100 GHz Externally Modulated Laser for Optical Interconnects
Summary (2 min read)
Introduction
- 4 pulse amplitude modulation (PAM) and 105 Gbps 8PAM optical transmitter using an InP-based integrated and packaged externally modulated laser for high speed optical interconnects with up to 30 dB static extinction ratio and over 100 GHz 3 dB bandwidth with 2 dB ripple.
- Large lane count increases complexity and power consumption resulting in higher costs [1].
A. Previous Experimental Demonstrations
- The optical transmitter is based on a monolithically integrated distributed feedback laser with traveling-wave electroabsorption modulator (DFB-TWEAM) designed by T > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 KTH, fabricated by KTH and Syntune and packaged by u 2 t Photonics [22],[23].
- In [23], the performance of already packaged DFB-TWEAM transmitter module for 100 Gbps data rate is evaluated experimentally using 100 meters long standard single mode fiber (SSMF) and 10 km optical link with dispersion management.
- In [21] successful field trial is demonstrated with transmission of 112 Gbps OOK signals using a purely electrical time division multiplexing-based system without DSP.
- A bit error rate (BER) performance below the forward error correction (FEC) threshold of 2·10-3 is achieved after transmission over 42 km field installed SSMF with dispersion management between Kista and Råsunda in Sweden.
- It is the first demonstration of a complete 112 Gbps ETDM system based on monolithically integrated transmitter [23] and receiver modules.
B. Device Structure and Characterization
- The total active length of modulator is 180 µm.
- The gain section of the DFB is based on 7 QWs 7 nm thick grown by metal vapour phase epitaxy coupled with buttjoint technique.
- The laser is 440 µm long with a grating designed to have most of the output power towards the modulator.
- The dynamic extinction ratio during modulation is related to modulator bias voltage and swing of driving voltage.
- The S21 curve of the device [22] (W1 connector) depicted in Fig. 1c clearly shows beyond 100 GHz 3 dB bandwidth, the highest reported bandwidth to the best of their knowledge, with less than 2 dB ripple in the pass band of the EML [22] which indicates high phase linearity.
III. EXPERIMENTAL SETUP
- Figure 2 shows the transmitter setup for 116 Gbps OOK, 4PAM and 105 Gbps 8PAM with receiver setup including DSP (see Fig. 2a) and output optical spectrum of modulated signals (see Fig. 2b).
- In the OOK setup, two pseudo-random bit sequences with a word length of 2 15 -1 (PRBS15) at 58 Gbps are first decorrelated and then multiplexed into a single 116 Gbps sequence.
- In the 4PAM setup, two PRBS15 are first decorrelated and then.
116 Gbit/s 4PAM Tx
- > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 3 passively combined into a 4PAM signal, which is then used to directly drive the EML.
- During measurements the bias voltage was kept at minus 2 volts and driving voltage was 2 Vpp for OOK, 8PAM and 1 Vpp for 4PAM.
- The signal is then downsampled to 1 sample per symbol for static/adaptive feed forward equalizer (FFE) with different number of taps to overcome inter symbol interference (ISI) or symbol-spaced adaptive decisionfeedback equalizer (DFE) with different configuration of feedforward taps (FFT) and feedback taps (FBT) to overcome ISI in presence of noise.
- A total number of 1.2 million bits are used for BER counting.
- The bandwidth limitation is due to limited effective 3 dB bandwidth on the transmitter side components used to generate electrical signals, while additional filtering occurs due to the limited DSO bandwidth, affecting the performance of OOK and 8PAM setups.
IV. RESULTS AND DISCUSSIONS
- Obtained results provide the quantitative and qualitative performance evaluation of the optical transmitter for its capabilities to enable the third generation 400 Gbps client-side links for optical interconnects.
- One can observe that the use of 3-FFT&3-FBT DFE allows achieving below 7% FEC limit performance, which was further improved with larger lengths of equalizers.
