The Fabry-Perot laser diode controlled by the fibre Bragg grating

High power single-mode pump laser diodes operating around 980nm are key components for Erbium-doped devices. Much effort is still currently devoted to improve both their wavelength stability and their achievable output power, while maintaining a stable single-mode operation. Usually, the emission wavelength is stabilized by an external Fiber Bragg Grating (FBG). This configuration requires free-space optics between the laser diode output facet and the fiber or a lensed fiber to ensure an efficient coupling efficiency. This constraint increases fabrication costs, dimensions and mechanical instabilities. Moreover, the maximum achievable output power is limited because a high optical power density can damage the laser facets. To increase the achievable output power, a solution consists in using Broad-Area Laser Diodes (BALD), which are multimode emitters that are composed of large active ribbons with width of some hundreds of micrometers. The objective is then to improve the beam quality by locking the BALD emission on its transverse fundamental mode. We propose in this article to insert an integrated adiabatic transition between the multimode laser and a single-mode FBG. This taper, made by ion-exchange in glass, provides a coupling efficiency of -22.0dB from the multimode laser emission to the single-mode fiber. An optical feedback of -34dB demonstrates the stabilization of the BALD spectrum at the Bragg wavelength. The spectrum of the device is characterized by a maximum side-mode suppression ratio of 35dB, a RMS spectral width of (0.16 ± 0.04) nm and a frequency shift with current of -12GHz/100mA.

Results of a long-term research in spectral narrowing and transverse mode selection in semiconductor lasers by means of volume Bragg gratings recorded in a photo-thermo-refractive (PTR) glass are described. PTR glass is a multicomponent silicate optical glass which changes its refractive index after UV exposure followed by thermal development. This feature enables recording of volume holograms with efficiency exceeding 97% in visible and near IR spectral regions which tolerate high temperatures up to 400°C, high power laser radiation. Transmitting and reflecting volume Bragg gratings recorded in such manner have spectral and angular selectivity down to 0.01 nm and 0.1 mrad, respectively. These spectral and angular selectors were used as transmitting and reflecting elements of external resonators for high-power semiconductor laser diodes (LDs). Transmitting Bragg gratings provide tunability of LDs in the range up to 60 nm, spectral narrowing down to 200 pm, stabilization of wavelength within 500 pm. Reflecting Bragg gratings allow spectral narrowing down to 20 pm, stabilization of wavelength below 100 pm at temperature variations up to 75 K. A single transverse mode emission for wide stripe LDs is observed at pumping currents exceeding 10 thresholds. Narrowing and stabilization of emission spectra of LD bars is demonstrated. It is important that all these features are achieved by passive elements with efficiency exceeding 97% and unlimited lifetime while actual brightness increase exceeded two orders of magnitude.

We have demonstrated simultaneous generation of stable picosecond laser pulses at five close wavelengths at a pulse repetition rate of approximately 2 GHz by using a self-seeding configuration that consisted of a gain-switched Fabry-Perot laser diode (FPLD) with an external cavity formed by a tunable spectrum-split fiber Bragg grating (FBG). The FBG selected only one of the modes of the FPLD and, at the same time, filtered the selected FPLD mode such that the reflection from the FBG, tapped with a directional coupler, provided an output of dual-wavelength pulses and the transmission through the FBG provided an output of three-wavelength pulses. We could change the intensities and the wavelength separations of the pulses by adjusting the modulating radio frequency and the temperature of the FPLD and the separation of the FBG reflection peaks. In our experiments the highest side-mode suppression ratios of the pulses obtained were 44.5 dB for the dual-wavelength output and 25.0 dB for the three-wavelength output. Also, the laser was demonstrated to minimize pulse intensity imbalance and produce equally separated wavelengths (with a wavelength separation of 0.2 nm). This simple laser offers considerable flexibility for various applications.

