Front Facet Reflectivity Research Articles

Reliability study on 50-100-mW CW operation of 680-nm visible laser diodes with a window-mirror structure

We have studied reliability of 680-nm visible laser diodes (LDs) with a window-mirror structure formed by zinc-diffusion-induced disordering of GaInP quantum wells. Aging tests under the condition of CW output power in the range of 50-100 mW for ambient temperature from 40/spl deg/C to 70/spl deg/C have been carried out for 650-/spl mu/m-long LDs and 900-/spl mu/m-long LDs with three kinds of the front facet reflectivities (1.5%, 6% and 13%). In all tests, laser operation was stable for over 5000 h. For example, reliable 500-10000-h operation of the 900-/spl mu/m LDs was realized under the conditions of output power of 50 mW at 70/spl deg/C, 70 mW at 60/spl deg/C, and 100 mW at 40/spl deg/C for the first time. By various aging test results, dependence of the temperature and the output power on increase of the operating current has been investigated. Also, influences of the facet reflectivities and the cavity lengths on the reliability have been studied with focus on the optical power density and the current density. Consequently, it has been clarified that the degradation of the window LDs rather strongly depends on the operating current density than the optical power density. Moreover, it has been proven that the reliability of the window LDs is effectively improved by the reduction of the operating current density due to extension of the cavity length.

Study on Droop Characteristics of Laser Diodes as Light Sources of Laser Beam Printer

To obtain a small droop, which is the ratio of transient reduction of output power after the laser diode (LD) is turned on, the influence of quantum efficiency and transient thermal flow of laser diodes is investigated both theoretically and experimentally. The analysis shows that as the thermal resistance between active region and heatsink is decreased and the reflectivity of front facet is increased, the droop decreases. The results are applied to self-aligned structures with bent active layer laser diodes (SBA LD's). By employing a junction-down configuration for assembly of the SBA laser chips and silicon carbide (SiC) as a submount material, and by coating the front facet so as to have reflectivity of 50% with taking into reduction of catastrophic optical damage (COD) level, the droop is decreased to 7.0%. In contrast, in the conventional SBA LD's which have a junction-up configuration with silicon (Si) as the submount material and have only a passivation coating on the front facet, value of the droop is 17.5%. Our results serve as a guide to the design of the light source of laser beam printers (LBP's).

Optimum asymmetric mirror facet structure for high-efficiency semiconductor lasers

A design theory for asymmetric mirror facet structure lasers that takes into account the spatial hole burning (SHB) effect along the cavity is presented. It is shown that the SHB effect reduces the external quantum efficiency in asymmetric mirror facet structure lasers, and the front facet reflectivity has an optimum value depending on the internal loss for high-efficiency operation. A slope efficiency of more than 50% has been obtained in lasers based on the new design theory. It is noted that nonuniform current injection is one of the most useful methods for increasing the external quantum efficiency, which is reduced by the SHB effect. >

Analysis of wavelength conversion using a multisection DBR laser with a saturable absorber

The static performance of a wavelength converter obtained by a multisection distributed Bragg reflector (DBR) laser with a saturable absorber between two gain sections is analyzed. For this device an optical input signal at one given wavelength is converted into an output signal at the lasing wavelength. The performances are evaluated in terms of conversion efficiency, suppression ratio, and minimum input power needed for wavelength conversion. It is shown that a proper choice of the injected signal wavelength with respect to the lasing wavelength enhances the conversion efficiency and the suppression ratio and reduces the switching power. The performance can be enhanced also by increasing the electrical current applied to the first gain section provided that the second gain section is biased above the transparency level. The authors have shown that reduction of the front facet reflectivity results in a dramatic increase for the conversion efficiency. The sensitivity to the electrical bias condition and to the resonant condition of the injected signal with respect to the cavity has also been investigated. >

Over 245 mW 1.3 μm buried ridge stripe laser diodes on n-substrate fabricated by the reactive ion beam etching technique

1.3 μm GaInAsP/InP buried ridge stripe lasers have been developed using the reactive ion beam etching process for the realization of the mesa stripe and two-step metalorganic vapor phase epitaxy technique for the layers growth. A cw output power over 245 mW has been achieved in 500-μm-long lasers with front and rear facet reflectivities of 5% and 95%, respectively. Transverse-mode stability at high output power (100 mW) has been demonstrated. In addition, high reliability has been shown in preliminary high-power aging experiments.

Low-noise and high-power operation in high reflectivity coated nonabsorbing mirror GaAlAs lasers

We have successfully attained low-noise and high-power operation in GaAlAs lasers by applying high reflectivity facet coating approach to nonabsorbing mirror lasers. High reflectivity coating is used to reduce feedback induced noise coming from hopping between the diode cavity modes and between the external cavity modes. A nonabsorbing mirror structure is used to obtain high-power operation by suppressing mirror degradation and catastrophic optical damage effectively even with the use of high reflectivity coating. Nonabsorbing mirror lasers with the front facet reflectivity 50% showed the relative intensity noise level less than −130 dB/Hz at 3 mW under 0%–10% optical feedback and showed the cw output power as high as 50 mW in fundamental spatial mode operation. Under 30-mW cw operation at 50 °C, on the other hand, obvious degradation has not been observed over 4000 h in the nonabsorbing mirror lasers with 50% front facet reflectivity. Thus the present laser is useful as a low-noise source at a low-power level and also as a high-power source with high reliability.