- Adding small feedback in equalizer allows significantly improving the performance.
- BER curves are obtained using same equalizers as in OOK case.
- The distance for 116 Gbps 4PAM can be increased about three times compared to OOK for same penalty and equalizer parameters.
V. CONCLUSIONS
- The authors report on the performance of an EML with higher than 100 GHz bandwidth for optical interconnects.
- Furthermore, this is the first time [20] that 116 Gbps OOK is achieved on a single EML based > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 5 optical transmitter with low complexity DSP.
- The authors study the trade-off between power penalty and equalizer length using Monte Carlo simulations.
- According to simulation results, a transmission of 3 km standard single mode fiber with less than 1 dB dispersion penalty using only 3-tap static filter for 116 Gbps 8PAM seems to be possible.
- The authors conclude that, provided sufficient bandwidth and linearity on the electrical domain, this optical transmitter can be used for advanced modulation formats at higher than 100 Gbaud.
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Citations
142 citations
Cites background from "100 GHz Externally Modulated Laser ..."
...Transceiver developers have thus to weigh the high yield and facilitated electronic integration afforded by mature complementary metal– oxide–semiconductor (CMOS) processes, the high stability and material quality of silicon (Si), and relatively easy to handle capacitive or resistive loads associated to SiP modulators against the reduced insertion losses afforded by monolithic integration of externally modulated lasers (EMLs) [9]....
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99 citations
Cites background or methods from "100 GHz Externally Modulated Laser ..."
...(a) P(I) characteristics; (b) P(V) characteristics of the DFB-TWEAM module [66]; (c) picture of the packaged module; (d) small-signal transfer characteristics S21 [65]....
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...Besides the aforementioned works, the DFB-TWEAM reported in [65] was also employed for a 204 Gbaud OOK transmission,...
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...(DFB-TWEAM) of >100 GHz bandwidth [65], with which several high-speed transmissions were demonstrated [66], [67]....
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99 citations
68 citations
48 citations
Cites methods from "100 GHz Externally Modulated Laser ..."
...In this paper, we demonstrate a transmitter producing OOK optical data at record symbol rates of 140-GBaud and 204-GBaud with InP technologies, enabled by a large-bandwidth InP-based double heterojunction bipolar transistor (DHBT) selector [6] and an integrated EML [3]....
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...An IAF InP DHBTbased modulator driver module with gain of 16 dB and the 3-dB bandwidth of 110 GHz was used to achieve sufficient driving voltage swing between 2:1 Selector (SEL2) and for the DFB-TWEAM [8]....
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...Alternatively, the generation of a 116 Gbit/s OOK signal was reported, using an ultrabroadband monolithically integrated distributed feedback laser with traveling-wave electro-absorption modulator DFB-TWEAM) [3]....
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...of 140-GBaud and 204-GBaud with InP technologies, enabled by a large-bandwidth InP-based double heterojunction bipolar transistor (DHBT) selector [6] and an integrated EML [3]....
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...Such high symbol rate electrical generation is enabled by a very-high-speed 2:1-Selector in InP DHBT technology, which requires electrical inputs at half the output rates; and we used 100 GHz externally modulated laser (DFB-TWEAM)....
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References
6 citations
"100 GHz Externally Modulated Laser ..." refers background in this paper
...voltage and the frequency response taken at 22 °C [24]....
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3 citations
"100 GHz Externally Modulated Laser ..." refers background in this paper
...Advanced modulations formats are extensively studied for short reach applications: (1) pulse amplitude modulation (PAM) [6]-[16], (2) carrierless amplitude phase (CAP) modulation [17], or (3) discrete multi-tone (DMT) [13],[18]....
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...Advanced modulations formats are extensively studied for short reach applications: (1) pulse amplitude modulation (PAM) [6]–[16], (2) carrierless amplitude phase (CAP) modulation [17], or (3) discrete multi-tone (DMT) [13], [18]....
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