Signal transmission experiments were performed at 170 Gbit/s in an integrated Al2O3:Er3+ waveguide amplifier to investigate its potential application in high-speed photonic integrated circuits. Net internal gain of up to 11 dB was measured for a continuous-wave 1532 nm signal under 1480 nm pumping, with a threshold pump power of 4 mW. A differential group delay of 2 ps between the TE and TM fundamental modes of the 5.7-cm-long amplifier was measured. When selecting a single polarization open eye diagrams and bit error rates equal to those of the transmission system without the amplifier were observed for a 1550 nm signal encoded with a 170 Gbit/s return-to-zero pseudo-random 27-1 bit sequence.
©2009 Optical Society of America
One of the key driving forces behind integrated optics, and particularly silicon photonics, is the potential for higher data transmission rates compared to integrated electronic circuits . Data rates as high as 160 Gbit/s have been demonstrated in long-haul optical fiber networks  and in the future transmission at this speed can also be expected at the chip level. In such integrated photonic circuits, several enabling elements would be required, including an amplifier to boost the optical signal at various stages. Semiconductor optical amplifiers (SOAs) are well established as packaged components. However fabrication of such devices is complex, requiring many processing steps, and for incorporation of a SOA between silicon-based passive components hybrid integration methods are required. In addition, due to their short carrier lifetimes of ~50 ps, when they are operated in the saturated or quasi-saturated gain regime SOAs exhibit transient gain suppression and recovery which is dependent on both the bit rate and the bit sequence . This can result in cross-talk between different wavelength channels when amplifying wavelength division multiplexed (WDM) signals. Alternatively, planar erbium-doped waveguide amplifiers (EDWA)  offer a potential solution for amplification at high speed on a chip. Er-doped amplifiers with their long excited-state lifetime of ~10 ms and accordingly longer gain recovery time do not exhibit such transient channel cross-talk. EDWAs can be deposited directly on thermally oxidized silicon or other materials allowing all-optical signal enhancement with a straightforward process flow . Furthermore, the widespread availability of compact, low-cost laser diode pump sources begins to make such devices more attractive. Compared to other EDWA materials, Al2O3:Er3+ offers increased integration potential. Because of its relatively high refractive index contrast (Δn ≈0.2 compared to the SiO2 cladding), more compact waveguide cross-sections and thus higher pump intensities can be achieved, allowing higher integration density and reducing pump power requirements . Recently we have demonstrated significantly improved continuous-wave gain with a wavelength range of 80 nm and 2 dB/cm at the 1532 nm peak in this material .
In the past, transmission experiments at up to 10 Gbit/s using an EDWA have been carried out [8–10] and a packaged photoreceiver using an EDWA for 40 Gbit/s applications has also been reported . In this letter we demonstrate amplification at 170 Gbit/s in an Al2O3:Er3 + EDWA deposited on thermally oxidized silicon.
2. Al2O3:Er3+ channel waveguide gain
A 5.7-cm-long reactively co-sputtered Er-doped aluminum oxide (Al2O3:Er3+) straight channel waveguide amplifier was fabricated on a thermally oxidized silicon substrate. The fabrication procedure has been reported elsewhere [12, 13]. The cross-sectional dimensions of the ridge-type channel waveguide were 0.9 µm × 4.0 µm with an etch depth of approximately 50 nm. The uncladded waveguide was single mode at wavelengths of 1480 nm and above. The doping concentration, uniform throughout the Al2O3:Er3+ core layer, was 2.1 × 1020 cm−3 .