A qualitative comparison between two semiconductor laser amplifier equivalent circuit models

Two different forms of equivalent circuit models have been independently proposed for semiconductor laser amplifiers. These have interesting similarities in their equivalent circuits. A comparison is made of the models in terms of derivation, completeness, applications, and computing speed. Results from the transmission line laser model (TLLM) are presented and show the effects of input power, carrier inhomogeneities, and front facet reflectivity on two-input intermodulation distortion. >

Optimum cavity length for high conversion efficiency quantum well diode lasers

The cavity length which maximizes the peak power conversion efficiency is determined for quantum well diode lasers. These calculations are based upon simple models of the diode injection laser’s electrical and optical behaviors, including saturation in the quantum well gain-current characteristic. Here the influences of the distributed optical cavity loss, electrical resistivity, and facet reflectivity on the optimum cavity length are described. Although a lower facet reflectivity results in increased threshold current, there are advantages to longer devices, as the peak conversion efficiency is not reduced. Since the optimum cavity length is greater for low reflectivity, the diode series resistance is smaller. Furthermore, when operating at the point where conversion efficiency is a maximum, the power output of the device with low facet reflectivity exceeds that of the device with higher facet reflectivity. Therein lies the principle advantage of reduced front-facet reflectivities in high power, high efficiency quantum well diode lasers. Good agreement results when these predictions are applied to a strained InGaAs/AlGaAs single quantum well laser (λ=0.93 μm).

Yield and device characteristics of DFB lasers: statistics and novel coating design in theory and experiment

Uncoated 1.55- mu m InGaAsP distributed-feedback (DFB) lasers show superior properties e.g. smaller linewidth and lower feedback sensitivity, in comparison to devices with low reflective facet(s). However, the phases of both front and rear mirror reflectivities, with respect to the grating, must be within sharp tolerances to get a stable dynamic single longitudinal mode device. These sharp tolerances lead to rather poor yield figures. A type of coating that allows the change of effective mirror phase by an arbitrary angle while preserving the superior high facet reflectivity is presented. To know what coating is optimum when applied to the batchwise process, a thorough statistical investigation in lasing behavior as a function of random facet phases, kL product, and front facet reflectivity is needed. Theoretical predictions of these quantities are presented, and also more general validity. The predictions on optimum phase rotation and the influence of absolute reflectivity values are verified experimentally and are found to very accurate. >

Effect of front facet reflectivity on transmission characteristics of asymmetric reflectivity DFB lasers

We experimentally investigated the dependence of bit-errorrate of DFB lasers on facet reflectivity. We found that stable single-longitudinal-mode operation was obtained by lowering the front facet reflectivity below 1%. These DFB lasers showed no bit-error-rate floor and no mode-partition.

Longitudinal mode selectivity above threshold for asymmetric mirror structure DFB lasers with low front facet reflectivity

We analysed longitudinal mode selectivity above threshold for asymmetric mirror structure DFB lasers which have a low front facet reflectivity by taking spatial hole burning into account. As a result we found that single longitudinal mode (SLM) operation yield cannot be improved by an introduction of front facet reflectivity for KL of 0.75. There was no strong KL dependence of the SLM operation yield for a low-noise DFB laser with a front facet reflectivity of 0.05.

Dependence of spectral linewidth of DFB lasers on facet reflectivity

We studied theoretically and experimentally the dependence of the spectral linewidth of DFB lasers on the facet reflectivity. It is shown theoretically that a higher facet reflectivity gives a narrower spectral linewidth, but gives a smaller yield of single-longitudinal-mode operation. We also show experimentally that the spectral linewidth takes its minimum value at a front facet reflectivity of about 5%.

High-power high-reliability operation of 1.3 µm p-substrate buried crescent laser diodes

Stable high-power CW operation of 1.3-μm InGaAsP/InP p-substrate buried crescent laser diodes (PBC-LD'S) has been realized, by controlling the front and rear facet reflectivities of the laser diode chips. The front facet reflectivity is reduced to 17 percent and the rear facet reflectivity is increased to 90 percent, by evaporating multilayer dielectric films (Si/Al 2 O 3 SiO 2 :17 percent, SiO 2 /Si/SiO 2 /Si/SiO 2 :90 percent) on each facet. CW light output power of 50 mW is achieved up to 60°C. Aging tests have been carried out under automatic power control (APC) mode conditions of 50°C-30 mW, 40 mW, 50 mW, and 30°C-50 mW. All samples are operating stably in spite of junction-up configuration. The lifetimes are estimated to be more than 2 \times 10^{4} h for all conditions.

1.MU.m wavelength DFB lasers.

1μm wavelength distributed feedback (DFB) lasers are suitable for high-bit-rate lightwave transmission systems. Spatial hole burning is a key factor to obtain the good single longitudinal mode lasing yield. Their corrugation depth has been optimized to raise the reproducibility of stable single longitudinal mode lasing. For asymmetric reflectivity structure, it is important to suppress Fabry-Perot mode lasing. Optimized front facet re-flectivity has minimized relative intensity noise and linewidth. Typical characteristics of these lasers emitting at 1.3μm wavelength include a threshold current of 13m A, a differential efficiency of 0.29 m W/m A, and a 3-d B bandwidth of 2-3 GHz. Long term reliabity is under test.