Applications using Packaged MEMS VCSEL

After successfully dicing, gluing to the submount wafer and bonding the MEMS and the VCSEL pads to the submount, a few devices have been packaged to form TOSA (Transmitter Optical Sub-Assembly) with a standard LC connector. A flex cable is later soldered to the pins of the TOSA, via which the modulation signal can be fed. The MEMS VCSEL used for this experiment is characterized with a continuous tuning of ≈ 92 nm (from 1518 nm to 1610 nm). The fiber coupled optical power Popt, SMSR and corresponding MEMS actuation power PMEMS = I2MEMS× RMEMS for a constant Ibias are recorded as a function of the tunable spectrum and shown in Fig. 5.7. As can be seen, the actuation power is not perfectly linear (orange line with star symbol), which is due to partial ARC damage the half-VCSEL. Popt is over 1 mW in a range of 52 nm, which reaches the specification mentioned in AppendixD. The SMSR is > 45 dB across the entire tuning range.

Fiber Transmission

Before explaining the measurement setup and the experimental results, a low-cost tunable MEMS VCSEL based WDM-PON concept is briefly discussed. The details of the setup

5.1. Optical Data Transmission 117 1520 1530 1540 1550 1560 1570 1580 1590 1600 1610 1620 0.4 0.6 0.8 1.0 1.2 1.4 I bias = 32 mA O u t p u t O p t i ca l P o w e r ( m W ) Emission W av elength (nm) Packaged MEMS VCSEL T = 25 °C 0 5 10 15 20 25 M E M S A ct u a t i o n P o w e r ( m W ) 46 48 50 52 54 56 58 S M S R ( d B )

Figure 5.7: _{Fiber-coupled power, SMSR and corresponding MEMS actuation power} PMEMS = I2MEMS× RMEMS of a packaged MEMS VCSEL plotted against its tunable

emission wavelength.

and the experiment are explained in [94]. The WDM-PON is illustrated in Fig.5.8(a). The main aspect of the proposed WDM-PON system is a centralized wavelength-locker enabled autonomous tuning of the MEMS VCSEL in the tail-end equipment (TEE). The centralized locker is shared by all TEEs. In this way, it eliminates the necessity of wavelength-controlling for individual transmitter and reduces the system cost [95]. The optical distribution network (ODN) can be based on a tree or dropline architec- ture. The wavelength multiplexing (MUX) and demultiplexing (DEMUX) are realized by arrayed waveguide gratings (AWG). In case the feeder fiber length is significantly longer than the drop fibers (the differential end-to-end reach is typically under 5 km), a single dispersion-compensating fiber (DCF) in the head-end allows optimization of the chromatic dispersion for all channels. Droplines use optical add/drop multiplexers in several locations. In case the differential reach matches the for MFH specifications of 20 km. a single DCF is installed and optimized for the entire range of relevant reach. Thus, both architectures can be supported [94].

Figure 5.8(b) shows the experimental setup. The experiment is mainly focused on the upstream application for MFH and MBH. Therefore, a limited reach of up to 40 km is considered. The packaged MEMS VCSEL is directly modulated at 10.3125 Gbit/s with 231_{− 1 bit long PRBS signal. The tuning range has been limited to 33 nm (1529 nm to} 1592 nm) to compensate a decrease of the optical power at two edges of the tuning range. The Ibias and the Vpp are set to 22 mA and 1 V, respectively and are kept constant for all measurements. The modulated signal is coupled into an SSMF and is transmitted over a maximum distance of 40 km. The accumulated dispersion at the OLT is compensated by a DCF matched to 40 km for all transmission lengths. It is worth to mention that the G.metro standard considers an optional erbium-doped fiber amplifier (EDFA) in upstream direction to reduce the requirements at the tail-end tunable transmitter. As the cost of EDFA and DCF will be distributed among all subscribers, the approach is

EDFA (optional) Splitter λ-locker/ Etalon Controller TEE 3 PD PT Data L- ba nd C-ban d BBU Tx array VM Tx BBU Rx array VM Rx C-band L-band Communication channel Head-End / CO ODN TEE 2 TEE 1 TEE 5 TEE 4 TEE N RRH RRH RRH RRH RRH RRH RRH TEE 6 B B U Pilot tone MEMS VCSEL RRH RRH (a) BERT EDFA DCF-40 AWG

VOA AWG MEMS VCSEL

Rec e iv e r SSMF 0-40 km

Head end (OLT) ODN Tail-end (ONU)

(b)

