We propose a novel waveguide-coupled germanium lateral uni-traveling carrier photodetector (L-UTC PD) structure, which offers tremendous potential for coherent communication applications at 130 Gbaud and beyond. This PD structure is very simple and can be easily fabricated in a commercial silicon photonics (SiPh) foundry. The fabricated L-UTC PD has a very low dark current (~4.2 nA), a high responsivity (~0.66 A/W@1550 nm) and a small capacitance (<14.6 fF). Moreover, the L-UTC PD exhibits excellent tolerance to high optical input power, with a 3-dB bandwidth exceeding 67 GHz, even at a high photocurrent of 3.0 mA.
AlGaInAs-MQW distributed reflector (DR) laser with semi-insulating buried-heterostructure (SI-BH) has been
developed as a high speed light source for short reach data transmissions. The DR laser has DBR mirrors on both sides of
a DFB active region. Owing to the increased optical feedback with these DBR mirrors, we can obtain short-cavity lasers
without increasing threshold gain. The small active region of the DR laser, which comes from the combination of the
short cavity structure and the SI-BH waveguide, makes driving current lower. The fabricated DR lasers have shown
excellent characteristics such as stable single mode operation, and high values of relaxation oscillation frequencies under
low driving current in a wide temperature range. We have achieved 25- and 40-Gbps direct modulation with low driving
current. Using four different wavelength DR lasers, whose lasing wavelengths coincide with LAN-WDM grid, 25.8-
Gbps direct modulations with sufficient mask margins was obtained. Using 1.55-μm-wavelength DR lasers, we achieved
40-Gbps direct modulation with 5 dB dynamic extinction ratio with the driving current less than 50 mA even at 85°C.
40-Gbps fiber transmission over 10-km single mode fiber under the operation conditions up to 70°C was also confirmed
in 1.3-μm wavelength.
We explain how a semiconductor optical amplifier in a Sagnac-interferometric arrangement can be used for switching of 200 fs optical pulses. The switching principles are based on gain and index saturation dynamics on a sub-picosecond timescale. We present a model that accounts for bi-directional propagation of ultrashort optical pulses through the amplifier as well as free-carrier absorption and two-photon absorption. We have also carried out pump and probe experiments to measure the ultrafast refractive index dynamics of a multi-quantum well InGaAsP-InGaAs semiconductor optical amplifier that is operated in the gain regime. The pump and probe pulses are cross-linearly polarized. We observe a phase shift of 200 degrees if the amplifier is pumped with 120 mA of current, but find that the phase shift vanishes if the injection current is increased to 160 mA. Our results indicate a contribution of two-photon absorption to the nonlinear phase shift that opposes the phase shift introduced by the gain. Finally, we observe that the phase shift comes up and disappears within a picosecond.
We have dramatically improved the optical properties of extremely thin QWs required for ISBT devices operating at optical communication wavelengths using novel InGaAs/AlAs/AlAsSb QW structures with 4-7 monolayers (MLs) of AlAs. The intersubband saturation intensity (Is) was reduced to 3fj/μm2. This represented an Is reduction of nearly 3 orders of magnitude relative to the previous samples whether or not such sample featured 1 ML of AlAs interface layer. This paper reviews the recent results of novel InGaAs/AlAs/AlAsSb quantum well properties grown by molecular beam epitaxy, and discusses the linear and nonlinear optical responses of ISBT.
Based on our line shape analysis of temperature dependent absorption spectra on InGaAs/AlAsSb single quantum wells, we expect a fast carrier redistribution with in the broad inhomogeneous intersubband absorption spectrum from a wavelength as short as 1.72 micrometers . In addition, due to large resonant 3rd order susceptibility but weak absorption, we expect small saturation intensity (Is) at this short wavelength. We present wavelength dependent saturation measurements to show that the Is is, indeed, lower by more than an order of magnitude compared to that at the main peak (1.88 micrometers ). We also show from the figure of merit estimates that the carrier relaxation time at 1.72 micrometers is expected to be faster at 1.72 micron, consistent with the line shape analysis predictions.
We demonstrated four-wave mixing (FWM) with two pumps in a wavelength selectable laser to achieve a wide conversion bandwidth. Non-degenerated FWM with dual pumps in SOAs has been proposed to flatten the conversion efficiency variation accompanied with the variation of detunings between input signal and output conjugate. By utilizing a wavelength selectable laser, this complex scheme can be easily realized by a single device. We used a wavelength selectable laser, which consists of an 8-channel DFB laser array connected to an multi-mode interference (MMI) combiner with an SOA at its output. The DFBs had different lasing wavelengths with 3.18 nm spacing. We introduced a signal wave through one of the DFB lasers, and two of the lasers were used as pump sources. The wavelength of the signal and one of the pumps were fixed. The wavelength of the output conjugate was changed by switching to a different second pump laser. Conversion efficiencies between -14 and -16 dB were observed with wavelength detunings between 8 and 27 nm. An efficiency variation as small as 2 dB over a 2 THz frequency detuning range was achieved, in contrast to more than 5 dB variation in single pump schemes.
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