Spectral linewidth reduction in partly gain-coupled 1.55 μm strained multiple quantum-well distributed feedback laser

The linear gain saturation effect is shown to be important in determining the dynamics of multiple-quantum-well (MQW) distributed-feedback (DFB) lasers. A more realistic logarithmic dependence of material gain on carrier density is assumed in a comprehensive MQW DFB laser model. It is found through simulation that because of the linear gain saturation, the interplay between modal gain and differential gain leads to an optimal kappa L for maximum small-signal modulation bandwidth in lambda /4-shifted MQW DFB lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Flat panel displays (FPD) are electronic viewing technologies used to let people see content in media and entertainment, consumer electronics, personal computer and mobile devices. Other applications and industries are medical, transportation and industrial equipment. FPD is often the preferred choice due to lower weight and less power consumption. Common types of FPD are liquid crystal display (LCD), plasma display panels (PDP) and organic light‐emitting diode (OLED). The global market for FPD continues to grow due to the high demand in the automobile and smartphone industries. As a result of the high demand, the FPD manufacturing industry continues to remain highly competitive. In this cost sensitive arena, FPD manufacturers need to find new ways to reduce the cost of ownership of lithography equipment. Most lithography systems in FPD manufacturing plants use high power mercury ARC lamps to generate i‐line (365nm) wavelength for the photoresist exposure process. This is a costly process as the mercury ARC lamps have a limited lifetime, typically less than one month, which results in downtime to swap lamps.. The cost of replacing the high power mercury ARC lamp, together with the high electricity consumption, and disposal of mercury waste, turns out to be a substantial operating cost. This paper will demonstrate how the use of ultraviolet light emitting diode (UV LED), in replacing the high power mercury lamp, will reduce the overall operating cost which therefore lowers the cost of ownership of the stepper. The breakdown of cost structure will be discussed in detail and show that UV LED is a good replacement solution for the high power mercury ARC lamp. The throughput improvement by using UV LED will also be discussed and compared to the conventional high power mercury arc lamp. The analysis will also show that the consistency of throughput, as compared to the degradation of mercury arc lamp intensity, as well as the time saved for replacement further reduces operating costs. This paper also demonstrates the improved performance of dose accuracy and repeatability, as well as the capability to achieve extremely low exposure doses without compromising dose accuracy. The improvement in stepper reliability by removing the conventional shutter moving parts will also be discussed. The performance data for illumination intensity, dose accuracy, uniformity, spectral line width, CD uniformity and cost reduction will be discussed in detail. The final analysis will show that moving to UV LED illumination offers lower cost and better performance.

External optical feedback sensitivity in distributed feedback (DFB) semiconductor lasers is analyzed in this paper with special attention to phase-shifted and complex-coupled lasers. The effects of variations of structural parameters such as index and/or gain coupling strengths, facet reflectivities, and corrugation phase angles on external optical feedback sensitivity are studied. Quarter-wave-shifted index-coupled DFB lasers exhibit low external optical feedback sensitivity for large index coupling coefficients and high facet reflectivities. Pure gain-coupled DFB lasers perform better than the uniform index-coupled DFB lasers without any phase-shifting structure but worse than /spl lambda//4 phase-shifted index-coupled lasers with high coupling strengths. External optical feedback sensitivity of complex-coupled lasers is a sensitive function of the structural parameters and varies significantly with the index-to-gain coupling ratio and the total coupling. Presence of a small amount of index coupling relative to the gain coupling in a gain-coupled laser improves the external optical feedback sensitivity.

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. >

The linewidth narrowing of a semiconductor laser due to weak optical feedback is analysed, taking into account both phase condition and threshold change for the feedback-induced modes. The achievable linewidth reduction lies in between two limiting cases, 1/(1 + X √(1 + α2))2 and 1/(1 + X)2, where α and X are the linewidth enhancement factor and the feedback parameter, respectively.

By exploiting electronic and thermal tuning by forward and backward biasing the tuning region, a record continuous tuning range of 11 nm has been achieved with an improved strained-layer (SL) multiquantum-well (MQW) TTG DFB laser at 1.55μm wavelength. Over a wavelength range of 9.2nm an optical power above 1mW and a spectral linewidth below 30MHz can be maintained with 400μm long devices.

