Low cost 112 Gb/s InP DFB-EAM for PAM-4 2 km ... - IEEE Xplore

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(2) Alcatel-Lucent Bell Labs France, miquel_angel.mestre_adrover@alcatel-lucent.com. Abstract A 112-Gb/s PAM4 transmitter module which integrates InP DFB ...
Ecoc 2015 - ID: 1011

Low cost 112 Gb/s InP DFB-EAM for PAM-4 2 km Transmission (1)

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C. Caillaud , M. A. Mestre Adrover , F. Blache , F. Pommereau , J. Decobert , F. Jorge , P. (1) (1) (1) (2) (1) (1) Charbonnier , A. Konczykowska , J.-Y. Dupuy , H. Mardoyan , K. Mekhazni , J.-F. Paret , M. (1) (1) (1) Faugeron , F. Mallecot and M. Achouche (1) (2)

III-V lab, a joint laboratory between ALBLF, TRT and CEA Leti, [email protected] Alcatel-Lucent Bell Labs France, [email protected]

Abstract A 112-Gb/s PAM4 transmitter module which integrates InP DFB and EAM demonstrated for the first time 2 km transmission with only 3-taps equalizer due to 50 GHz bandwidth, >13 dB extinction ratio and 1.5 mW output power. signals with a clear eye opening and without any digital signal processing technique and demonstrated 2 km amplifier free transmission with a small 3-taps FFE equalizer to compensate for limited bandwidth of the receiver setup. Transmitter design The transmitter module is based on a high speed shallow ridge InGaAlAs EAM and DFB laser packaged together as shown in the insert of Fig. 1. The waveguide stripe is 520 μm long and 1.5 μm wide. The modulator active section is 75-μm long, and is composed of 10 InGaAs/InGaAlAs tensile strained quantum 5 wells . The active section length is defined by H+ implantation. The active section is between 2 passive sections made of an InGaAsP layer (Ȝg=1.3 μm) grown by GSMBE and butt coupled to the active section to reduce propagation losses and to prevent early saturation of our device. The EAM losses and extinction ratio were measured using a broadband ASE source (Fig. 1). The insertion losses are 13.5 dB at 1605 nm, and 15.5 dB at 1547 nm, dominated by coupling losses (§10 dB). The use of a passive InGaAsP layer limits the absorption in the passive section at low wavelength. Therefore, our modulator presents a large optical bandwidth with an extinction ratio above -10 0V->-4V

-15 Transmission (dB)

Introduction The continuous growth of data center and interconnects traffic is driven by cloud computing, big data and smart mobile devices and leads an increasing demand for next generation ultra high-speed short reach optical interconnect at 100 and 400 Gb/s. These applications require low footprint, low cost and low power consumption transceivers to reach the largest bandwidth throughput in the limited space of data center equipment. Current 100GbE-LR4 solutions use 4 wavelength channels at 28 Gb/s with NRZ modulation format. The 8×50 Gbits/s 4-level pulse amplitude modulation (PAM-4) scheme with 25 Gbauds symbol rate and the 4×100 Gbit/s PAM-4 modulation scheme with 50 Gbaud symbol rate are widely considered by IEEE P802.3bs 400 Gbit/s Ethernet Task Force for 500m and 2 km 1 applications . The 50-Gbit/s 8 lanes scheme is attractive because it is compatible with existing 100 GbE building blocks (driver, laser, photodiodes, TIA) but doubling lanes count results in additional complexity, consumption and cost. The 4 lanes 100-Gb/s scheme is therefore attractive but requires new component technology developments to allow >50 Gbaud modulation speed. Recently several experiments of PAM-4 transmissions at 50-56 and 100-112 Gbit/s were 2-4 2 demonstrated with silicon modulator and InP 3-4 transmitters but most of them required digital signal processing due to the bandwidth limitation of transmitter and receiver, especially at 112 2 3 4 Gbit/s (>38 taps , 40 taps , 184 taps additioning TX and RX equalizer) and present low BER -4 2, 3 performances (>2×10 in ). In this paper, we present a transmitter module which integrates a high speed electroabsorption modulator and a high power laser. The transmitter module presents an output power >1.5 mW, a large extinction ratio of 13 dB and a broad bandwidth above 50 GHz. The module generated 112 Gbit/s PAM-4

-20 -25

0V -0.5 V -1V -1.5V -2V -2.5V -3V -3.5V -4V

-30 -35 -40 1520

1540

1560 1580 1600 Wavelength (nm)

1620

Fig. 1: Absorption spectra of the electro absorption modulator (insert: photograph of the transmitter module)

Ecoc 2015 - ID: 1011

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100 mA 150 mA 200 mA 250 mA 300 mA

