Design and analysis of step apodized coupling surface grating for narrow linewidth distributed feedback lasers with highly efficient optical field modulation

Analysis of second and third order intermodulation distortion characteristics of semiconductor DFB lasers is, for the first time, performed, including dynamic longitudinal spatial hole burning (LSHB) effect. We have shown theoretically that the third order intermodulation distortion can be lower than the calculated curve without LSHB effect, which is confirmed with experiments.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The spectral linewidth of a long distributed feedback (DFB) laser above threshold at high output power is investigated theoretically. Longitudinal spatial hole burning (LSHB) is shown to have important effects on the lasing characteristics and the broadening of linewidth. A four-wave coupled mode equation is developed to describe the LSHB phenomenon. The threshold gain and the carrier distribution are solved self-consistently above threshold at various output powers. The simulation results show that the power-linewidth product can increase by a factor greater than 2 due to LSHB for long DFB lasers. This theory has important implications on the limits of linewidth reduction by increasing the length of DFB lasers and on possible future device design for coherent optical system.

In gain-coupled (GC) multiple-quantum-well (MQW) distributed feedback (DFB) lasers, phenomena on the submicron scale corresponding to quantum-well active regions with etched Bragg gratings have been shown to affect overall laser performance. In this paper we present results combining submicron bi-dimensional modeling with a fine-scale longitudinal transfer matrix calculation, above threshold. This has the benefit of considering both the detailed bi-dimensional carrier behaviour at the Bragg-wavelength scale and the larger scale DFB effects such as the longitudinal spatial hole burning and the non-uniform current injection along the the laser cavity.

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.

This paper presents a systematic investigation of the third-order intermodulation distortion characteristics in distributed feedback (DFB) semiconductor lasers. The influence of several nonlinearities, such as longitudinal spatial hole burning, gain compression, and relaxation oscillation, is considered. Detailed analysis shows that it is possible to make different nonlinearities cancel one another to give a low intermodulation distortion by choosing the appropriate DFB structure and beat conditions. Specifically, conditions for cancellation between spatial hole burning and gain compression nonlinearities are introduced.

First investigation on spectral linewidth re-broadening due to photon density distribution has yielded optimized high-power (225mW) CW-DFB MQW lasers with very narrow linewidth (0.7MHz at 100mW).

A statistical yield analysis is presented for gain- and index-coupled distributed feedback (DFB) laser structures, allowing a comparison of their single longitudinal mode (SLM) yield capabilities. For the yield calculations, the threshold gain difference and the longitudinal spatial hole burning (SHB) are taken into account. By comparing the experimental and theoretical yield of index-coupled DFB lasers, the significance of SHB for correct yield predictions is illustrated. For the purpose of comparison, yield calculations for various lambda /4-shifted DFB lasers (with low facet reflectivities) are presented. The most emphasis, however, is on partly gain-coupled DFB lasers. Estimations of practical gain coupling coefficient values for gain and for loss gratings are discussed. >

For high-power semiconductor lasers, asymmetric reflectivities of facets are employed in order to improve slope efficiency. Cavity lengths of these laser diodes have been increased to better distribute heat in order to improve output power. However, these two methods result in an inhomogeneous longitudinal profile of photon density, which leads to a nonuniform gain profile and is typically referred to as longitudinal spatial hole burning (LSHB). In this work, we developed a model to self-consistently calculate the longitudinal photon density distribution, carrier density distribution, and gain distribution in a high-power semiconductor laser. The calculation is based on modified rate equations, and a finite difference method is used to solve the differential equations. Newton’s method is employed to obtain final results with residual error below 10^-6. The impact of LSHB was analyzed with different parameters, and we demonstrate that LSHB is expected to limit the maximum achievable output power of semiconductor lasers having cavity lengths in excess of several mm. The results are expected to be useful in the optimization of high-power semiconductor laser designs.

In the present paper, the comprehensive self-consistent VCSEL physical model is used to analyse excitation of successive transverse cavity modes in oxide-confined (OC) vertical-cavity surface-emitting diode lasers (VCSELs) using the double intra-cavity-contacted 5-λ-cavity double-quantum-well GaAs-based GaInNAs/GaAs OC VCSEL emitting the 1.3-µm radiation as a typical VCSEL example. As expected, many inter-related physical phenomena occurring within a VCSEL volume have an impact on the mode excitation, e.g. the current spreading between both annular contacts and the central active region, the carrier injection into the active region and radial diffusion within it, creation of optical standing waves within the VCSEL cavity, the heat-flux generation and extraction, creation of the active-region gain profiles, the radial wave-guiding mechanism, penetration passive areas by the mode optical field, spatial hole-burning effect etc. Therefore, for any VCSEL design, an analysis of its mode selectivity requires the fully self-consistent comprehensive simulation of all the above important phenomena together with numerous important interactions between individual physical processes, as it has been done in the present paper. Over-simplified theoretical models may lead to faulty results.

We have developed single-mode quantum cascade lasers (QCLs) emitting at 5.2μm and 7.7 Em. Single-mode emission with narrow line width is achieved using a distributed feedback (DFB) technology. A narrow line width is interpreted in terms of an extremely small effective coupling efficiency of spontaneous emission, caused by ultrafast nonradiative scatterings. We have demonstrated absorption signal detection of NO, N2O, and CH4. The highest spectral resolution can be achieved with a CW mode DFB-QCL. We have observed lines of isotopomer of nitrous oxide, and signal detection limit is about 50 ppb.

