     

Fabrication of a distributed-feedback dye laser with a grating structure in its plastic waveguide

Two approaches of fabricating grating structures for waveguided plastic dye lasers are described and compared for lasing performance. Rhodamine6G-doped poly(methyl methacrylate) (PMMA) film on a PMMA substrate was used for the waveguide, and a distributed-feedback (DFB) laser operation with a single-propagation mode was demonstrated. The performances of both types of permanent grating structured DFB dye laser were better than those of a DFB dye laser on a plain waveguide with a dynamic grating formed by the interference of two pump beams. Wide tuning range is expected by use of a multistripe DFB laser with different grating pitches.     

Wavelength tuning in optically mode-locked semiconductor fiber ring lasers with linearly chirped fiber Bragg gratings

We demonstrate wavelength tuning in single-wavelength and multiwavelength semiconductor fiber ring lasers that are mode locked with an optically injected control signal. A semiconductor optical amplifier is used to provide gain as well as to function as an optically controlled mode-locking element. Linearly chirped fiber Bragg gratings--single or superimposed--are used to define the lasing wavelengths as well as to provide wavelength tunability and allow for multiwavelength operation. We obtain pulses of tens of picoseconds in duration when we inject a sinusoidal optical control signal into the laser cavity, and we can tune the lasing wavelength(s) over the reflection bandwidth(s) of the grating(s) by simply changing the frequency of the injected control signal.     

光纤饱和吸收体稳频窄线宽光纤激光器

结合光纤饱和吸收体与光纤光栅法布里-珀罗标准具,研制出了全光纤结构1550nm单频窄线宽掺铒光纤环形激光器.采用两个976nm激光二极管双向抽运作为抽运源,高掺杂浓度铒光纤作为增益介质,以行波腔消除空间烧孔效应,利用光纤光栅法布里-珀罗标准具窄带选模特性,以10m长低掺铒光纤饱和吸收体稳频,得到了十分稳定的窄线宽激光输出.激光器抽运阈值功率21mW,在抽运光功率为145mW时输出光功率39mW,斜率效率30%,信噪比大于50dB.采用延迟自外差方法精确测量光纤激光器线宽小于10kHz.     

Diffractive optical element used in an external feedback configuration to tune the wavelength of uncoated Fabry-Pérot diode lasers

Abstract

A diffractive optical element (DOE) has been designed, fabricated and used in an external feedback configuration to set the wavelength of operation of uncoated Fabry-Pérot diode lasers. The DOE was designed to replace the conventional elements of an external feedback system, which are a collimating lens and grating in the Littrow configuration. The goal was to simplify use of the external cavity laser by replacing the lens and grating with a single optical element while maintaining the performance that is achieved with a grating and lens. Four DOEs were fabricated with two different focal lengths and two different reflectivities. DOE external cavity lasers were tested for maximum tunable range and stability of the wavelength of operation and compared with an external cavity laser based on a lens and grating in the Littrow configuration. A 40 nm tuning range was achieved with the DOE external cavity laser and this is comparable with the tuning range of the external cavity lasers based on a grating and collimating lens in the Littrow configuration.     

Radio frequency controlled synthetic wavelength sweep for absolute distance measurement by optical interferometry

We present a new technique applied to the variable optical synthetic wavelength generation in optical interferometry. It consists of a chain of optical injection locking among three lasers: first a distributed-feedback laser is used as a master to injection lock an intensity-modulated laser that is directly modulated around 15 GHz by a radio frequency generator on a sideband. A second distributed-feedback laser is injection locked on another sideband of the intensity-modulated laser. The variable synthetic wavelength for absolute distance measurement is simply generated by sweeping the radio frequency over a range of several hundred megahertz, which corresponds to the locking range of the two slave lasers. In this condition, the uncertainty of the variable synthetic wavelength is equivalent to the radio frequency uncertainty. This latter has a relative accuracy of 10(-7) or better, resulting in a resolution of +/-25 microm for distances exceeding tens of meters. The radio frequency generator produces a linear frequency sweep of 1 ms duration (i.e., exactly equal to one absolute distance measurement acquisition time), with frequency steps of about 1 MHz. Finally, results of absolute distance measurements for ranges up to 10 m are presented.     

Simple high-resolution wavelength monitor based on a fiber Bragg grating

A compact and low-cost device for monitoring the peak wavelength of single-peak spectral distributions is presented. The system is based on the transmission properties of a fiber Bragg grating when its period is modulated. Different types of optical signal, such as the emission of distributed-feedback lasers and the reflection of a broadband optical source produced by fiber gratings used in sensor systems, can be measured with this device. We demonstrate that a high wavelength resolution of micro 1 pm can be achieved and that our proposal can be used for real-time monitoring.     

Stability of short, single-mode erbium-doped fiber lasers

We conducted a detailed study of the stability of short, erbium-doped fiber lasers fabricated with two UV-induced Bragg gratings written into the doped fiber. We find that the relative intensity noise of single-longitudinal-mode fiber grating lasers is approximately 3 orders of magnitude lower than that of a single-frequency 1.523-mum helium-neon laser. The frequency noise spectrum contains few resonances, none of which exceeds 0.6 kHz/Hz(1/2) rms; the integrated rms frequency noise from 50 Hz to 63 kHz is 36 kHz. We also demonstrate a simple method for monitoring the laser power and number of oscillating modes during laser fabrication.     

Quasi synchronous tuning for grating feedback lasers

A general analytical form of the round trip phase shift in grating feedback diode lasers is proposed. Using the new form, it is obvious that the round trip phase shift can be independent of rotation angle in first order approximation when only one restriction condition is met. We call this the quasi synchronous tuning (QST) condition. In the QST region, a considerably large mode hopping free tuning range can be obtained. An adjustment structure with only one freedom is needed to accurately find and locate the quasi synchronous pivot, which is not strictly confined on the grating surface and its extension. It means that the external cavity diode lasers design can be easier and the laser can be more stable and reliable.     