Prior to transmission experiments, the continuous-wave small-signal gain of the amplifier was measured at 1532 nm (the 4I13/2 → 4I15/2 Er transition gain peak) and 1550 nm (the wavelength at which transmission experiments were performed). Pump light from a 1480 nm Raman laser source and signal light at 1532 nm or 1550 nm from a tunable laser were combined using a WDM fiber multiplexer (MUX) and coupled to and from the chip via lensed fibers . Both microlensed fibers have a mode-field diameter of 2.9 µm at 1/e2 of the intensity profile. This mode-field diameter appears to be a good compromise to the asymmetry of the waveguide. At the output, the residual pump light and amplified spontaneous emission (ASE) were filtered using a 0.25 nm tunable bandpass filter and the signal power was measured with an optical power meter. Both co- and counter-propagating pump cases were investigated. The internal net small signal gain was calculated by subtracting the total propagation loss from the measured signal enhancement with pump on and pump off. The total small-signal propagation loss was calculated at each wavelength using the known background loss of 0.13 dB/cm, calculated confinement of the signal light within the Al2O3:Er3+ waveguide core (85%) and measured 4I15/2 → 4I13/2 absorption cross-sections . The residual pump light and ASE were measured by shifting the bandpass filter 2 nm from the signal wavelength and determined to be negligible (> 20 dB lower than the amplified signal).
The resulting gain vs. pump power curves are shown in Fig. 1 . The average gain for both TE- and TM-polarized signal light is shown, since similar signal enhancement was measured for each polarization (± 0.2 dB). In the co-propagating case, up to 11 dB total internal net gain at 1532 nm was measured for a launched pump power of 63 mW, while 6.6 dB gain was measured at 1550 nm. The threshold launched pump power for internal net gain was 4 mW and 3 mW at 1532 nm and 1550 nm, respectively. For a launched pump power of 65 mW constant internal net gain was measured when varying the launched signal power from 1 µW to 1 mW, demonstrating unsaturated gain for launched signal powers in this range. While high internal net gain was demonstrated, external fiber-chip-fiber net gain was not obtained, because the amplifier was designed for on-chip amplification, hence achieving low fiber-chip coupling loss was not considered in the design. However, with improved lensed fibers and appropriate waveguide design fiber-chip coupling losses on the order of 1 dB/facet are predicted, thus principally also allowing for fiber-chip-fiber net gain. Furthermore, in a longer amplifier internal net gain of more than 20 dB is expected across the entire C-band wavelength range (1525-1565 nm) . The net internal gain measured at 1550 nm using a counter-propagating pump setup and the 170 Gbit/s modulated signal is also shown in Fig. 1. A higher threshold pump power is observed for counter-propagating pumping as compared to the co-propagating pump measurement. The difference in the two curves is probably due to a difference in pump coupling efficiency between the two lensed fibers. Net gain of up to 6.3 dB at 1550 nm was achieved in the counter-propagating setup when the launched pump power was increased to 110 mW.
3. 170 Gbit/s transmission measurements
Transmission experiments were performed at a bit rate of 170 Gbit/s on the Platform for tEst and Research on optical telecommunications SYSTems (PERSYST) . The experimental setup is depicted schematically in Fig. 2 . The transmitter was composed of a 42.5 GHz optical clock generated from a filtered and chirp-compensated quantum dash Fabry-Perot mode-locked laser source at 1550 nm [16, 17]. Following this, a LiNbO3 Mach-Zehnder Interferometer (MZI) was modulated electronically by a pattern generator, specified for up to 27-1 bit sequences, to obtain a 42.5 Gbit/s 27-1 return-to-zero pseudo-random bit sequence (PRBS). The transmitter ended with an optical time division multiplexing (OTDM) bit rate multiplier consisting of two stages. In each stage the signal was split into two channels with one channel delayed, after which the two signals were recombined. In this manner four delayed versions of the 42.5 Gbit/s signal were multiplexed, resulting in a 170 Gbit/s data stream consisting of pulses with a full width at half maximum (FWHM) of 1.5 ps. An erbium-doped fiber amplifier (EDFA) was then used to boost the signal followed by an optical attenuator and polarization controller to adjust the amount of power and the polarization of the signal light coupled to the device, respectively. An optical isolator was used to eliminate any back-scattered pump or signal light. The 1550 nm 170 Gbit/s signal was coupled to and from the chip via the microlensed fibers. 1480 nm pump light from a Raman pump laser source was coupled to the chip in a counter-propagating approach using a fiber multiplexer (MUX). A reverse-pumping scheme was selected because it simplified the transmission setup by eliminating the need for filtering of residual pump light in front of the receiver. The transmitted signal was amplified by an EDFA before entering the receiver part of the setup. The first part of the receiver was an electro-optical loop which optically demultiplexed the 170 Gbit/s data stream to 42.5 Gbit/s and recovered the 42.5 Gbit/s optical clock . The loop was composed of an electro-absorption modulator (EAM), an optical delay line, an electrical clock recovery and a high finesse electrical band-pass filter, and was used to select and transmit one of the four multiplexed 42.5 Gbit/s signal pulse trains. Following this, the signal light was detected by a 42.5 Gbit/s electrical time division demultiplexing receiver (ETDM). An optical sampling oscilloscope (with 1 ps resolution) was used for visualization.