In this paper we present the fabrication of optical mode field adaptors at the end of single mode and multimode optical fibers, which act as a micro lens, for fiber optical communications devices, capable up to 40Gbit/s data. The mode field adaptors were used to focus the optical output field (1550nm wavelength) of the fiber to receiver and transmitter OEICs. Based on the measurement of a singlemode fiber in accordance with ITU Recommendation G.652 the optical mode fields are measured in a new set-up, which is demonstrated and discussed in comparison to conventional methods. The work was performed in cooperation with the Heinrich-Hertz-Institute in Berlin.

One of the fundamental limitations in photonics is the lack of a bidirectional transducer that can convert optical information into electronic signals or vice versa. In acoustics or at microwave frequencies, wave signals can be simultaneously measured and modulated by a single transducer. In optics, however, optical fields are generally measured via reference-based interferometry or holography using silicone-based image sensors, whereas they are modulated using spatial light modulators. Here, we propose a scheme for an optical bidirectional transducer using a spatial light modulator. By exploiting the principle of time-reversal symmetry of light scattering, two-dimensional reference-free measurement and modulation of optical fields are realized. We experimentally demonstrate the optical bidirectional transducer for optical information consisting of 128 x 128 spatial modes at visible and short-wave infrared wavelengths.

Several important applications in optical communications require high power fiber coupled laser sources operating at 1.3 /spl mu/m, 1.48 /spl mu/m and 1.55 /spl mu/m. Semiconductor diode lasers for these wavelengths can be fabricated using the InP/lnGaAsP material system. High power operation requires careful selection of the active region, p and n doping and the lateral waveguide structure. Additionally, distributed feedback (DFB) lasers require appropriate choice of grating feedback. High power, narrow linewidth and low noise single mode diode laser sources operating at 1.3 /spl mu/m and 1.55 /spl mu/m wavelengths have applications in cable television transmission systems, microwave photonics, interferometric sensors and as an alternative to diode-pumped solid state lasers. High output power is desirable to overcome losses in external modulators and for large scale distribution systems. It is particularly important for 1.3 /spl mu/m applications as reliable and efficient fiber amplifiers are not commercially available at this wavelength. 1.3 /spl mu/m DFB lasers with 440 mW chip power, over 150 mW fiber coupled power and 100 kHz linewidth have been demonstrated.

Quantum cascade lasers are unipolar semiconductor lasers based on intersubband transitions in heterostructures. These lasers, which have demonstrated continuous wave operation at room temperature in the mid-infrared spectral range, are well suited for the realization of compact, ultra-sensitive, trace-gas sensors based on absorption spectroscopy. Up to now, only distributed feedback (DFB) single-mode devices have been used for such applications. DFB quantum cascade lasers have proven to be effective for gas sensing, but their relatively narrow tuning range, smaller or equal to about 1% of the wavelength, makes them not very versatile and limits their usefulness for spectroscopic investigations. In this thesis we developed broadly tunable external cavity quantum cascade lasers. The main advantage of these sources compared with DFBs is their broader tuning range, which is limited only by the spectral bandwidth of the gain element. We particularly studied broad gain bandwidth active regions based on bound-to-continuum designs. With that kind of active region, we have demonstrated a tuning range equal to 15% of the center wavelength at ~ 10 m, which was three times broader than the best values reported in the literature at that time, as well as good performance in pulsed mode at room temperature. Using a strain-compensated bound-to-continuum design emitting near 5.2 m, we have demonstrated for the first time continuous-wave operation of an external cavity quantum cascade laser on a thermoelectric cooler. The tuning range was comparable to that of pulsed devices, but with a much better side-mode suppression ratio and a much narrower linewidth. This continuous-wave device has successfully been applied to the spectroscopy of nitric oxide in collaboration with Prof. Tittel's Laser Science Group at Rice University. High resolution absorption spectra of that gas could be acquired over a large wavelength range. We also studied heterogeneous cascade structures, that is quantum cascade structures in which the various stages emit at several different wavelengths, as a way to increase further the tuning range of external cavity quantum cascade lasers. A pulsed laser tunable from 8.2 to 10.4 m, that is over 24% of the center wavelength has been realized using an active region composed of two substacks of bound-to-continuum designs. We conclude on the feasibility of room temperature, continuous-wave, broadly-tunable external cavity quantum cascade lasers in the mid-infrared spectral range. These devices will be useful for the analysis of multi-component gases, for high resolution spectroscopic investigations in fundamental science, and for the detection of heavy molecules with broad absorption features which DFB quantum cascade lasers cannot scan entirely.

We propose and experimentally demonstrate a turbulence-resilient free space chaotic optical communication system based on vector optical field manipulation. In this scheme, the utilization of vector beam multiplexing technology not only enhances transmission fidelity in the turbulence link but also increases the transmission capacity of the free-space chaotic communication system. A 22 Gbps non-return-zero on-off keying spatial multiplexed free-space chaotic optical communication that is resistant to turbulence is successfully demonstrated. We investigate and compare the effect of using different optical field modes on the chaotic synchronization quality and communication performance under different turbulence strengths, which includes the scalar Gaussian modes with X and Y polarizations, and V-point 1st-order cylindrical vector modes with azimuthal and radial types. The experimental results reveal that employing vector mode leads to a 10% ∼ 13% improvement in the probability of high-quality synchronization and an 8% ∼ 12% reduction in the communication interruption probability compared to the Gaussian mode. Passive turbulence-resilient technique based on vector optical field manipulation is expected to provide new solutions for developing future FSO chaotic communication systems with large capacity and high transmission reliability.