We report on edge-emitting InAs/GaAs quantum dot laser promising as multiple wavelength light source for dense
wavelength-division-multiplexing systems in future generation of silicon photonic integrated circuits. Broad and flat gain
spectrum of quantum dots as well as pronounced gain saturation effect facilitate simultaneous lasing via a very large
number of longitudinal modes with uniform intensity distribution (comb spectrum). A very broad lasing spectrum of
about 75 nm in the 1.2-1.28 μm wavelength range with a total output power of 750 mW in single lateral mode regime is
achieved by intentional inhomogeneous broadening of ground state transition peak and contribution of lasing via excited
state transitions. Average spectral power density exceeds 10 mW/nm. A bit error rate less than 10-13 is demonstrated for
ten spectrally filtered and externally modulated at 10 Gb/s Fabry-Perot modes owing to a low (<0.3% in the 0.001-10 GHz range) relatively intensity noise of each individual mode. This result shows aptitude of a multimode quantum dot
laser for high bandwidth wavelength-division-multiplexing systems.
Vertical-cavity surface-emitting lasers (VCSELs) emitting in the 1530-1565 nm region of flat gain in Er-doped fibers offer the potential for low-cost transmitters for wavelength division multiplexing (WDM). Methods are described to produce precisely-defined vertical-cavity surface-emitting laser arrays which: 1) efficiently utilize wafer real estate; 2) have precise and uniform wavelength distributions despite wafer thickness nonuniformity and wafer-to-wafer thickness variation; 3) are compatible with known multiplexing technologies; 4) have minimum wavelength variation with temperature. Epitaxial growth on patterned substrates with varying-size mesas has been shown to produce multiple-wavelength VCSEL arrays by Iga's group at the Tokyo Institute of Technology. This can be combined with additional refinements to fine tune the wavelengths, increase yield, and to maximize VCSEL efficiency, manufacturability and performance. Multi-wavelength VCSEL arrays represent a much lower cost, more controllable alternative to distributed-feedback laser arrays for WDM sources. The difference in laser output powers can be largely compensated via use of an Er-doped fiber amplifier within the transmitter. Reports such as that by ElectroniCast point to transmitters and receivers as being the most vital WDM components, in terms of both cost and technology.
Optics has the fundamental capability of dramatically improving computer performance via the reduction of capacitance for intrinsic high bandwidth communications and low power usage. Yet optical devices have not displaced silicon VLSI in any measure to date. The reason is clear. When placed into systems, the optical devices have not had significantly greater performance in equally complex information processing circuits and similarly low manufacturing cost. An approach demonstrated here uses the same system integration techniques that have been successful for silicon electronics, only applied to optics. Essential for creation of very large scale integrated optics (VLSIO), with over 50,000 high speed logic gates per square centimeter, is a new class of ultra high confinement (UHC) waveguides. These waveguides are created with high index difference (as high as 4.0 to 1.0) between guide and cladding. The waveguides have been demonstrated with infrared cross sections less than 5% of a square free space wavelength. These waveguides can be manufactured today only in the mid-infrared, but the concepts should scale to the near-infrared as lithography improves. Waveguide corners have been designed and demonstrated with a bend radius of less than one free space wavelength. Resonators have been designed which have over 100 times smaller volume than VCSELs, yet efficiently inter-connected laterally in high densities. A connector to the UHC waveguides has been developed and demonstrated using diffractive optical element arrays on the back side of the substrate. The coupler arrays can allow up to 10,000 Gaussian beam connections per square centimeter. This connectivity also has advantages for low cost three dimensional packaging for reduced cost and thermal dissipation. Experimental results on the above concepts and components are presented.
