High-brightness narrow-line laser diode source with volume Bragg-grating feedback

Thick Bragg gratings recorded in the volume of photo-thermo-refractive (PTR) glass have found wide application in high-power laser beam control. They have been successfully used for spectral beam combining, angular magnifying for beam scanning, selection of transverse and longitudinal modes in laser resonators etc. Laser damage was studied in PTR glass blanks and gratings fabricated under different conditions. No threshold reduction was found after grating recording in PTR glass. The laser damage threshold of a volume Bragg grating for a single mode 1 ns pulse at 1054 nm focused on inclusions was 7 J/cm 2 , while irradiation of inclusion-free sites resulted in surface damage at 20 J/cm 2 . N-on-1 laser damage tests 1 for 10 Hz repetition rate 1064 nm multimode 8 ns pulses focused to a 78 µm diameter spot have shown a 32 J/cm 2 threshold for a Pt-melted sample with detected inclusions, and a surface damage threshold greater than 42 J/cm 2 for a sample with no detected Pt-inclusions.

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).

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.

Spectral beam combining (SBC) has been extensively used for power scaling of laser systems. SBC is an incoherent technique of combining laser radiation from multiple sources with offset wavelengths into a single near-diffractionlimited beam with increased energy brightness. SBC by means of volume Bragg gratings (VBGs) recorded in photo-thermo- refractive (PTR) glass has been shown to be a simple and robust technique for combining high-power laser radiation. High-efficiency large-aperture VBGs were fabricated in PTR glass wafers. While being photosensitive in the UV, PTR glass offers high transmittance in the near-IR and visible parts of spectrum. Excellent mechanical properties and refractive index independent of temperature enable VBGs in PTR glass to withstand high-power laser radiation, making them ideal elements for high-power SBC. We report spectral combination of five randomly polarized fiber lasers with 0.5 nm spectral separation between channels around 1064 nm using reflecting VBGs in PTR glass. Maximum output power of the system is 773 W, corresponding to 91.7% combining efficiency. It is shown that VBGs introduce no significant beam distortions under high-power operation. Additionally, a common-cavity configuration for SBC with automatic wavelength control of sources by intra-cavity VBGs is suggested. Two fiber lasers are combined using this technique and automatic wavelength control is demonstrated. We show how simple power scaling allows obtaining multi-kW near-diffraction-limited laser radiation via SBC with volume Bragg gratings in PTR glass.

In this work spectroscopic and laser characteristic of photo-thermo-refractive (PTR) glasses doped with different concentrations of rare-earth ions (ytterbium-erbium and Neodymium) were comprehensively carried out. Spectroscopic parameters were obtained using some theoretical techniques like Judd-Ofelt theory and Fuchtbauer-Landenburg (F-L) theory. Results show that the optimal concentration of neodymium oxide in PTR is 0.5 mol%, at which the glass demonstrates the best spectral characteristics. It was found that PTR glass doped with 0.1 mol% of erbium oxide and codoped with 2 mol% of ytterbium oxide also shows good spectral-luminescent properties. Laser action on those two samples was demonstrated. optical losses were found to be 0.34 % for neodymium- doped PTR active element and 0.28 % in the case of erbium and ytterbium co-doped active element. These values are quite low and compared to that obtained in commercial laser glasses. It was concluded that the rare-earth ions doped PTR glass isIn this work spectroscopic and laser characteristic of photo-thermo-refractive (PTR) glasses doped with different concentrations of rare-earth ions (ytterbium-erbium and Neodymium) were comprehensively carried out. Spectroscopic parameters were obtained using some theoretical techniques like Judd-Ofelt theory and Fuchtbauer-Landenburg (F-L) theory. Results show that the optimal concentration of neodymium oxide in PTR is 0.5 mol%, at which the glass demonstrates the best spectral characteristics. It was found that PTR glass doped with 0.1 mol% of erbium oxide and codoped with 2 mol% of ytterbium oxide also shows good spectral-luminescent properties. Laser action on those two samples was demonstrated. optical losses were found to be 0.34 % for neodymium- doped PTR active element and 0.28 % in the case of erbium and ytterbium co-doped active element. These values are quite low and compared to that obtained in commercial laser glasses. It was concluded that the rare-earth ions doped PTR glass is a promising material that can be a good candidate for producing the DFB lasers. a promising material that can be a good candidate for producing the DFB lasers.

A simple configuration of a multimode laser diode with external cavity terminated by a fibre Bragg grating is presented. The improvement of its spectral purity and wavelength stabilisation is demonstrated.

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.

