High reliability, high power, single mode laser diodes

The development of fiber optic interferometric sensor systems has created new demands for single mode semiconductor diode lasers with low output noise power. Recently, several single longitudinal and transverse mode GaAlAs laser diodes have become available. Several experimenters have reported the deleterious effects of optical feedback resulting from fiber optic coupling on the laser's mode structure and high frequency noise spectrum.1-3 Our interest for interferometric sensors is in the low frequency noise, up to a few kilohertz. We report here the first such low frequency noise measurements in fiber coupled and uncoupled single mode diode lasers. Additionally, we also show that the use of index matching fluid can be used to greatly reduce optical feedback and its associated low frequency noise, upon coupling into an optical fiber.

The amplification properties of a tunable single transverse mode laser diode can be exploited to be used as an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n \times 1</tex> wavelength division switch. This kind of switch is expected to play an important role in telecommunications systems, especially when signals are not synchronous or when signal bit rates are various. The light amplification ratio between a desired main-mode wavelength and undesired submode wavelengths are improved by decreasing injection light power, and a ratio of 8.5 dB is achieved. This experimental result is in good agreement with theory. It is predicted theoretically using rate equations that the ratio is also improved by increasing the laser cavity mirror loss difference between the main mode and the submodes. A light amplification ratio of more than 11 dB is obtained when the cavity mirror loss difference is more than 50 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .

Previous works on photorefractive self-organizing laser cavities were about lasers that oscillate, prior the self-organization process occurs, on a set of axial modes sharing the same transverse structure. We have now demonstrated that the insertion of a photorefractive crystal can also create a dynamic spatial filter. In a well-designed extended cavity broad-area laser diode, such a filter can affect the transverse modal structure to force a laser, initially oscillating on several transverse modes, to oscillate on a single transverse and longitudinal mode. A large increase of the single-mode operating range of the laser has been observed.

We demonstrate an electrically pumped, single-mode, large area, edge-emitting InGaAsP/InP two-dimensional photonic crystal (PC) Bragg laser with triangular lattice. The laser operates in the single transverse and longitudinal modes with a single lobe, near-diffraction-limited far field. We compare the performance of the triangular-lattice PC Bragg laser with the rectangular-lattice PC Bragg laser fabricated from the same wafer and find that their performances are comparable. Then, we combine two single triangular-lattice PC Bragg lasers that tilt to opposite directions by taking advantage of the symmetry of the single emitter cavity mode. The measurement results show that the combined PC Bragg lasers provide the near-diffraction-limited output beam, and the single wavelength operation is also maintained in the coherently combined broad-area PC Bragg lasers.

GaInAsP/InP terraced substrate (TS) lasers (λ=1.3 µm) have been fabricated and stabilized single transverse mode operation has been demonstrated. The wafers were prepared by a single-step LPE growth on (100) n-InP terraced substrates which was terraced along <011> orientation. Pulsed threshold current was less than 50 mA at room temperature. Single transverse and longitudinal mode operation was obtained up to 2.8 times the threshold. Cw operation was also achieved at room temperatures.

Semiconductor lasers with ring cavities are attractive as possible sources for future fibreto-the-home systems that require affordable components. Ring resonator lasers can readily be integrated and do not require gratings or facets for feedback. They can be operated cw [1] in a single transverse and longitudinal mode [2] without the need for accurate gratings or overgrowth and hence offer themselves strongly for low-cost mass-production.

Tapered lasers are extensively employed in high power single-mode lasing, yet still suffering from the potential formation of higher-order transverse modes in the tapered region, which severely degrades the laser beam quality. Herein, we propose a novel supersymmetric segmented tapered laser to achieve stable fundamental transverse mode operation. Our design features a three-segmented tapered waveguide along the light propagation direction, with sub-waveguides of varying widths flanking each main waveguide segment. The main waveguides measure 8 μm, 12 μm, and 16 μm in width. Specifically, the 8 μm main waveguide has a corresponding subsidiary waveguide of 2.67 μm; the 12 μm main waveguide has two subsidiary waveguides measuring 4.82 μm and 6.63 μm; and the 16 μm main waveguide has two subsidiary waveguides measuring 7.13 μm and 3.68 μm. Based on coupled-mode theory, we systematically analyzed the coupling coefficients under different inter-waveguide spacings, leading to an optimized spacing of 1.2 μm through comprehensive theoretical simulations and fabrication compatibility constraints. Furthermore, phase-shifted gratings were integrated between tapered waveguide segments to enable precise longitudinal mode selection. This synergistic design could successfully realize robust single-mode operation, offering a scalable framework for on-chip integrated high-power, high-beam-quality tapered lasers.

