Single longitudinal mode operation in phase-shifted DFB laser diodes

We have reported that in aysmmetric mirror reflectivity GaInAsP/InP DFB laser, a high single longitudinal mode (SLM) operation yield was achieved by adjusting a KL to to be 0.5 to 1 and a power reflectivity of a low reflecting end to be 0.05 when the other end is as cleaved [1]. This is because that an axial hole burning effect which breaks SLM operation at high output power is not serious as to reduce the gain difference. In such a weakly coupled case, there is a possibility of unnecessary Fabry-Perot (FP) mode lasing due to a mismatch between a gain peak wavelength and a lasing wavelength of DFB mode when a temperature is changed. Here we have examined an effect of the mismatch for 1.3µm DFB lasers, and made clear an optimum region of the wavelength mismatch. With this optimization, low threshold high efficiency DFB lasers with high SLM operation yield over a wide temperature range have been realized.

We propose and experimentally demonstrate an all-pole-type cavity structure to force single longitudinal mode (SLM) operation in fiber lasers. The proposed structure is based on the modified version of the Smith predictor, which compensates delay in a closed-loop feedback system. This structure is applicable to arbitrary wavelengths and gain media, does not limit the tunability range of the laser, and does not require few centimeters long loops or Fabry-Pérot tunable filters to achieve SLM operation. Thus, the major drawbacks of the conventional SLM forcing mechanisms are alleviated. Therefore, this structure can be considered as a universal approach to SLM operation in fiber lasers. To exemplify this structure, an erbium-doped fiber ring laser in continuous-wave mode is implemented. The SLM operation is confirmed from the RF spectrum over a 10 GHz bandwidth, and the laser is tunable over 62 nm from 1512 to 1574 nm.

Enhancement of asymmetry in light output from front/rear facets in resonance-shifted DFB-LDs

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.

Summary form only given. For the reduction of the spectral bandwidth of optical parametric oscillators (OPO's) to values below 1 GHz the use of grazing-incidence (GI) grating has been demonstrated successfully. Even single longitudinal mode (SLM) operation is feasible. A GIOPO device uses the favourable intrinsic properties of nonlinear crystals and can be tuned over a wide spectral range. SLM operation and a large scan range have been demonstrated with a BaB/sub 2/O/sub 4/ GIOPO, where a signal range of 460-630 nm could be covered.

SLM operation of a high repetition rate BBO Optical Parametric Oscillator pumped by DPSSL at 355 nm

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.

A tuneable single longitudinal mode fibre ring laser with dual-taper fibre filter is proposed and experimentally demonstrated in this paper. The single longitudinal mode operation, and power limitations for a Mach–Zehnder interferometer filter generated from a single mode fibre, are verified for the first time. Incorporating an in-line taper fibre Mach–Zehnder interferometer filter inside the laser ring cavity causes a spatial mode beating interference, resulting in a passive narrow band filter with the ability to generate stable single longitudinal modes. The single longitudinal mode achieves a side mode suppression ratio of more than 60 dB using low pump power. The tuneability of the fibre laser ranges from 1525 to 1562 nm using a passive band pass filter. A study of the stability and limitation of the single longitudinal mode in the Mach–Zehnder tapered fibre is also presented.

We propose a new kind of distributed feedback (DFB) semiconductor laser based on the sampled Moire grating (SMG) integrated with a grating reflector (GR). Coupling coefficient along the laser cavity can be controlled, and $\pi $ phase shift ( $\pi $ -PS) can be introduced by SMG. The GR, which is a uniform sampled grating, can provide light reflection instead of high reflection (HR) facet coating. Thus, the front and rear facets are both coated with anti-reflection coatings (AR/AR) to avoid the influence of random grating phase at the HR facet. Therefore, more output power, better single longitudinal mode (SLM) operation, and higher wavelength precision can be simultaneously achieved, compared with traditional DFB lasers with AR/HR facet coatings. As an example, an 8-channel DFB laser array based on the proposed structure has been simulated. The proposed laser may benefit the applications requiring both high output power and wavelength precision, such as wavelength division multiplexing (WDM) transmission systems and gas sensors.

A three phase shifted (3PS) distributed feedback (DFB) semiconductor laser based on Reconstruction-Equivalent-Chirp (REC) technique is experimentally demonstrated for the first time. The simulation results show that the performances of the equivalent 3PS DFB semiconductor laser are nearly the same as that of the true 3PS laser. However, it only changes the μm-level sampling structures but the seed grating is uniform. So, its cost of fabrication is low. The measurement results exhibit its good single longitudinal mode (SLM) operation even at high bias current and surrounding temperature.

Laser radar oscillators are necessarily low output power devices since (1) short optical cavities and low pressure operating conditions are required for minimum fluctuations, (2) short laser cavities and low pressures are required for single frequency, single longitudinal mode operation, and (3) small optical beam cross-sections are required for single transverse mode operation. All of these combined restrictions severely limit the attainable output power and stability of oscillators. Amplifiers, on the other hand, have no such restrictions in discharge length, gas pressure or optical cross-section. With regard to frequency stability, amplifiers are more forgiving than oscillators since only the time variation of the electron and gas density gradients affects the frequency stability. TETIi oscillator-amplifier configurations are used whenever moderate to high power (greater than about 50 watts) applications are desired. In this paper the design of amplifiers for CO, laser radar applications will be presented. This discussion will examine the issues of discharge technique selection, gain requirements, frequency stability requirements/achievability, optical folding schemes to obtain high gain-length products, backward wave feed-back minimization/suppression, and flow requirements/achievability for high average power operation. These are some of the issues that must be considered in the design of moderate power CW and pulsed laser radar amplifiers for short and long range applications.

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.

The influence of nonlinear spectral bandwidth on single longitudinal mode intra-cavity second harmonic generation

Summary form only given. As a suitable epitaxial structure for high power and high brightness LDs, we have proposed a decoupled confinement heterostructure (DCH) and reported on the characteristics of the devices. Here we present the results from a grating integrated structure, narrow-stripe DFB LD, based on DCH technology, and reveal the potential for single longitudinal and single spatial mode operation.

The laser feedback interference in gain-coupled DFB laser diodes (Distribute Feedback LD) is analyzed. The threshold gain and frequency variation in the DFB laser diode are theoretically deduced. Considering the experiment in practice, we analyze the laser feedback interference at the condition of multiple external cavities. Some parameters have influences on the feedback interference, e.g., the coupling coefficient, the linewidth enhancement factor of gain-coupled DFB laser, the reflection coefficient of external reflector. We discussed these different parameters value in numerical analysis, including the simulation at the condition of multiple external cavities. Compared with the effect of laser feedback interference on F-P and λ/4 phase-shifted DFB lasers, the gain-couple DFB lasers are more sensitive to optical feedback for a specific κL value (coupling coefficient and L cavity length). These results show that the gain-couple DFB lasers can be used as laser feedback sensors with a high accuracy.