One route to generating mid-infrared (mid-IR) radiation is through a two-stage non-linear conversion process from the near-IR, exploiting powerful neodymium lasers operating at wavelengths close to 1 μm. In the first stage of this process non-linear conversion within a degenerate optical parametric oscillator (OPO) is used to double the wavelength of the 1 μm laser. The resultant 2 μm radiation is then used to pump a second OPO, based on a material such as ZGP, for conversion into the 3 to 5 μm mid-IR waveband. Periodically poled lithium niobate (PPLN) is a useful material for conversion from 1 to 2 μm due to its high non-linear coefficient (d_eff ~ 16 pm/V) and the long crystal lengths available (up to 50 mm). Slope efficiencies in excess of 40% have readily been achieved using a simple plane-plane resonator when pumped at 10 kHz with 3.5 mJ pulses from a 1.047 μm Nd:YLF laser. However, the OPO output was spectrally broad at degeneracy with a measured full-width-half-maximum (FWHM) linewidth of approximately 65 nm. This output linewidth is significantly broader than the spectral acceptance bandwidth of ZGP for conversion into the mid-IR. In this paper techniques for spectral narrowing the output from a degenerate PPLN OPO are investigated using two passive elements, a diffraction grating and an air spaced etalon. Slope efficiencies approaching 20% have been obtained using the grating in a dog-leg cavity configuration producing spectrally narrow 2 μm output with linewidths as low as 2 nm. A grating-narrowed degenerate PPLN OPO has been successfully used to pump a ZGP OPO.

On-line measurement of fiber core-temperature is very important for the investigations of high-power fiber lasers. Using thermal infrared imager is currently the most common approach to detect the temperature of fiber, even though it only monitor the surface temperature. So the measurement result is greatly different from the actual temperature of fiber core. Fiber Bragg Gratings (FBGs) have been widely used in detecting many physical quantities, such as strain, temperature, compression strength and curvature due to its intrinsic advantages, including simple manufacture technology, low insertion loss. Here, a new method based on FBG for measuring the temperature of fiber core is presented and experimentally demonstrated. We use a mutiple inscription method that inscribe a low reflectivity FBG on the high reflectivity chirp grating location as a fiber core temperature on-line monitor. Due to the resonant wavelength of chirp grating (1080nm) is far away from FBG's resonance peak (1550nm), it means that the presence of FBG will not impact the performance of the chirp grating. Alongside this composite optical fiber structure is applied to fiber oscillator we inject a amplified spontaneous emission (ASE) light with the wavelength range near 1550 nm from the signal fiber of (2+1)*1 combiner. And then, the reflection spectrum of FBG can be record from the port 2 of circulator. The temperature of chirp grating and low reflectivity FBG can be measure by recording the shift of FBG's resonance peak after the lineal relation between resonance wavelength and temperature are cofirmed. In order to detect the accuracy of this new structure, we also use Optical Frequency Domain Reflectometer (OFDR) to measure the temperature of the chirp grating. Experiment results prove the high accuracy of proposed temperature monitor. It is strongly believed that the novel proposed structure can be used to achieve measurement of fiber core temperature in high-power fiber lasers. This work also provides a novel idea for manufacturing multi-functional composite fiber structure.

Chirped Volume Bragg Gratings (CBGs) in photo-thermo-refractive (PTR) glass allow stretching and compression of ultra-short pulses with high pulse energy and power and minimal beam distortions. PTR glass based CBGs can stretch pulses up to 1 ns while the duration of the recompressed pulses is nearly transform limited. In this work we report for the first time on multiplexed chirped volume Bragg gratings in PTR glass. These novel optical elements allow shaping of an ultrashort pulse spectral profile with a Moire pattern. Such elements can be used for comb-generation, shaping of ultrashort pulses, and output couplers.

Depending on their application high-power semiconductor laser devices have to be optimized with respect to various design parameters. We discuss the simulation and optimization of the device performance with respect to electrical and thermal properties, output power, wavelength stability, and device reliability using the example of our latest pump-laser generation that has been developed and qualified to support the high-end market of erbium-doped fiber amplifiers. Modeled and measured device properties are compared at each design step. Based on an overall optimization of the device performance fiber Bragg grating (FBG) stabilized telecom-grade pump-laser modules have been realized yielding 750 mW fiber-coupled light output power.

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%.

Role of bromine on the thermal and optical properties of photo-thermo-refractive glass

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.

: The purpose of this work was to explore the continuous wave (cw) Raman laser for generating high-power laser radiation in the Infra-Red (IR) spectral regime. This was done by numerically modeling cw Raman lasers. The numerical models were compared with results from recent experiments to confirm their accuracy. A survey was conducted to identify necessary components for this technology, including mirrors and pump lasers. Results indicate that the cw Raman laser can produce useful low-power laser radiation across the spectral range from 1 micrometers to 5 micrometers using commercially available diode lasers as pumps. Available pump lasers limit the ranges of wavelengths that can be produced at high-powers. Using real-world parameters, theory indicates photon conversion efficiencies of approximately 90% are possible. Independent experimental measurements have recorded efficiencies of over 60% without achieving optimal operating conditions. Additionally, the laser can be built to be compact, with longest dimensions on the order of 10 cm. Potential applications of the cw Raman laser include generation of high-power eye-safe radiation, generation of high-power mid-IR laser radiation for IR countermeasures, and generation of a broad range of mid-IR wavelengths for spectroscopy applications.