Optics has the fundamental capability of dramatically improving computer performance via the reduction of capacitance for intrinsic high bandwidth communications and low power usage. Yet optical devices have not displaced silicon VLSI in any measure to date. The reason is clear. When placed into systems, the optical devices have not had significantly greater performance in equally complex information processing circuits and similarly low manufacturing cost. An approach demonstrated here uses the same system integration techniques that have been successful for silicon electronics, only applied to optics. Essential for creation of Very Large Scale Integrated Optics, with over 50,000 high speed logic gates per square centimeter, is a new class of Ultra High Confinement (UHC) waveguides. These waveguides are created with high index difference (as high as 4.0 to 1.0) between guide and cladding. The waveguides have been demonstrated with infrared cross sections less than 5% of a square free space wavelength. These waveguides can be manufactured today only in the mid- infrared, but the concepts should scale to the near-infrared as lithography improves. Waveguide corners have been designed and demonstrated with a bend radius of less than one free space wavelength. Resonators have been designed which have over 100 times smaller volume than VCSELs, yet efficiently interconnected laterally in high densities. A connector to the UHC waveguides has been developed and demonstrated using diffractive optical element arrays on the back side of the substrate. The coupler arrays can allow up to 10,000 Gaussian beam connections per square centimeter. This connectivity also has advantages for low-cost 3D packaging for reduced cost and thermal dissipation. Experimental results on the above concepts and components will be presented.
A high speed modulator at low voltage is created in the mid-infrared at 10 micrometers wavelengths by using field-induced absorption on otherwise forbidden intersubband transitions. The physical effects could scale to 1.5 micrometers wavelength light. This modulator is packaged into a unique 350 micrometers long ultra high confinement (UHC) waveguide for low capacitance and high speed. The modulator quantum wells are at the interface of a 2.1 micrometers thick by 3.75 micrometers wide UHC Ge waveguide and the GaAs substrate. The quantum wells have a 17% power coupling to the evanescent fields of the Ge waveguide. A connector to the UHC waveguides, with dimensions much smaller than a free space wavelength, has been developed and demonstrated using diffractive optical element arrays on the back side of the substrate and non-uniform grating couplers. Fields are applied across the modulator quantum wells via an ohmic contact to the side of the Ge waveguide on the top of the QWs. The ground is on the other side of the waveguide and lower towards the substrate. The 7 micrometers wide mesa supporting the quantum wells on the bottom of the Ge waveguide is slightly wider to accommodate a gold electrode.
The Fourier Transform Infrared (FTIR) absorption spectrum for the range of 500 to 4000 cm-1 wavenumbers was measured for several Ge films deposited on GaAs using ultra high vacuum E-beam deposition at various substrate temperatures ranging from room temperature (RT) to 500 degree(s)C. Using transmission electron microscopy, we show that Ge films deposited at room temperature and 100 degree(s)C on a (100) GaAs surface that did not have the oxides removed are amorphous while those deposited at 100 degree(s)C with the oxide removed are crystalline, but are highly defective. Secondary ion mass spectroscopy (SIMS) measurements show that the amorphous films at RT contain more than two orders of magnitude more oxygen than the films deposited at 100 degree(s)C or a single crystal film deposited at 400 degree(s)C. The oxygen-18 diffusion studies definitively show that the excess oxygen in the amorphous films percolates in from the atmosphere. SIMS studies further reveal that thermally removing the GaAs substrate surface oxide or depositing a Au film on top of the Ge film has little effect on the incorporation of oxygen.
A possible laser device is designed with the use of classically free quasibound electron states. An asymmetric semiconductor electron wave Fabry-Perot interference filter is designed with an upper electron state having much stronger confinement than the lower electron state. This structure also allows for direct current pumping of the upper state and rapid depletion of the lower state under the presence of a field. Spectroscopy experiments demonstrate the existence of the upper quasibound state in a test structure. This laser filter structure, designed for infrared gain with current pumping, is combined with a special injector filter for room temperature narrow energy current injection into the upper lasing state. A stack of 54 periods of this electrically pumped structure is placed within a waveguide geometry. A laser device is fabricated by etching mesa structures from 50 to 100 micrometers wide. End cleaved facets serve as reflectors for mesas from 2 to 5 mm long. Tests are performed on these devices to determine their electrical properties and suitability for lasing.
Short infrared pulses have been generated by nonlinear mixing of the wavelengths from a two color femtosecond modelocked Ti:Sapphire laser. The pulses can be tuned from 7 to 12 micrometers and are used to perform a direct measurement of the speed of a GaAs/Al0.2Ga0.8As quantum well infrared photodetector. The electrical response of the detector has rise and decay times of less than 95 ps and the FWHM is 115 ps.
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