Figure 5.8: _{(a) A concept of a WDM-PON system based on low-cost tunable MEMS} VCSEL. TX: transmitter, RX: receiver, AWG: arrayed waveguide grating, PD: pho- todetector, PT: pilot-tone. (b) Experimental setup. VOA: variable optical attenuator, EDFA: Erbium-doped fiber amplifier, SSMF: standard single-mode fiber„ DCF: disper- sion compensating fiber, AWG: arrayed waveguide gratings, BERT: bit-error rate tester, OLT: optical line terminal, ODN: optical distribution network, optical ONU: optical

network unit.

still cost efficient. Now, the dispersion compensated signal is finally launched into an SFP receiver on an evaluation board via a VOA. The electrical signal is connected back to the BERT to evaluate the BER performance.

Figure5.9(a) shows the BER as a function of the received optical power for BTB without DCF for four wavelengths (1529 nm, 1548 nm, 1567 nm, and 1592 nm), thus constituting a tuning range of of 63 nm. The BTB measurements show a wavelength dependent receiver sensitivity. In order to evaluate the system performance in an actu- al application case, the Ibias and the Vpp are not changed while tuning the wavelength. Figure5.9(b) shows the experimental results for the SMF transmission between 0–40 km with a constant dispersion compensation matched to 40 km SMF. That means, a trans- mission length of 0 km refers to simplest case where the signal is launched directly into the DCF. Comparing the BERs at 10−9, the receiver sensitivities after 40 km SSMF transmission and BTB show similar results. This is expected due to the DCF matched to 40 km. For transmission lengths between 0 km and 30 km the receiver sensitivity varies < 1 dB for a particular wavelength. For G.metro MFH applications, the differen- tial reach is limited to 20 km. This leads to a system optimization of the DCF. From Fig.5.9(b), it can be seen that the smallest deviation of the receiver sensitivity for a 20 km transmission range is between 10 km and 30 km. This means that the optimum DCF should be matched to 30 km SSMF. The receiver sensitivity varies within 1.75 dB

5.1. Optical Data Transmission 119 -20 -19 -18 -17 -16 -15 -14 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 B E R

Received optical Power (dBm)

1529 nm 1548 nm 1567 nm 1592 nm (a) 0 5 10 15 20 25 30 35 40 -19 -18 -17 -16 -15 -14 -13 R e ce i ve r S e n si t i vi t y ( d B ) a t B E R = 1 E - 9

Fiber Transmission Length (km) 1529 nm

1548 nm 1567 nm 1592 nm

(b)

Figure 5.9: _{(a) Back-to-back BER performance vs. received power for various wave-} lengths. (b) Receiver sensitivity measured as BER vs. receied optical power for different emission wavelengths after 1-40 km standard single-mode fiber (SSMF) and dispersion

compensating fiber (DCF) matched to 40 km SSMF [? ].

over the whole tuning range and all transmission reaches. Taking into consideration that the VCSEL settings are kept constant for all wavelengths and transmission lengths, the results are very promising. In other applications such as MBH and enterprise access where longer differential reaches are required, transmission is still possible with slightly increased penalties [? ].

Application as Colorless ONU Transmitter

A. Gatto et al. has employed the fabricated MEMS VCSEL in a so-called colorless optical network unit (ONU) transmitter. The experimental results are published in [96]. Implementation of discrete multi-tone (DMT) modulation in combination with direct detection facilitate to overcome the bandwidth limitation of the MEMS VCSEL (≈ 7 GHz small-signal modulation bandwidth) and allow to employ a standard receiver suitable for 10 Gb/s operation. As a result, an upstream transmission beyond 25 Gbit/s using an SSMF of 20 km has been archived without any chromatic dispersion compensation. This shows the potential of the MEMS VCSEL as a high-bandwidth, low-cost, reduced- footprint ONU source in time and wavelength multiplexed-PON (TWDM-PON) system [96].

Application in SDN-Enabled Flexible Optical Metro Networks

M. S. Moreolo et al. has proposed and experimentally validated for the first time the potential of a directly modulated MEMS VCSEL in software-defined networking (SDN)- enabled sliceable bandwidth variable transceiver (S-BVT) flexible optical metro networks [97]. The integration of the fabricated MEMS VCSEL provides improved performance

and functionalities which include saturating the spectral voids with fine granularity. Us- ing the proposed module, a bit rate up to 33 Gbit/s with DMT in BTB configuration and

> 20 Gbit/s up to 185 km 2-hop path using single-sideband (SSB) orthogonal frequency

division multiplexing (OFDM) has been demonstrated.