0 -3 -6 -9 0

2 1,8 1,6 1,4 1,2 1 0,8 0,6 0,4 0,2 0

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100 150 200 250 300 350 I (mA)

characteristics of our transmitter for various laser current at 25°C. Large SER from 13 to 16.5 dB are achieved at a low bias voltage below 2.5 V. The increase of the extinction ratio for high laser drive current is probably due to 7 self-heating of the modulator . The frequency response was measured using a vector network analyzer with a calibrated photodiode for various laser currents as represented in Fig. 4. The bias is adjusted from -0.9 V at 100 mA to -1.1 V at 300 mA to maximise the signal in the VNA. The laser 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20

100 mA 150 mA 200 mA 250 mA 300 mA

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Fig. 4: Frequency response of the laser-modulator module for various laser drive current

Fig. 2: P(I) characteristics of hybrid InP laser modulator transmitter

P (dB)

current has negligible effects on the transmitter response and no saturation effects are visible. The 3-dB bandwidth is 50 GHz and the 6-dB bandwidth is 58 GHz.

S21 Response (dB)

10 dB over a large spectral range (1525-1595 nm). The laser is a 3.5 μm width InP DFB shallow ridge laser with asymmetrical cladding and 6 6 quantum wells . The laser wavelength is 1547 nm with an output power of 18 dBm at 300 mA and a threshold of 67 mA due to the low optical confinement in the quantum well. The modulator is coupled to the high power laser and to the output optical fiber with microlenses for maximum coupling efficiency. Fig. 2 shows the P-I curve of the transmitter at 25°C. The transmitter output power is 0.5 mW with the EAM turned off at 140 mA drive current and can reach 1.5 mW at 300 mA drive current. Fig. 3 shows the static extinction ratio (SER)

Large signal eye diagram measurements are conducted using PAM-4 modulation formats. The transmitter module is driven by a Power DAC (pDAC) integrated circuit which generates 112 Gbit/s PAM-4 signals. The principle of 8 Power DAC operation was presented in . The optical signal generated by our transmitter is converted into an electrical signal by a high speed photodiode (f-3dB=70 GHz) and then recorded on a 70 GHz sampling oscilloscope. The photodiode photocurrent is kept constant at 4 mA in all experiment and the transmitter temperature is regulated at 25°C with a peltier cooler. Fig. 5 shows the transmitter optical output eye diagram when driven with a pDAC 4level electrical signal with a swing of 1.9 Vpp (see insert in Fig. 5), a laser drive current of 300 mA and a modulator bias of -1.15 V. The transmitter output power is -1.2 dBm. To our knowledge, this is the first time that a clear and open 112 Gbit/s PAM-4 eye diagram is obtained using a compact and low cost InP EAM modulator without any digital signal processing.

T=25ƒC Ȝ=1547 nm

-2 Bias (V)

-1

0

Fig. 3: P(V) characteristics of the transmitter module

Fig. 5: PAM-4 optical eye diagram measurement of the transmitter module (insert: DAC output eye diagram)

Ecoc 2015 - ID: 1011

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Finally, a real time 80 GS/s oscilloscope (keysight DSAV334A) was used to measure the bit error rate performances of our transmitter. The experimental setup is shown in Fig. 6. The data were sent to the pDAC which drive our optical transmitter. Then the 112 Gbit/s PAM-4 optical signal of our transmitter is launched into a SMF fiber and detected by a PIN-TIA receiver connected to the real time oscilloscope which digitized and stored the RX waveform for offline processing. We first resample the waveforms to obtain 2 samples/symbols; then we perform feed-forward equalization to recover the signal, mainly impaired by the limited bandwidth of the oscilloscope (33 GHz) and the PIN-TIA receiver (40 GHz). Finally PAM-4 signal is decoded to carry out bit error counting.

We wish to thank Keysigth for their support

The results are summarized in Fig. 7. With a very low 3 taps FFE which is significantly less than the number of taps required in previous 112 Gbit/s PAM-4 experiments (§40 taps 2-4 -3 minimum ), we achieved a BER of 2.1×10 in -3 BtB (PRX=-2 dBm) and 3.2×10 after 2 km transmission (PRX=-5 dBm). This is compatible with threshold of the 6.7% FEC used in OTU4 -3 standards (3.8×10 ). With a larger equalizer (25 taps) to better compensate the limited bandwidth of the setup, we achieved a very low -5 -4 BER of 1.8×10 in BtB, 1.2×10 after 1 km SMF -4 fiber and 9.6×10 after 2 km SMF fiber which will allow to use lower complexity FEC to reduce power consumption (like KP4 FEC with a -4 threshold of 3×10 for