a 50 gb/s nrz modulated 850 nm vcsel transmitter operating error free to 90 °c

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  • 0733-8724 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.

    This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JLT.2014.2363848, Journal of Lightwave Technology

    > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <

    1

    A 50 Gb/s NRZ Modulated 850nm VCSEL Transmitter Operating Error Free to 90C

    Daniel M. Kuchta, Senior Member, IEEE, Alexander V. Rylyakov, Clint L. Schow, Senior Member, IEEE, Jonathan E. Proesel, Member, IEEE, Christian W. Baks, Petter Westbergh, Johan S. Gustavsson,

    Anders Larsson, Fellow, IEEE

    AbstractWe report on the properties of an 850nm VCSEL

    transmitter running at 50Gb/s with NRZ modulation from 30C to 90C. This is the highest transmitter operating temperature at 50Gb/s for a VCSEL link of any wavelength. This achievement is made possible by a combination of a high speed VCSEL design and driver and receiver circuits that incorporate equalization. Without equalization, the highest temperature attained with BER

  • 0733-8724 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.

    This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JLT.2014.2363848, Journal of Lightwave Technology

    > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <

    2

    hole mobility for AlGaAs with temperature [29]. Fig. 3 shows the extracted VCSEL threshold current over the same temperature range. At room temperature (RT) the threshold current is 0.73mA and increases to 1.24mA at 90 oC. The threshold minimum for this device is below RT. The slope of the L-I curve is 0.72mW/mA at RT and decreases to 0.55mW/mA at 90 oC. Measurements below RT were not performed to avoid condensation on the sub-assembly.

    0 2 4 6 8 100

    1

    2

    3

    4

    5

    6

    Current (mA)

    Opt

    ical

    Pow

    er &

    Vol

    tage

    (mW

    , V)

    25 oC

    95 oC

    Fig. 2. Light-current-voltage curves as a function of VCSEL substrate temperature from 25 to 95 oC in steps of 5 oC.

    20 40 60 80 1000.7

    0.8

    0.9

    1

    1.1

    1.2

    1.3

    1.4

    Temperature (oC)

    Thre

    shol

    d C

    urre

    nt (m

    A)

    Fig. 3. Threshold current versus VCSEL substrate temperature.

    840 845 850 855 860-70

    -60

    -50

    -40

    -30

    -20

    -10

    Wavelength (nm)

    Am

    plitu

    de (d

    Bm

    )

    Fig. 4. VCSEL optical spectrum at two substrate temperatures: Left = 30 oC, Ibias=2mA, IC=off and Right = 90 oC, Ibias=6.9mA, IC=on.

    20 40 60 80 1002600

    2800

    3000

    3200

    3400

    3600

    Temperature (oC)

    Ther

    mal

    Impe

    danc

    e (K

    /W)

    Fig. 5. Measured VCSEL thermal impedance as a function of substrate temperature. Fig. 4 shows the optical spectrum at substrate temperatures of 30 and 90 oC. For the 30 oC measurement, the bias current is just above threshold at 2mA and the IC is not powered on. This is intended to be a reference point where the VCSEL junction temperature is not much higher than the substrate temperature. Using a measured thermal impedance of 2700K/W, the junction is found to be only 6 oC above the substrate under these conditions. For the 90 oC measurement, the bias current is 6.9mA and the driver IC is powered on. The 6.9mA current is the bias current used for the high speed measurements. The RMS spectral width is 476pm and the mean wavelength is 854nm. The junction temperature under this condition is found to be 137 oC. The thermal impedance of a VCSEL is a function of temperature due in part to the temperature dependence of the thermal conductivity of AlGaAs. Fig. 5 shows the measured thermal impedance of the VCSEL as a function of substrate temperature from 25 to 95 oC. The thermal impedance is extracted from LIV curve and the shift in the peak wavelength of the optical spectrum from 2 to 10mA. In making this measurement, a measured wavelength shift with temperature of 62pm/K was used which

  • 0733-8724 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.