High power 1.02 μm single spatial mode laser diodes with low-loss (3.3 cm -1 ) GaInP buried waveguides have been developed for pumping Pr 3+ -doped optical fibre amplifiers. A maximum CW light output power of 415 mW and an optical fibre output of 71 mW at 200 mA have been achieved. A preliminary lifetest showed stable operation for over 2300 h under 100 mW CW conditions at 50 o C

The generation of tunable narrow-band light in the mid-infrared region is of considerable interest in pollution monitoring, remote sensing and trace gas detection. Nonlinear difference frequency mixing (DFM) is one of several techniques which allow access to the important 3-20 μm region, where many molecules have fundamental absorptions. Recent advances in single longitudinal mode high-power diode-laser technology, coupled with new developments based on periodic poling of non-linear crystals and improved growth techniques for birefringently phasematched crystals (such as GaSe), has made DFM a promising approach for a compact broadly tunable, narrowband infrared source. A judicious selection of pump laser wavelengths and non-linear crystals will allow most of the mid-infrared spectral region to be accessible by DFM. Our approach involves DFM of two high power single longitudinal mode diode lasers in the non-linear crystal, GaSe. We combine the broad tuning-range of the diode lasers with the extended critical phasematching range of GaSe to produce a widely tunable system with which we have implemented computer controlled diode frequency tuning, automatic crystal angle positioning, and automatic alignment of the pump beams into the crystal.

ALADIN TXA is the first in the world All-Solid-State, Compact, Transmitterlaser Assembly for the first in the world Doppler Wind Lidar inside the ESA Aeolus mission. Its optical architecture is that of a MOPA, medium energy, pulsed, frequency tripled, tunable, almost single transverse and single longitudinal mode Nd:YAG lasers with 50 Hz PRF and a three years in-orbit lifetime. A brief resume of the design, together with the qualification approach and the main experimental results obtained with the two flight models are presented. The main technological challenges faced during the program development and the lesson learnt for future space All-Solid-State lasers will complete the paper.

A GaAs-Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> As strip buried heterostructure laser with distributed feedback Bragg reflectors is described. In this structure, the active strip is present in the form of an isolated segment completely buried in wide band gap Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> As. The diode is shown to operate stably in single transverse (fundamental) and longitudinal modes when the Fabry-Perot modes are suppressed by saw cutting the mirrors.

A longitudinally laser diode (LD)-pumped Nd:YAP microchip master oscillator power amplifier (MOPA) system operating at 1.08 μm was demonstrated. The system consisted of a longitudinally LD-pumped monolithic Nd:YAP microchip master laser and a two-stage disc amplifier. The cavity length of the master oscillator was 500 μm. The short cavity length lead to a large frequency separation between the resonator modes and hence single longitudinal mode operation was achieved. Pumped by a high-brightness LD an output power of 155 mW in both single longitudinal and transverse mode was generated. To scale to a high power, the output power from the master oscillator was amplified by amplifier discs. The amplified output power was 1.43 W. The central wavelength of the laser system was tuned by changing the cavity length and refractive index via temperature. Using a shorter master oscillator (L = 310 μm) a continuous output wavelength tuning over the gain profile of 0.9 nm was realized. To our knowledge this is the first study of a MOPA system based on an LD-pumped microchip Nd:YAP laser.

Single-mode optical fiber communication systems demand lasers to operate in a single longitudinal and fundamental transverse mode. Therefore, various single-mode lasers have recently been studied and fabricated(1), and the stable single-mode operation for a laser under high speed modulation has been realized by means of optical injection locking(2). This paper describes a twin-mesa GaAs-(AlGa)As laser integrated with a passive waveguide, which can be controllablly ocsillated in a single longitudinal and fundamental transverse mode with a Δλ of about 1-1.6 Å.

Previous works on photorefractive self-organizing laser cavities were about lasers that oscillate, prior the self-organization process occurs, on a set of axial modes sharing the same transverse structure. In a well-designed broad-area laser diode extended cavity, we theoretically and experimentally demonstrate that the insertion of a photorefractive crystal can also affect the transverse modal structure to force the laser, initially oscillating on several transverse modes, to oscillate on a single transverse and axial mode. This spatial self-organization process leads to an enhancement of the single mode operating range of the laser.

The effects of nonlinear gain on the single longitudinal mode are analyzed by means of multilongitudinal mode and multitransverse mode rate equations and compared with the linear gain calculations and experments. It is shown that further suppression of the third order nonlinear gain on nonlasing modes always serves to accelerate the process of the single longitudinal mode operation and that results obtained with long cavity length are entirely contrary to those where only linear gain is considered. The experiments also show that single longitudinal mode lasers with even larger output power can be realized with long cavity length; and, with further proper reduction of the stripe width, single mode (single frequency) operation can be realized. But the results obtained from both gain considerations indicate that single longitudinal mode operation will be facilitated if the active layer is not too thin and the end face reflectivity is increased.