KEYWORDS: Transceivers, Transmitters, Energy efficiency, Electronics, Modulation, Sensors, Signal attenuation, Silicon, Manufacturing, Receivers, Modulators, Photonics, Diodes, Silicon photonics, Data centers, Optical interconnects, Back end of line
We report on an ultra-compact co-integrated transmitter and receiver in SiGe BiCMOS technology for short reach optical interconnects. A fully integrated EPIC transceiver chip on silicon photonics technology is described. The chip integrates all photonic and electronic devices for an electro-optic transceiver and has been designed to be testable on wafer-scale. A node-matched diode modulator based on carrier injection is a key building block in the chip design. Its operation performance is presented with respect to insertion loss, signal-to-noise-ratio and power consumption at a 25.78125 Gbit/s in NRZ operation. A novel SiGe based photodetector exhibits a -3 dB bandwidth of up to 70 GHz and a responsivity of >1 A/W. Details are given about the process technology of co-integration of photonic and electronic integrated circuits using both silicon-on-insulator and bulk silicon. The implemented co-integration process requires only few additional process steps, leading to only a slight increase in complexity compared to conventional CMOS and BiCMOS baselines.
Frequency conversion of a single longitudinal mode Nd:YAG laser operating at 1.064 μm is realized by an external third
order Stokes barium nitrate Raman laser providing narrowband radiation at 1.599 μm wavelength which coincides with a
CO<sub>2</sub> absorption line. Transmission measurements employing a multi-pass absorption cell yield good agreement with
theoretical simulations of the CO<sub>2</sub> absorption characteristics.
Stimulated Raman scattering (SRS) has been observed in various crystals generating a multitude of wavelengths
covering the range from 280 nm to 3 μm with a mean spacing of 1 nm. Barium nitrate crystals pumped by two different
pulsed Nd:YAG laser systems have been used to demonstrate Raman laser action achieving a high average power of 5 W
or an output energy of up to 23 mJ with a quantum efficiency of 43% at 1.599 μm intended for CO<sub>2</sub> detection in longrange
LIDAR systems. Spectral narrowing of the pump radiation reduced the Raman laser emission bandwidth to
Measurement of the three-dimensional distribution of atmospheric trace gases, especially CO<sub>2</sub>, is an important factor to
improve the accuracy of climate models and to understand the global effects of the greenhouse effect. This can be
achieved by differential absorption Lidar (DIAL).
The absorption spectrum of CO<sub>2</sub> features several suitable absorption lines for a ground-based or air-borne DIAL system
working at wavelengths between 1.57 μm and 1.61 μm. An appropriate laser transmitter must emit laser pulses with
pulse energies of more than 10 mJ and pulse duration in the nanosecond range. For high spectral purity the bandwidth is
required to be less than 60 MHz.
OPOs and Er-doped solid-state lasers emit around 1.6 μm, but we describe here alternatively Nd:YAG and Nd:glass laser
systems with Raman converters. The use of stimulated Raman scattering in crystalline and ceramic materials is a
possibility to shift the wavelength of existing lasers depending on the size of the Raman shift. After the investigation of a
large number of Raman-active materials some of them could be identified as promising candidates for the conversion of
typical Nd:YAG emission wavelengths, including LiNH<sub>2</sub>C<sub>6</sub>H<sub>4</sub>SO<sub>3</sub>•H<sub>2</sub>O, Ba(NO<sub>3</sub>)<sub>2</sub>, Li<sub>2</sub>SO<sub>4</sub>•H<sub>2</sub>O, Y(HCOO)<sub>3</sub>•2H<sub>2</sub>O, β-BBO and diamond. Our experiments with Ba(NO<sub>3</sub>)<sub>2</sub> showed that the choice of the material should not be restricted to
those with an adequate first order Stokes Raman line position, but also second or third order Raman shift should be
Development of Raman frequency converters for high pulse energies concentrates on linear and folded resonator designs
and seeded Raman amplifiers using the Raman material as a direct amplifier. With Ba(NO<sub>3</sub>)<sub>2</sub> pulse energy up to 116 mJ
and 42 % quantum efficiency at the third Stokes wavelength with 1599 nm has been demonstrated. High power operation
at 5 W with compensation of thermal lensing was achieved.
