Joint Research on Gain-Coupled DFB Semiconductor Laser

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1. Measurement of gain coupling coefficient : Although the gain-coupling coefficient is the most imporant device parameter in the gain-coupled (GC) distributed-feedback (DFB) laser, there has been no measurement method for it. Here we studied, with the University of Gent, extraction of parameters including index and gain coupling coeffcients from DFB lasers by minimum-square-fitting theoretical subthreshold spectrum with the experimental one. As a result, it has become possible for the first time to measure and extract, for example, an index-coupling coefficient of 56.4cm_<-1> and a gain-coupling coefficient of 22.3cm_<-1> out of a complex-coupled DFB laser. Also, this method permits to determine the facet reflectivity and phase. Moreover, the linewidth enhancement factor can be extracted if the subthreshold spectrum is measured at different bias levels.2. Disordering of stratined InGaAs/InGaAIAs quantum wells on InP substrates : The above-mentioned quantum wells were grown by molecula … More r beam epitaxy at the University of California. The samples were next capped with SiO_2 and/or SiN films by plasma-assisted chemical vapor deposition (p-CVD) that were made into patterns later. Then, rapid thermal annealing (RTA) was applied to the whole sample to disorder the quantum wells. Photoluminescence (PL) from such samples was taken so that the degree of disordering could be evaluated by the PL wavelength shifts. Consequently, it was verified that the SiO_2 worked as a disordering promoter whereas the SiN worked as a protective cap which suppressed the disordering. The stress existing in the strained quantum wells was found to cause the disordering the RTA heating process regardless of whether the cap film was deposited or not. Finally, the in-plane spatial resolution of the selective disordering was evaluated through microscopic PL measurement. As a result, it turned out that microfabrication down to 2mum was possible by such processing.3. Analysis and fabrication of distributed forward-and backward-coupling wavelength-tunable laser : In wavelengthdivision multiplexed (WDM) optical communication systems, wide-range wavelength tunability is required in semiconductor lasers. However, in the GC DFB laser, tunable oscillation wavelength has not been possible so far. Here we proposed and studied, in collaboration with University of Gent, "distributed forward-and backward-coupling (DFBC) " structure that brings about wide-range wavelength tunability in the GC DFB laser. This structure has a sampled grating above the active layr, and a codirectional passive waveguide for shifting refractive index below the active layr. The sampled grating provides backward feedback necessary for laser oscillation, whereas the codirectional forward coupling between the active and the passive waveguides provides wavelength-filtering function. A small change in the index of the codirectional passive waveguide results in a large change in the wavelength selected by the filter, and thereby a wide-range tuning is made possible. Making use of the F-matrix analysis, we simulated wavelength tuning characteristics of a 1.55mum purely-gain-coupled DFBC laser, and found that index change as small as 0.03 in the codirectional passive waveguide could give rise to lasing wavelength shift as large as 100nm. Next, we designed a DFBC laser in 0.8mum wavelength region, and did preliminary fabrication experiment. After the first step MOCVD (organo-metallic vapor phase epitaxy), the sampled grating was formed by a newly-developed method of dual photoresist coating. Then, the second step MOCVD as well as a number of photolithography steps were carried out to complete the DFBC structure. We verified electrical diode characteristics and spontaneous emission in the fabricated sample, and thereby the fundamentals of the DFBC laser fabrication were established.4. Monolithic integration of a GC DFB laser and an external EA modulator : In collaboration with Tsinghua University, we fabricated a GC DFB laser integrated with an electro-absorption (EA) modulator. By detuning the DFB wavelength toward the longer side with respect to the gain profile peak, we could use identical quantum wells in both the laser and the modulator sections. As a result, whole structure was made by only two MOCVD growth steps. Due to the facet reflection insensitivity of the GC DFB,the laser oscillated in a single mode, and the modulator had sufficient extinction ratio.5. Analysis of analog modulation distortion in GC DFB lasers : The second-and third-order harmonic distortions which affect analog applications very much were analyzed in collaboration with University of Gent. Consequently, GC DFB lasers were found to generally possess a little smaller distortion than conventional DFB lasers due to smaller longitudinal spatial hole burning. The improvement could be as large as 10 dB in some cases. Less