    This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JLT.2014.2363848, Journal of Lightwave Technology

    > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <

    3

    is consistent with the value reported in [30]. Over the measurement range of interest, the thermal impedance was found to increase by 24%.

    B. Dynamic VCSEL Characteristics Fig. 6 shows the small signal modulation response for the VCSEL at RT and at 85C. The modulation bandwidth at 6.8 mA is approximately 24 GHz at RT. At the higher temperature, thermal saturation causes a reduction of the bandwidth to 21 GHz at the same bias current. The D-factor decreases from 10.4 to 9.6 GHz/mA when moving from RT to 85C while the K-factor increases slightly from 0.15 to 0.16 ns.

    Fig. 6. Small signal modulation response for increasing currents at (a) room temperature and (b) 85C.

    III. TRANSMITTER AND RECEIVER ASSEMBLIES The full optical link consists of a transmitter subassembly and a separate receiver subassembly which are described in the following two sections.

    A. Transmitter Subassembly The transmitter consists of a driver IC fabricated in IBMs 0.13 m SiGe BiCMOS process, co-mounted on a printed circuit board with two VCSELs and eight decoupling capacitors. See Fig. 7. The high speed input to this printed circuit board is through two vertical SMP connectors and two single ended 50 Ohm transmission lines which do not appear in the figure. There are dc blocking capacitors on the inputs. The driver IC, which has been described in [14], is a two tap feed forward equalization design with a 50 Ohm load in the differential output stage. Because the output stage is differential, it is terminated with two VCSELs in a common

    anode configuration. In the common anode configuration, the VCSEL appears in parallel with the output stage load resistor. Using a lower resistance improves the speed but also reduces the modulation amplitude. One of the two VCSELs is not used for these experiments and is not powered on. The VCSELs are attached using Ag epoxy to an AlN diving board submount as shown Fig. 8. This submount has half its thickness removed under the VCSELs to form an air gap, giving it a diving-boardlike structure. The purpose of the air gap is two fold. For high speed testing, the air gap acts to reduce the parasitic capacitance of the VCSELs to ground. The air gap also helps to thermally isolate the VCSELs from the driver IC. To monitor the temperature, there are two chip thermistors, a large one on the printed circuit board next to the IC and a small one on the AlN submount close to the VCSELs. Both thermistors were calibrated to 100 oC and have an accuracy of

  • 0733-8724 (c) 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. Seehttp://www.ieee.org/publications_standards/publications/rights/index.html for more information.

    This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI10.1109/JLT.2014.2363848, Journal of Lightwave Technology

    > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) <

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    reduce the parasitic capacitance to ground a bit and also facilitates the wirebonding. The IC has been described in [32] with some improvements mentioned in [14]. It has a transimpedance amplifier with a 480 Ohm feedback resistor, several limiting amplifiers and a two tap FFE output driver. The printed circuit board used for the receiver subassembly is the same design as the transmitters. A thermistor is present on the receiver printed circuit board and the receiver is mounted over a single layer TEC however, neither is used in these experiments. The temperature of the receiver circuit board rises to 40C when the receiver is powered on without air flow.

    Photodiode

    SiGe Receiver IC

    Photodiode

    SiGe Receiver IC

    Fig. 9. Top view of receiver subassembly showing the SiGe receiver IC, photodiode and six decoupling capacitors.

    IV. TRANSMISSION RESULTS The transmission link begins with an SHF 12103A pattern generator with both data and databar outputs connected to the transmitter printed circuit board through 10cm coaxial cables. The pattern is set for PRBS7. The light output from one of the VCSELs is coupled to a 50/125 m graded index multimode fiber (OM3 grade) first using a diffractive lens (similar to the one described in [33]) and then through a PRIZM LightTurn connector1for a coupling efficiency of approximately -5dB. Two meters of fiber connect the transmitter to a variable optical attenuator (JDSU MAP200) and 5m of fiber connects