An efficient fiber Bragg grating (FBG) sensor integration system, which utilizes a small number of standard Telecom opto-electronics components, is presented in this paper. Wavelength-swept optical pulses were generated by driving a standard telecom DFB laser diode by high amplitude (>3 A) and short duration (300 ns) current pulses. A total laser diode's wavelength sweep in excess of 10 nm was achieved, which allows for simultaneous interrogation of several FBGs within similar wavelengths. The application of short duration wavelength-swept optical pulses also allows for time division multiplexing of small FBG arrays containing FBGs with overlapping characteristic wavelengths. When a short, wavelength-swept optical pulse is launched into the fiber containing single or multiple FBGs along its length, fractions of the launched pulse are back-reflected towards the detector. Since the launched pulse wavelength changes over the time, the reflections from the FBGs occur in the time moments that depend on the FBGs' characteristic wavelengths. Measurement of the time delay among back-reflected optical pulses is then used to determine the FBGs characteristic wavelengths. The experimental system demonstrated FBG wavelength readout resolution, which exceeded 3 pm at 1 KHz sampling rate, maximum sampling rate of over 40 KHz, and capability to readout/demultiplex over 30 FBGs located down a single optical fiber.

A temperature measurement system and algorithm method based on Fiber Bragg Grating (FBG) sensors and Fabry-Perot Filter (FPF) is put forward. Three reference FBGs were used to eliminate the change of the FPF induced by the environment temperature and the nonlinearity of the FPF. Two FBG temperature sensors with different temperature sensitivity were used to measure the temperature, without the expensive standard light wave length component that provide absolute stable light wave length. Arithmetic was induced how to using the fixed wavelength difference of the reference FBGs to eliminate the influence of the environment temperature, and how to using the difference sensitivity of the temperature FBGs to get the absolute temperature of the transmission lines. Experiments with a glass of water were made to contrast the measurement results of this system with that of a mercury temperature meter whose precision is 0.1°C. The experiment results shown that this FBG system has precision bigger than 99.5%.

A hybrid dense‐wavelength‐division‐multiplexing (DWDM) system using a combination of a Fabry–Perot (FP) laser diode, an optical circulator (OC), and a fiber Bragg grating (FBG), as well as half‐split‐band and WDM techniques, is demonstrated. It is a four‐wavelength system, in which two wavelengths are for AM–VSB video, one wavelength is for a 256‐QAM digital signal, and one wavelength is for OC‐48 digital baseband signal transmission. © 2002 John Wiley & Sons, Inc. Microwave Opt Technol Lett 32: 390–393, 2002.

We demonstrate a simple fiber-optic sensor for the measurement of transverse force, where a fiber Bragg grating (FBG) is used as a sensing element as well as a wavelength-selection element for a self-seeded Fabry-Pe/spl acute/rot laser diode (FP-LD). When a transverse force is applied on the FBG, the birefringence induced in the fiber causes the FP-LD to emit short pulses at two close wavelengths. The wavelength separation of the pulses is converted into a time difference between the two pulse trains by using a dispersive single-mode fiber, so that wavelength measurement is replaced by the much less expensive time-difference measurement. To extend the measurement range, the laser is made to emit at the two wavelengths alternately by using a polarization-switching technique. The sensor also allows compensation for any isotropic drift on the Bragg wavelength of the FBG.

A reliable method for measurement of the nonlinear refractive index through application of multi wavelength phenomenon. Multi wavelength realisation based on Erbium doped fibre laser (EDFL) is proposed and experimentally demonstrated. A combination of 15 m high efficiency Erbium doped fibre (EDF) and a 20 m Photonic Crystal Fibre (PCF) as main catalyst to suppress the homogenous broadening of EDF and to obtain highly stability of multi wavelength through insertion of a set of fibre Bragg gratings (FBGs) in the cavity. This PCF has zero dispersion of 1040 nm which mismatch from transmission window of 1550 nm. A reliable repeatability of multi wavelength based on multiple configuration of FBGs less than 0.2% obtained. This consistent results influence in determination of nonlinear refractive index by relation of four wave mixing (FWM).

In this work we report on high-power diode laser modules with enhanced spectral brightness by means of volume holographic gratings for wavelength stabilization. High-power diode laser modules typically have a relatively broad spectral width of about 3 to 6 nm. In addition the center wavelength shifts by changing the temperature and the driving current, which is obstructive for pumping applications with small absorption bandwidths. Wavelength stabilization of high-power diode laser modules is an important means for more efficient pumping of solid-state lasers with a narrow absorption bandwidth. However, for efficient and reliable wavelength stabilization the parameters of the volume holographic grating and the parameters of the diode laser bar have to be adapted carefully. Important parameters are the reflectivity of the volume holographic grating, the reflectivity of the diode laser bar and the angular and spectral emission characteristics of the diode laser bar. In addition, the lateral structure of the diode laser bar and the microoptical elements for beam shaping have to be considered. In this paper we present a detailed characterization of different diode laser systems with wavelength stabilization in the spectral range from 790 - 1000 nm. The laser modules are divided into systems with and without fiber coupling. We will present data for a wavelength stabilized single diode laser bar with an output power of 69 W at a wavelength of 808 nm. Another example is a wavelength stabilized fiber-coupled diode laser module with an output power of 456 W for a fiber with a core diameter of 400 μm (NA 0.22).

