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 multi-frequency low-noise semiconductor laser sources for resonant fiber optic gyroscope (RFOG) interrogation that have enabled excellent gyro stability over temperature. Each laser source includes three distributed feedback semiconductor laser chips coupled with micro-lenses to multi-component silicon photonics (SiP) chips. A first laser, the master, is locked to the RFOG with a Pound-Drever-Hall loop. Two slave lasers are optically phase-locked to the master laser with electrical loop bandwidths of 100 MHz. The SiP chips perform beat note detection and several other functions, such as phase and intensity noise suppression. The lasers and SiP chips are packaged in an optical engine that is controlled by compact low noise electronics. The fiber pigtails are connected to the RFOG so that light is sent in clockwise and counterclockwise directions. Tracking of the RFOG resonance frequencies in both directions allows rotation sensing. An ultra-stable differential frequency noise floor of 0.05 Hz/rt-Hz was obtained between the lasers and the coil resonator which was instrumental in achieving results for the RFOG over 60˚ C operating temperature range. The corresponding angle random walk level is less than 0.01 ˚/rt-hr and was not limited by laser differential frequency noise. The gyroscope bias drift over the tested temperature range was maintained within 0.005 ˚/hr, the best-ever published RFOG performance over temperature to date.
Monolithic distributed feedback semiconductor lasers (1550 nm) for FMCW LiDAR applications have been designed, fabricated and tested. The strong optical frequency modulation distortion observed when a standard DFB laser is modulated with a triangular current waveform is significantly mitigated in our laser. A 100 kHz frequency modulation with amplitude of 0.9 GHz and nonlinear distortion of 0.3%, calculated as the standard deviation of the optical frequency after removal of a linear fit, was measured through an unbalanced fiber interferometer. This was achieved without electronic pre-distortion of the triangular waveform. The 60 kHz intrinsic linewidth of the laser was unaffected by the modulation. Two lasers were co-packaged in a 2.6 cm3 multi-layer ceramic package and coupled to fiber pigtails with micro-lenses. The pins of the ceramic package were soldered to a printed circuit board containing the current sources driving the lasers. This optical source was used in a two-channel LiDAR demonstrator built from off-the-shelf fiber optic components and a twodimensional gimbal scanning mirror. This demonstrator enabled detecting a target with 10 % Lambertian reflectivity up to a distance of >120 m and recording point clouds of different scenes. This shows that FMCW LiDAR in combination with highly coherent and linear DFB laser sources is a very promising technology for long range sensing. A version under development will include a silicon photonics chip for further integration and functionality including I/Q detection.
Narrow-linewidth semiconductor lasers, micro-optics, silicon photonics (SiP), low noise electronics and high-density packaging are key elements for the development of compact high-end light sources for sensing.
A laser module for the interrogation of an RFOG (Resonant Fiber-Optic Gyroscope) includes three distributed feedback lasers coupled with micro-lenses to a multi-component SiP chip that performs beat note detection and several other functions. The lasers and SiP chip are packaged in a 2.6 cm3 multi-layer ceramic package, a 4x volume reduction over a first generation module. The package interfaces with 92 electrical pins and two fiber pigtails, one carrying the signals from a master and slave lasers, another carrying that from a second slave laser. The complete laser source including electronics is 60 mm in diameter and 23 mm in height, a 10x volume improvement over a previous version. The master laser can be locked to the RFOG resonator with a loop bandwidth greater than 1 MHz. The slave lasers are offset frequency locked to the master laser with loop bandwidths greater than 100 MHz. This high performance source is compact, automated, robust, and remains locked for days.
A lighter version of this laser module for FM-CW LIDAR applications produces an output optical frequency that varies linearly as a function of the electrical drive. A triangular modulation at 100 kHz with a greater than 1 GHz amplitude has been demonstrated with a linearity noise near 1 MHz as measured through a 150 m unbalanced interferometer.
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 compact three-laser source for optical sensing is presented. It is based on a low-noise implementation of the Pound Drever-Hall method and comprises high-bandwidth optical phase-locked loops. The outputs from three semiconductor distributed feedback lasers, mounted on thermo-electric coolers (TEC), are coupled with micro-lenses into a silicon photonics (SiP) chip that performs beat note detection and several other functions. The chip comprises phase modulators, variable optical attenuators, multi-mode-interference couplers, variable ratio tap couplers, integrated photodiodes and optical fiber butt-couplers. Electrical connections between a metallized ceramic and the TECs, lasers and SiP chip are achieved by wirebonds. All these components stand within a 35 mm by 35 mm package which is interfaced with 90 electrical pins and two fiber pigtails. One pigtail carries the signals from a master and slave lasers, while another carries that from a second slave laser. The pins are soldered to a printed circuit board featuring a micro-processor that controls and monitors the system to ensure stable operation over fluctuating environmental conditions.
This highly adaptable multi-laser source can address various sensing applications requiring the tracking of up to three narrow spectral features with a high bandwidth. It is used to sense a fiber-based ring resonator emulating a resonant fiber optics gyroscope. The master laser is locked to the resonator with a loop bandwidth greater than 1 MHz. The slave lasers are offset frequency locked to the master laser with loop bandwidths greater than 100 MHz. This high performance source is compact, automated, robust, and remains locked for days.
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