An analytic expression for the linewidth of distributed-feedback (DFB) lasers is derived. Local fluctuations in carrier density are shown to provide a significant contribution to laser linewidth in the presence of spatial holeburning. In a single-electrode DFB laser the effect can be avoided by careful optimization of the kl-product, but for a multielectrode DFB laser the linewidth can also be reduced by adjusting the injected current profile to compensate for spatial holeburning. Numerical results for a three-electrode lambda /4-shifted DFB laser are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An efficient and accurate method is developed for the analysis of ridge-waveguide multiple-quantum-well (MQW) distributed-feedback (DFB) lasers. Based on the combination of the mode-matching transfer-matrix method for the longitudinal grating-assisted coupling and the discrete spectral-index method for the transverse mode characteristics, this method provides a semianalytical approach to the modeling of three-dimensional ridge-waveguide semiconductor lasers. Compared with the conventional combination of effective-index method and the coupled-mode theory, as well as other numerical methods, this approach offers versatility, simplicity, and a compromise between the accuracy and the computation effort. >

Optical frequency stabilisation and linewidth reduction by the current feedback technique were studied with a multi-electrode DFB laser. In the current feedback technique, phase rotation in the feedback loop, particularly induced by a laser, is the fundamental problem. This problem was resolved by using multielectrode DFB lasers without phase inversion. We simultaneously succeeded in optical frequency stabilisation and linewidth reduction with only one feedback electrical circuit. Optical frequency drift and spectral line-width were reduced to 1/14 and 1/126, respectively, of those in the free-running state. The minimum spectral linewidth obtained was 95 kHz.

In-phase gain-coupled distributed feedback (DFB) lasers with etched quantum-well active-layers emitting at 1.3-/spl mu/m wavelength have been fabricated for the first time. High optical output power up to 50 mW was achieved while single-mode operation was maintained at the longer wavelength side of the Bragg stop band because the in-phase gain coupling effect favors this wavelength. With the optimization of the strained-layer multi-quantum wells (MQW) in the active region, a large effective differential gain of 7.5-19/spl times/10/sup -16/ cm/sup 2/, a high modulation bandwidth of 18 GHz, and high temperature operation up to 100/spl deg/C were obtained when the DFB wavelength was not detuned from the material gain peak.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The spectral linewidth of distributed feedback (DFB) laser diodes is theoretically studied. Numerical calculation shows that DFB lasers with long cavity lengths and large coupling coefficients have very narrow spectral linewidth less than 1 MHz, The effects of the phase shift and mirror facets on the spectral characteristics of DFB lasers are also analyzed, It is shown that the phase-shifter further narrows the spectral linewidth of DFB lasers. Its numerical result and physical meaning are also shown.

Simultaneous generation of several wavelengths in a steady-state distributed feedback (DFB) laser on jelly-like dyedoped gelatin is reported. It is shown that such lasing regime of the device is possible when a set of the stationary spatial gratings of different periods is prerecorded in the same volume of its active medium. Holographic characteristics of the employed photosensitive medium have been studied. The 2nd harmonic radiation (&lgr; = 532 nm) of the nanosecond (&tgr;_0.5 &ap; 17 ns) YAG:Nd laser was used for recording the gratings. The obtained holograms were readout with the help of a single-mode He-Ne laser radiation. Both picosecond and nanosecond YAG:Nd lasers were used for DFB laser excitation. The stable multiwavelength operation of a DFB laser at two and three narrow spectral lines simultaneously was obtained. The minimum and maximum clearance between simultaneously emitted spectral lines was measured to be ~ 0.09 and ~25 nm, respectively, at spectral width of separate lines of ~ 0.01 nm.

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.

There is a need to determine the suitability of laser sources for use in coherent transmission systems and to study the spectral characteristics and processes that determine phase and amplitude noise of these sources. A novel method for determining the laser lineshape, linewidth, and phase noise is presented. An optical-fibre, 4 x 4 multiport is used to derive amplitude and phase information of an optical signal by mixing it with a reference optical signal. Signal processing then yields the complex cross-correlation function of the two signals, (or for a single source, the complex auto-correlation function) in the frequency domain. It is intended that information, concerning the amplitude and phase noise of the source or sources, may then be derived or inferred by comparison. The initial system has been designed to operate at 1.55 μm wavelength with a distributed feedback laser.

Distributed feedback lasers emitting in a stable dynamic single mode at 1.5 μm wavelength in a temperature range of more than 50 °C have been developed using the so-called flat-surface buried heterostructure. Threshold currents as low as 14 mA in cw (8 mA in pulse) operating and output powers as high as 20 mW/facet were obtained. By detuning the Bragg wavelength to the shorter wavelength side of the gain peak, and by reducing the confinement factor, a wavelength chirp as a function of modulation current as low as 0.003 nm/mA has been obtained. This value corresponds to a total chirp of 0.1–0.2 nm under normal modulation conditions.