Tuning of external-cavity semiconductor lasers with chirped diffraction gratings

We present a novel scheme of tunable semiconductor laser based on the use of a chirped grating in an external cavity. The chirped grating is fabricated using a simple holographic technique: two Gaussian beams having wavefronts with different radii of curvature are brought to interfere on a photoresist layer. The tuning properties of chirped gratings have been investigated with semiconductor lasers operated with an external cavity. With this type of grating positioned in Littrow configuration, the wavelength selection can be done by translating the grating without any need to rotate it. This cavity configuration provides a tunable output beam with an angle of propagation that is independent of the wavelength. The translation of chirped gratings was shown to tune a visible diode laser and an infrared diode laser over the same spectral band as the conventional tuning scheme where an unchirped grating is rotated.     

Demonstration of a rapidly strain tuned Er3+-doped fiber laser for sensitive gas detection

We describe a novel combination of a diode-pumped, wavelength-modulated Er(3+)-doped fiber laser light source with a sensitive noise cancellation circuit for detection of acetylene and ammonia. The laser tuning element, a fiber Bragg grating, was mounted in such a way that it could be strained controllably and rapidly, allowing noise cancellation techniques to be applied to the wavelength-modulated output of the fiber laser. The experimental setup is relatively simple and can be extended to other fiber laser wavelengths for which semiconductor lasers are not readily available by selection of a different fiber Bragg grating and gain medium.     

Experimental method based on wavelength-modulation spectroscopy for the characterization of semiconductor lasers under direct modulation

An experimental method is presented for characterization of the combined intensity and frequency modulation produced when the injection current of a laser diode is modulated. The reported technique is based on the analysis of the harmonic signals produced when a modulated laser is used to probe a gas absorption line by the so-called wavelength-modulation spectroscopy method. Based on a theoretical model of this technique, we present two methods that facilitate the determination of (i) the deviation in laser frequency and (ii) the phase shift between intensity and frequency modulation. These methods are illustrated experimentally by measurement of the modulation parameters of a 2-microm distributed-feedback laser by use of a CO2 absorption line. The experimental results have been compared with those obtained with another traditional method and have shown full agreement in the frequency range (400 Hz-30 kHz) considered.     

Surface-grating distributed bragg reflector quantum-well lasers fabricated in AlGaAs-GaAs asymmetric epitaxial waveguides

Ridge waveguide lasers, integrated with single deep-surface distributed Bragg reflectors (DBR's) in the passive section, were fabricated with a GaAs-AlGaAs double-quantum-well structure in an asymmetric waveguide. Third-order gratings, with a period of 389 nm and defined by holographic lithography, were formed by low-damage reactive ion etching processes. The grating losses and optical coupling coefficients were estimated, in particular, by use of the relationship between the real and the effective grating lengths that were computed and reexamined by measurements of grating periodicity and mode spacing. By use of two different geometries, we produced guide lines for obtaining high-performance lasing properties for these surface-grating DBR lasers. Additionally, a detailed analysis of lasing wavelength shifts was carried out for this study. It was found that injected-carrier-induced effects shift the lasing wavelength much more than gain-loss competition within an extended DBR laser cavity.     

Tuning a grating-controlled CO2 laser by means of a scanning slit

The use of diffraction gratings as tuning devices in laser resonators is well known and has been demonstrated abundantly for CO2 lasers. As the grating rotates, the lines of the infrared spectrum of the CO2 molecule reach threshold and oscillate one by one. Here it is shown that it is possible to obtain the same spectrum by moving an aperture inside the cavity perpendicularly to the laser axis while keeping the grating stationary. The conditions for obtaining a well-resolved spectrum are formulated for both the rotating grating and the moving aperture case.     

Wavelength switching in an actively mode-locked Fabry-Perot laser diode with a highly dispersive external cavity

We demonstrate a simple and reliable scheme for wavelength switching in an actively harmonic mode-locked Fabry-Perot laser diode (FPLD) that is coupled to a highly dispersive external fiber cavity. Wavelength switching between FPLD modes is achieved by detuning of the modulation frequency applied to the FPLD. A side-mode suppression ratio of as much as 40 dB is maintained, and the supermode noise suppression ratio is more than 50 dB for the whole wavelength switching operation range of 4.7 nm when the pulse repetition rate is ~2 GHz. The wavelength-switching mechanism of our laser is based on the wavelength-dependent cavity resonance frequency shift that is due to chromatic dispersion, and we verify that our experimental results closely coincide with analytical results.     

Analysis of external optical feedback characteristics of asymmetric, quarter-wave-shifted, distributed-feedback semiconductor lasers

External optical feedback sensitivity is analyzed for a quarter-wave-shifted, index-coupled, distributed-feedback semiconductor laser with asymmetries in reflectivity of facets and in the position of a lambda/4 phase shift. Proper asymmetric structures can withstand higher levels of external optical feedback compared with symmetric structures. However, the mode discrimination and yield are reduced for asymmetric lasers because of statistical variation of the corrugation phases at the reflecting facets.     

High-power widely tunable thulium fiber lasers

Applications requiring long-range atmospheric propagation are driving the development of high-power thulium fiber lasers. We report on the performance of two different laser configurations for high-power tunable thulium fiber lasers: one is a single oscillator utilizing a volume Bragg grating for wavelength stabilization; the other is a master oscillator power amplifier system with the oscillator stabilized and made tunable by a diffraction grating. Each configuration provides >150 W of average power, >50% slope efficiency, narrow output linewidth, and >100 nm tunability in the wavelength range around 2 μm.