In initial transmission experiments with a randomly polarized signal it was observed that two polarization modes were present. In order to investigate the polarization dependence of the amplifier, the differential group delay (DGD) between the fundamental TE and TM modes in the pumped and unpumped EDWA was measured using a continuous tunable laser source and a DGD analyzer. In both cases the DGD was found to be approximately 2 ps corresponding to a waveguide birefringence of ~7 x 10-3. The impact of DGD on the 170 Gbit/s signal was also clearly observed in the eye diagram as shown in Fig. 3 , by varying the polarization controller state. In Fig. 3(a) the input polarization controller state was adjusted such that both modes propagated with almost equal intensity, while in Fig. 3(b) one mode was more strongly excited, and in Fig. 3(c) almost a single polarization mode was excited. The two overlayed pulse trains corresponding to the fundamental TE and TM modes are visible in each image. This polarization dependence should not affect the system when used in a transmitter in a photonic integrated circuit, for instance, even though it would affect signal quality in an in-line use of the device. However, by careful design of the waveguides the birefringence can be largely reduced  so that polarization-independent amplification even at these bit rates can be obtained. For all bit error rate (BER) measurements a single polarization state was selected by adjusting the polarization controller on one fundamental mode. Typical eye diagrams without the EDWA and with the EDWA and a single polarization are shown in Figs. 4(a) and 4(b), respectively. The eye pattern is open and the pulse FWHM is 2 ps in both cases.
In order to determine the compatibility of the device in a telecommunication application, BER assessments were performed on the device. The BER was measured as a function of the input power on the ETDM 42.5 Gbit/s receiver for 0.5 mW and 0.1 mW of signal power and a pump power of 65 mW launched into the EDWA. A reference was measured with the same input power in EDFA 2 as with the EDWA included in order to take into account the fiber-to-fiber losses (~10 dB when pumped) of the EDWA. The results are shown in Fig. 5 . For each input signal power, the curve with the EDWA is superposed with the reference, showing that when the polarization state is properly adjusted, the EDWA does not add any penalty to the system. A minimum launched signal power of 0.1 mW was investigated in this setup in order to ensure sufficient power reached the receiver. However, based on the negligible ASE in the small-signal gain measurements and with improved coupling losses, no penalties can also be expected in the EDWA for launched signal powers in the 1-10 µW range.
Net internal gain of up to 11 dB at 1532 nm and data transmission at 170 Gbit/s at 1550 nm have been demonstrated in an integrated Al2O3:Er3+ amplifier when pumping at 1480 nm. Despite a DGD of 2 ps between the two guided polarization modes of the amplifier, no BER power penalty was observed at 170 Gbit/s by injecting the signal in a single fundamental polarization state. The ability to amplify signals at high data rates, when combined with straightforward fabrication techniques and the possibility to integrate with various materials, confirm such amplifiers as potential components in future high-speed photonic integrated circuits.
We would like to thank the “Centre Commun Lannionais d’Optique” of CNRS Foton for the fabrication of the microlensed fibers. This work was supported by funding through the European Union's Sixth Framework Programme (Specific Targeted Research Project “PI-OXIDE”, contract no. 017501) and the European FP6 Network of Excellence ePIXnet High Speed Characterisation Platform.
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