Stimulated Raman scattering (SRS) has been observed in more than 100 crystals generating about 2000 different
wavelengths covering the ultraviolet, visible and infrared spectral regions with a mean spacing of 1 nm. Barium nitrate
crystals have been used to demonstrate high Raman shifted output energy up to 156 mJ or high average power of 10 W at
1.197 μm, 1.369 μm and 1.599 μ;m wavelengths with quantum efficiencies of up to 66 %.
Water vapour absorption wavelengths have been directly generated by diode pumped Nd:YGG crystals emitting at 935 nm and with Nd:GSAG crystals emitting at 942 nm in cw and pulsed operation. In addition the 1064 nm fundamental wavelength from Nd:YAG pump lasers with pulse lengths of 10 or 20 ns was shifted using Stimulated Raman Scattering (SRS) or Ti:Sapphire (TiSa) lasers. The potential of Nd:GSAG, Nd:YGG, SRS and TiSa laser systems is compared for future incorporation into a satellite based Lidar system. High output energies are possible by recent advances of fiber coupled diode sources allowing pulsed longitudinal pumping of Q-switched solid state lasers.
Results of our SRS investigations of the organic crystals &agr;-Ca(HCOO)<sub>2</sub> (alpha calcium formate), LiNH<sub>2</sub>C<sub>6</sub>H<sub>4</sub>SO<sub>3</sub> • H<sub>2</sub>O
(lithium sulfanilate monohydrate) and N(CH<sub>2</sub>CH<sub>2</sub>NH<sub>3</sub>)<sub>3</sub>Br<sub>3</sub> (tren trihydrobromide) are presented.
Currently a promising development in solid-state laser physics is the use of highly transparent ceramics. We have
demonstrated efficient SRS in three ceramics based on cubic rare earth sesquioxides RE<sub>2</sub>O<sub>3</sub> (RE = Sc, Y and Lu) with
Raman shifts in the range of 378 cm<sup>-1</sup> to 419 cm<sup>-1</sup>.
Cascading &khgr;<sup>(3)</sup> → &khgr;<sup>(2)</sup> → &khgr;<sup>(3)</sup> lasing effects, self-SHG, self-SFM and cascading Stokes and anti-Stokes generation between
phonons of different energies has been observed in Li<sub>2</sub>SO<sub>4</sub> • H<sub>2</sub>O (lithium sulphate monohydrate), CsLiMoO<sub>4</sub> (caesium
lithium molybdate) and CsLiMoO<sub>4</sub> • 1/3H<sub>2</sub>O.
The three-dimensional measurement of the global water vapor distribution in the atmosphere considerably improves the reliability of the weather forecast and climate modeling. A spaceborne Differential Absorption Lidar (DIAL) is able to per-form this task by use of suitable absorption lines of the broad absorption spectrum of water vapor. Because no interference with the absorption of other molecules exists, the range of 935/936 nm, 942/943 nm are the most preferred wavelength ranges for a water vapor DIAL. The challenge is to develop a dedicated efficient high power laser source emitting at these wavelengths. The comparison between frequency converters based on stimulated Raman scattering (SRS) and Ti:Sapphire and the directly generated Mixed Garnet laser shows the favorable properties of each concept and helps to evaluate the most suitable concept. Development of Raman frequency converters for high pulse energies concentrates on linear resonator de-signs and seeding using the Raman material as a direct amplifier based on Raman four-wave-mixing. In addition a seeded and frequency stabilized pulsed Ti:Sapphire laser system with output pulses up to 22 mJ injection-seeded at the water vapor absorption line at 935.684 nm with a spectral purity up to 99.9 % has been developed. Direct generation of the wavelengths 935/936 nm and 942/943 nm required for water vapor detection is possible with diode-pumped, Nd-doped YGG- and GSAG-crystals. First experiments resulted in pulse energies of 18 mJ in Q-switched and 86 mJ in free-running operation at 942 nm wavelength.