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
7 citations
6 citations
Cites background from "100 GHz Externally Modulated Laser ..."
...It is worthwhile mentioning that traditional PAM signals generated in the electrical domain complicate the transmitter and require LEDs with good linearity [8]....
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5 citations
5 citations
Cites background from "100 GHz Externally Modulated Laser ..."
...In the case of 20 GHz bandwidth, at the best sampling point, two samples per symbol achieved about six times better performance than single sample MLSE....
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...It must be noted that the 3 dB bandwidth of the whole system in the PAM-4 experiment is around 20 GHz, such that the bandwidth limitations are less severe than in the case of the OOK experiment....
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...When lowering the bandwidth to 18 GHz the values of the first pre- and post-cursors are no longer 0....
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...Then, the pre-distorted data are up-sampled to two Sam/s and are loaded to a DAC with an 8-bit resolution and 13 GHz 3-dB analog bandwidth....
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...The DAC operates at 56 GSam/s leading to the generation of 28 Gbps NRZ-OOK electrical signal at its differential outputs which are fed into a 25-GHz 3-dB bandwidth RF driver and then sent to a 10- GHz transmitter optical sub-assembly (TOSA)....
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5 citations
Additional excerpts
...The optical spectrum of DFB-TWEAM with different modulation amplitudes (peak-to-peak voltage) is shown in Fig....
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...The EML is composed of a monolithically integrated distributed feedback (DFB) laser with a travellingwave electro absorption modulator (TWEAM) [21], [22]....
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References
274 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]....
[...]
76 citations
74 citations
"100 GHz Externally Modulated Laser ..." refers background in this paper
...Hence, consensus seems to evolve towards 4PAM [6]–[16] and on-off keying (OOK) [14], [19]–[23] as it saves complexity on the transmitter side....
[...]
...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]....
[...]
70 citations
"100 GHz Externally Modulated Laser ..." refers background or methods in this paper
...1(c) clearly shows beyond 100 GHz 3 dB bandwidth, the highest reported bandwidth to the best of our knowledge, with less than 2 dB ripple in the pass band of the EML [22] which indicates high phase linearity....
[...]
...A 65 GHz linear amplifier is used to drive the EML....
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...The S21 curve of the device [22] (W1 connector) depicted in Fig....
[...]
...The optical transmitter is based on a monolithically integrated distributed feedback laser with traveling-wave electroabsorption modulator (DFB-TWEAM) designed by KTH, fabricated by KTH and Syntune and packaged by u2t Photonics [22], [23]....
[...]
...We report on the performance of an EML with higher than 100 GHz bandwidth for optical interconnects....
[...]
67 citations
"100 GHz Externally Modulated Laser ..." refers background in this paper
..., 8 × 50 Gbps/λ [4]) thanks to compatibility with existing building blocks for 100 GbE (drivers, lasers, and photodetectors...
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...Four optical lanes at 100 Gbps net rate (i.e., 4×100 Gbps/λ [5]) are a promising solution in order to reduce complexity, power consumption and costs....
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...One attractive solution is based on eight optical lanes (i.e., 8×50 Gbps/λ [4]) thanks to compatibility with existing building blocks for 100 GbE (drivers, lasers, and photodetectors with transimpedance amplifiers)....
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