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Analysis of High Speed Operation of a VCSEL (Vertical Cavity by lindahy


Analysis of High Speed Operation of a VCSEL (Vertical Cavity

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									                       Analysis of High Speed Operation of a VCSEL
                         (Vertical Cavity Surface Emitting Laser)

                                        Andrew R Armstrong
                     School of Information Technology and Electrical Engineering
                                      University of Queensland

                                                         different modes were able to be identified in the laser
                     Abstract                            output at currents up to 15mA. The spectral output
                                                         was centred around 858nm, with a maximum RMS
    Vertical Cavity Surface Emitting Lasers              spectral width of 0.12nm. The spectral shift was
(VCSELs) are commonly used in optical                    calculated to be 0.103nm/mA.
communication systems.           There are many
sources of limitation on the speed of operation of       3 RIN characteristics
VCSELs, including package parasitics and the
                                                             The RIN of a device is defined as [1]:
intrinsic noise limit of the device.
    This paper presents the characterisation of a                                        δP 2 ,
                                                                                 RIN =      2
Honeywell HFE4080-321 VCSEL. The light                                                    P
current characteristic of the device is presented        the ratio of the mean square of the optical fluctuation
as well as some basic spectral characteristics.          to the mean of the square of the average power [2].
The Relative Intensity Noise (RIN) of the VCSEL          As a received electrical signal is proportional to the
is investigated, and determined not to limit the         square of the optical signal, the value of RIN can be
speed of operation until well above 2Gb/s.               obtained in the electrical domain by [2]:
                                                                                        P (ω )
    The small signal response of the device is                              RIN (ω ) = E
also investigated. It is shown that the maximum                                        P.C (ω )
data rate of the device is limited to around             where PE( ) is the measured noise after subtraction

2.8Gb/s by packaging parasitics.                         of the thermal noise floor, P is the received DC power
                                                         and C( ) is the response of the system.

                                                         The system used for determining the RIN of the
                                                         device was similar to that used by Obarski and Hale
1 Introduction                                           [2]. The first stage involved converting the received
                                                         optical signal to an electrical signal with a 5GHz
    The use of VCSELs (Vertical Cavity Surface           detector (Newport D-100) which included a DC
Emitting Lasers) is becoming increasingly common         monitor output signal. The output from the photo-
in optical communication systems due to their many       detector was then passed through a DC block (Narda
advantages including low cost of manufacture. There      4564) and two wideband amplifiers (Miteq JS2-
are many sources of limitations of the bit rate          00100800-17-OA, B&H AC5120HL) before being
achievable with a VCSEL, including parasitic             displayed on an RF spectrum analyser (HP8565e).
capacitances due to packaging. The relative intensity              The data from the spectrum analyser was
noise (RIN) of a VCSEL, while very low, can also be      captured to a PC using a GPIB interface. This data
a significant factor in limiting the maximum bit rate.
    In this paper a Honeywell HFE4080-321 VCSEL
is characterised. Basic light-current and spectral
characterisations are presented, as well as the RIN of
the device and its small signal response.

2 Basic Characteristics
    The obtained power versus current characteristic
of the investigated VCSEL was obtained using an
Ando AQ-1B5E power meter. The device was found
to have a threshold current of 3.5mA and a slope
efficiency of 0.214mW/mA.
    The spectral output of the device was also
investigated at different currents using an Agilent      Figure 1: RIN of investigated VCSEL at selected
Optical Spectrum Analyser (86145B). A total of six       currents

                              Analysis of High Speed Operation of a VCSEL

was then used to calculate the RIN of the device. The         500MHz. At this point the laser response begins to
resulting values of RIN for some selected currents is         dominate and thus the observed response begins to
displayed as figure 1.                                        rise.
    Figure 1 displays some of the expected trends of              This figure shows that the useable bandwidth of
RIN. One of these trends is that the RIN will peak at         the device, taken as the 6dB point, is around 1.4GHz
the relaxation frequency of the device, the value of          at currents above 8mA. This implies a maximum
which will increase as the drive current increases.           data rate of 2.8Gb/s. This figure is lower than the
This peak is clearly visible at around 1GHz at 4mA            maximum bit rate predicted from the RIN of the
and shifts to approximately 2GHz at 4.5mA. As the             device, which was well above 2.0Gb/s.
drive current increases, this peak is no longer visible           Packaging of a laser introduces parasitic
as it moves beyond the bandwidth of the system.               capacitances [5]. As the laser is driven at higher
    Another expected trend is that the value of RIN           frequencies, these capacitances are no longer
will decrease as the drive current increases. This is         negligible and thus affect the high frequency response
clearly visible in figure 1, with the level of RIN at         of the device. Thus, while the laser itself is capable
higher currents clearly below that of lower currents.         of higher data rates, the packaging limits the data rate
    A value of RIN of –121dB/Hz was observed at               to 2.8Gb/s.        This package limiting effect was
10mA of drive current, in agreement with the devices          expected at frequencies greater than 1GHz from the
data-sheet [3]. This value of RIN will allow a data           devices datasheet [3].
rate well in excess of 2Gb/s [4].
                                                              5 Conclusion
4 Small Signal Modulation
Characteristics                                                  In this paper, the results of the characterisation of
                                                              a Honeywell HFE4080-321 VCSEL have been
    The small signal modulation characteristics of the        presented. The RIN of the device was found to be
device were investigated to determine the usable              –121dB/Hz at 10mA, implying a maximum data rate
bandwidth of the device. The laser was driven from a          well in excess of 2Gb/s. However, small signal
bias tee, allowing an AC signal from a network                analysis showed that the maximum data rate would
analyser (HP8360, HP8517A, HP8530A) to be placed              be package limited to approximately 2.8Gb/s.
on top of a DC bias current. The output from the
laser was sent through the same photo-detector, DC            References
block and amplifier as used in the RIN
measurements. The electrical signal was then sent             1. K. Petermann, Laser Diode Modulation and Noise.
back to the Network Analyser.                                    Tokyo: KTK Scientific Publishers, 1991.
    The resulting transfer characteristics obtained at        2. G. E. Obarski and P. D. Hale, "How to measure
various currents is displayed as figure 2. It is                 relative intensity noise in lasers," Laser Focus
expected that a mismatch in the feed circuit caused              World, pp. 273-7, 1999.
some discrepancies in the obtained data.                      3.
    The response of the laser was expected to rise to a          321.pdf, "HFE4080-321 Datasheet", Accessed
clear peak before rolling off [1]. It is believed that           17/10/01
the peak observed most clearly in the response at             4. L. A. Coldren and S. W. Corzine, Diode Lasers
4mA is the expected peak, and that the mismatches in             and Photonic Integrated Circuits. New York: John
the circuit are dominant over the laser response up to           Wiley & Sons, 1995.
                                                              5. D. Wiedenmann et al., "Design and analysis of
                                                                 single-mode oxidized VCSELs for high-speed
                                                                 optical interconnects," IEEE Journal of Selected
                                                                 Topics in Quantum Electronics, vol. 5, pp. 503-
                                                                 511, 1999.

                                                                  Andrew Armstrong was born in 1979 in Brisbane,
                                                              Australia. After graduating as the Dux of Corinda
                                                              State High School in 1997, Andrew has undertaken a
                                                              Bachelor of Electrical Engineering (Honours) degree.
                                                              After graduating in 2001, Andrew will be employed
                                                              in the electronics industry.
Figure 2: Small Signal response of VCSEL


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