A narrow-linewidth semiconductor laser chip with highly linear frequency modulation response is presented and validated in two coherent sensing test experiments. This distributed feedback laser monolithic chip has an intrinsic linewidth of less than 10 kHz and an output power over 60 mW. When its injection current is modulated by a triangular function, the laser optical frequency can be modulated by more than 7 GHz at rates up to 100 kHz. The laser frequency modulation response is extremely flat up to 100 MHz, which allows correcting the residual sweep nonlinearities by a proper pre-distortion of the modulation signal. In a first test experiment, the laser was used into a monostatic FMCW lidar system. A point cloud was acquired with a field of view of 20°(H) × 10°(V) and an angular resolution of 0.05° along both axes. The acquisition was performed without averaging using a 7 mm diameter output beam of 100 mW. A high-quality point cloud including several objects of varying reflectivity was measured. In a second test experiment, the laser was used into an OFDR system for a distributed acoustic sensing (DAS) experiment. A short portion of a 50 m long SMF-28 fiber was exposed to a 2 kHz acoustic signal. Processed data clearly shows a strong 2 kHz tone at the location of the acoustic perturbation. In both test experiments, the laser was successfully linearized using modulation signal pre-distortion based on interferograms obtained with a Mach-Zehnder interferometer.
We present an efficient way to remove unwanted Amplified Stimulated Emission (ASE) in high-power fiber lasers and amplifiers using intracavity Chirped Tilted Fiber Bragg Grating (CTFBG) filters. The grating is written with tilted fringes so that the unwanted ASE is reflected into the fiber cladding where it is no longer amplified. Depending on the desired emission wavelength and active fiber, one or several filters are spliced within the active fiber to suppress ASE before it reaches a detrimental power. Numerical simulations clearly show that adding the filters allows amplification in configurations that would just be impossible due to the onset of ASE. The filter bandwidth and extinction, and the maximum allowed active fiber length between each filter are also computed depending on the core/cladding diameter ratio of the active fiber used and the targeted emission wavelength. As an example, a fiber laser at 1018 nm is assembled in a 20/400μm core/cladding diameter ytterbium fiber that is cladding pumped at 976 nm. Two CTFBGs with 20 dB attenuation from 1025 nm to 1070 nm are spliced within the 6-meter-long ytterbium fiber. 432 W of laser emission at 1018 nm is efficiently achieved at 77% slope efficiency with respect to the absorbed pump power. The extinction between the 1018 nm signal and the ASE is greater than 50 dB. Removing the ASE filters from the cavity clearly leads to only self-pulsation of the ASE between 1030 nm and 1050 nm, no generation of 1018 nm light was possible. The measured thermal slope of the filters shows scalability above the kW level. Demonstration at 1908 nm with a 25/400 core/cladding diameter thulium doped fiber is also done. Tests were done to inscribe the CTFBG directly in an ytterbium fiber for simpler implementation and avoid additional splicing.
We present recent developments regarding fiber Bragg gratings for kilowatt-level fiber lasers. First, we show that writing grating reflectors through the fiber coating using an ultrafast laser improves reliability and enables higher pump power handling. The use of ultrafast laser technology also offers more options to produce gratings in larger core fibers. Finally, we show that Raman suppression gratings are a good solution for SRS mitigation with their large (<20 dB) rejection over 15 nm and low reflectivity at Raman wavelengths, and negligible insertion loss at the laser wavelength.
An optical coherent receiver for the down conversion of radio frequency (RF) signals from 10-18 GHz to 2 GHz is presented. Light from a distributed feedback semiconductor laser is split between two lithium niobate Mach-Zehnder modulators driven either by a tunable local oscillator (LO) tone or a RF signal coming, for example, from a receiving antenna. The modulated light signals are combined with an optical coupler and filtered by two fiber Bragg gratings (FBG) that select one optical sideband from each signal. Detection of the filtered light by a balanced photo-detector produces an electrical signal at an intermediate frequency equal to the beat difference between the RF and LO frequencies.
Most current RF photonic systems are made from individually packaged devices that are interconnected with fiber-optic cables. In order to reduce size and weight and make the coherent receiver suitable for use in smaller airborne and mobile platforms, optical and opto-electronic components are packaged within a common enclosure where light routing is performed by micro-optics. A printed circuit board (PCB) is included within the module. It comprises a micro-processor to control and monitor the laser, the FBGs and thermo-electric coolers to ensure a robust operation over time and fluctuating environmental conditions. The module including the PCB, laser, modulators, optics, optical filters and balanced detector has a size of 89 x 64 x 32 mm3.
A CW kilowatt fiber laser numerical model has been developed taking into account intracavity stimulated Raman scattering (SRS). It uses the split-step Fourier method which is applied iteratively over several cavity round trips. The gain distribution is re-evaluated after each iteration with a standard CW model using an effective FBG reflectivity that quantifies the non-linear spectral leakage. This model explains why spectrally narrow output couplers produce more SRS than wider FBGs, as recently reported by other authors, and constitute a powerful tool to design optimized and innovative fiber components to push back the onset of SRS for a given fiber core diameter.
We review the improved performances of a narrow linewidth laser using negative electrical feedback obtained through
advances on narrowband FBG filters. Noteworthy, the tolerance of the laser to vibrations is significantly improved. As
an extension of this work, these narrow filters are proposed for filtering optical signals in RF photonics systems.
We present an alternative method to fabricate multi-wavelength distributed-feedback fiber lasers made of superstructured chirped fiber Bragg gratings in a single writing laser scan with a custom period-profiled phase mask and a tailored amplitude apodization profile produced by phase mask dithering. This method simplifies the fabrication process and increases the yield of samples having the right number of laser lines and a small frequency error with respect to a reference grid.
Photoinscription of superstructured Bragg gratings in Er-Yb codoped fiber is a promising and cost-effective approach to
produce high-quality multi-wavelength fiber lasers for various applications like radio-over-fiber systems, fiber-optic
sensors or low-cost WDM testing source. However, a good understanding of the noise properties of the laser source is
required before these applications can be addressed. Previous modeling has shown that these devices are similar to
compact cascades of single wavelength DFB fiber lasers in which the modes at each wavelength are almost nonoverlapping
along the fiber. In this paper, we further examine the independence of each channel by performing relative
intensity noise (RIN) measurements on a multi-wavelength fiber laser, a dual-polarization fiber laser and a dualwavelength
fiber laser. In each case, we estimate the correlation between the laser lines.
From RIN measurements performed on each channel of a multi-wavelength laser, as well as on the full spectrum, we
compute an average degree of correlation between the RIN of neighboring lines and observed no correlation in most of
the cases. Moreover, each channel displays a single relaxation frequency which is different from those of the other
channels. On the other hand, we observed strong partition noise, with negative correlation, between polarization modes
of a single wavelength fiber laser. Finally, we measured the RIN of the two modes of a dual-wavelength fiber laser with
modes having a greater overlap than the multi-wavelength laser. The results show that the lines share two common
relaxation frequencies, an indication of a dynamic link between them.
An ultra-compact single-mode six-wavelength DBR fiber laser, with potential as a source for multi-channel fiber sensors, is demonstrated. Pumped with a single 150 mW 980-nm diode laser, the average power is 5 dBm per line.
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