Tatsuyuki Shinagawa

Polarity inversion in high Mg-doped In-polar InN epitaxial layers

To investigate the Mg-dopability in In-polar InN epilayers grown by molecular beam epitaxy, polarity inversion dependence on Mg-doping level is studied. A multiple-InN layer-structure sample with different Mg-doping levels is grown and analyzed by transmission electron microscopy. Formation of high density V-shaped inversion domains is observed for the Mg-doped InN with Mg concentration ([Mg]) of 2.9×1019cm−3. These domains lead to polarity inversion from In to N polarity. Further study for Mg-doped InN epilayers shows that polarity inversion takes place when [Mg] increases above 1.6×1019cm−3. It is also shown that the Mg-sticking coefficient is almost independent of the polarity.To investigate the Mg-dopability in In-polar InN epilayers grown by molecular beam epitaxy, polarity inversion dependence on Mg-doping level is studied. A multiple-InN layer-structure sample with different Mg-doping levels is grown and analyzed by transmission electron microscopy. Formation of high density V-shaped inversion domains is observed for the Mg-doped InN with Mg concentration ([Mg]) of 2.9×1019cm−3. These domains lead to polarity inversion from In to N polarity. Further study for Mg-doped InN epilayers shows that polarity inversion takes place when [Mg] increases above 1.6×1019cm−3. It is also shown that the Mg-sticking coefficient is almost independent of the polarity.

Recent advances in DFB lasers for ultradense WDM applications

State-of-the-art distributed feedback (DFB) laser modules integrated with a wavelength monitor are presented that provide excellent wavelength stability. By adopting unique and compact configuration, wavelength deviations of as small as a few picometers have been achieved. The laser modules are improved also in the scope of high power, high reliability, and wavelength tunability. Reliability test results of the DFB laser diodes and modules confirm a sufficiently long lifetime of more than 25 years and a small wavelength drift of less than /spl plusmn/3 pm. The developed laser modules are fully applicable to ultradense wavelength-division multiplexing applications with the current narrowest channel spacing of 25 GHz.

Detailed investigation on reliability of wavelength-monitor-integrated fixed and tunable DFB laser diode modules

A highly reliable integration of a wavelength monitor (WM) in an industry standard 14-pin butterfly package has been successfully achieved by using soldering and YAG welding techniques. Mechanical integrity and endurance tests for fixed and tunable distributed-feedback (DFB) laser diode modules (LDMs) were performed according to an extended Telcordia GR-468-CORE in order to appreciate the fixtures of the WM part composed of a prism, a Fabry-Pe/spl acute/rot etalon, power, and WM photodiodes. Wavelength and a fiber output power were evaluated as a function of duration time for each test. Incident light angles change against an etalon and an optical coupling deviates due to the separation between a laser diode part and a WM part for tunable DFB LDMs. The two occurrences have a profound influence on wavelength drifts. It was found that the wavelength drifts were less than /spl plusmn/5 pm under mechanical and thermal stresses for both types of DFB LDMs. It was also confirmed that the coated and mounted etalon itself was also highly reliable under thermal stresses. These results show that the WM-integrated fixed and tunable DFB LDMs were fully applicable to next-generation dense-wavelength-division-multiplexing (DWDM) systems of 50- and 25-GHz channel spacing.

A highly stable and reliable wavelength monitor integrated laser module design

Three existing wavelength monitor integrated laser module designs are evaluated. The shortcomings of these designs are resolved by a unique design that eases alignment tasks and greatly enhances the wavelength stability and the wavelength tunability. For example, the wavelength drift over case temperature is 16 times smaller than the best result of previous reports. With the incorporation of thermal compensation, the temperature-induced wavelength drift of the etalon is eliminated. In anticipation, this design enables a worst overall wavelength-drift of 4.41 pm after 25 years of usage to be achieved.

Wavelength monitor integrated laser modules for 25-GHz-spacing tunable applications

Integration of 25-GHz-spacing wavelength monitors into industry standard butterfly laser modules are successfully achieved through the use of a unique optics. The 2-thermoelectric cooler (TEC) design enables independent temperature control of the laser diode and the wavelength monitor, hence eliminating the temperature dependence of the etalon as well as wavelength variation due to temperature variation and aging. As a result of the independent temperature control, narrowband tuning is easily achieved. Results from a series of thorough wavelength stability tests show that the wavelength variation of these laser modules are well within the ITU-T recommended requirement for 25-GHz channel-spacing dense wavelength-division multiplexing (DWDM) applications throughout 25 years of usage.

High output power widely tunable laser module

We have developed highly reliable widely tunable module, whose performances were comparable with fixed-wavelength DFB laser module. To realize wide tunability, 12 l/4-shifted DFB laser array, S-bend waveguides, MMI coupler and an SOA were integrated on a chip. We could achieve 37nm tunability by controlling each chip temperature in the range of 5 to 45°C. High output fiber coupled power of 30mW and very uniform L-I curves out of 12 DFB lasers were achieved even at 50°C. Good quality of lasing spectrum was obtained. Side mode suppression ratio (SMSR) > 45dB. The well-suppressed reflection at chip front facet contributed to the lower noise characteristics, such as RIN < -140dB/Hz and linewidth < 4MHz. The shift of locked frequency was less than 0.4GHz as the case temperature varied from -5 to 75°C. Very small frequency shift was realized by controlling the temperature of locker part, independently. By optimizing TEC design, we could achieve low TEC power consumption less than 4W under Tcase=75°C and the end of life condition of SOA current. The new function by incorporating SOA was VOA. By changing the operating SOA current, we could vary output power from 1mW to 20mW, maintaining SMSR > 40dB, RIN < -135dB/Hz, linewidth < 4MHz. We also performed optical blocking > 40dB, when SOA current was turned off. We examined modules reliability under high temperature storage of 85°C. The change of output power was < ± 10%, and the shift of locked frequency was < ± 5pm after 2000 hours.