The characteristics of transmission spectrum of the Fabry-Perot (F-P) cavity composed of two fiber Bragg gratings (FBG) are analyzed detailedly in this paper. The influencing factors of the position of the longitudinal mode are pointedout. The relationship and difference between the FBG F-P cavity and normal F-P cavity are discussed. The phase factor of the amplitude reflection coefficient in the range of the FBG main lobe is simulated linearly using the effective mirror surface model. The effective length of the FBG is defined and the expression of the effective length is obtained. Furthermore， the effective length of the FBG is brought into the equivalent cavity length of the FBG F-P cavity. The interval between longitudinal modes of the FBG F-P cavity is calculated using the equivalent cavity length. Both the numerical simulation and experimental results show that the interval calculated using the equivalent cavity length accords well with the true interval between longitudinal modes of the FBG F-P cavity.

A 1.3-μm vertical-cavity surface-emitting laser (VCSEL) with a fiber Bragg grating (FBG) as an external cavity was developed. VCSELs emit light vertically from the semiconductor substrate. They have a circular output beam and therefore have a merit of easy coupling with optical fibers. Long-wavelength VCSELs in the 1.3 or 1.55-μm bands have recently been developed, which are suitable for long-distance telecommunication using single mode fibers. In this study, we coupled a 1310-nm VCSEL with an FBG as a wavelength-selective external cavity. The FBG was inscribed into a single mode fiber and the reflectivity was 47%. A rod lens was used for coupling of the VCSEL and a fiber. Grating temperature was varied for tuning of the Bragg wavelength. The spectral width without using an FBG was 85 pm FWHM, then the spectral width decreased to 64 pm FWHM by using an FBG at a temperature of 50°C. Further narrowing to 52 pm FWHM was obtained by an optimized diode current. It is expected that the wavelength dispersion in optical fiber transmission can be reduced by using an FBG.

Electronic technique for temperature compensation of fibre Bragg gratings sensors

An analysis of Fiber Bragg Grating (FBG) sensor for measuring weight is presented here. In the design, FBG was bonded on the cantilever structure using insulating tape. The cantilever structure was loaded with a measurable object, and the transmitted intensity of laser diode was measured. In this study, we used tunable laser controller set to 1554.194 nm and uniform FBG with a center wavelength of 1554 nm. We also used a power meter to measure the intensity. The change in the transmitted intensity of laser diode was found linear, and it showed a repeatable pattern. The results showed that during the loading and unloading of the cantilever structure, the intensity response was detected almost the same for each load. Hence, the results confirmed that the sensor is free of hysteresis.

A three element, 15.3 cm, fiber Bragg grating array (FBGA) operating at 1550 nm wavelength is fabricated using a single mode photosensitive fiber. The FBGA is initially simulated using in-house developed software based on the Transfer Matrix Method, then fabricated using a double frequency Argon laser and a phase mask technique, and interrogated using Optical Frequency Domain Reflectometry. A single fiber Bragg grating (FBG) is accurately strain calibrated using a Fabry-Perot interferometer and piezoelectric actuation. The piezoelectric is linearly ramped, and the shifts in the Bragg wavelength along with the fringe count from the Fabry-Perot interferometer are recorded. The fringe count is then used to determine the strain on the FBG and compared to changes in the Bragg wavelength in-order to calculate the strain gage factor. This result is used to calibrate the FBGA for strain measurements. The FBGA is then bonded to a cantilever beam with three electric strain gages attached next to each FBG in the array. The axial strain results obtained from the electric strain gages and FBGA are compared for various displacements of the cantilever beam. The Fabry-Perot interferometer and piezoelectric calibration method is a non-destructive process that eliminates the need to bond the FBG to an external support during the calibration process, and can also be used to calibrate electric strain gages.

Design of feedback wavelength demodulation and compensation system for FBG-tuned CW fiber laser