The IEEE Std 802.3ba-2010 for 40 Gb and 100 Gb Ethernet was released in July, 2010. This standard will continue to
evolve over the next several years. Two of the challenging transmit/receive architectures contained in this standard are
the 100GBASE-LR4 (<10 km range) and 100GBASE-ER4 (<40 km range). Although presently envisioned for 1310 nm
optical wavelengths, both of these 4 lane, 25.78 GBaud formats may be adopted for the impending 850 nm short reach
optical backplane market, whose range is below 150 m.
Driven by major computer server companies, such as IBM, HP and Oracle, the 850 nm Active Optical Cable (AOC)
market is presently undergoing an increase of serial rates up to 25 Gbaud to enhance backplane interconnectivity. With
AOCs up to 16 channels, the potential for up to 400 Gbps backhaul composite data rates will soon be possible.
We report a 25 Gbps photodiode with quantum efficiency ~ 0.6 at 850 nm. This InGaAs/InP device was optimized for
high quantum efficiency at 850 nm. When pigtailed with multimode fiber and integrated with an application-specific RF
amplifier, the resultant photoreceiver will provide multiple functionalities for these 100 Gb Ethernet markets.
Optical synchronous coherent detection is attracting greater attention within the defense and security community because
it allows linear recovery both of the amplitude and phase of optical signals. Fiber-based transmission impairments such
as chromatic dispersion and polarization mode dispersion can be compensated in the electrical domain. Additionally,
synchronous detection offers the potential of improved receiver sensitivity and extended reach versus direct or
interferometric detection schemes. 28 Gbaud/112 Gb/s and 42.8 Gbaud transmissions are now being considered in fiber
networks worldwide. Due to the lack of broadband high frequency components centered at IF values of 56 GHz and 86
GHz, respectively, the coherent heterodyne approach is not viable for these baud rates. The homodyne approach remains
one of the choices available to fully exploit the advantages of synchronous coherent detection at these transmission data
rates.
In order to implement the homodyne receiver, optical phase locking between the signal and local oscillator laser (LO) is
required. Digital approaches for this task rely upon very complex, fast, and high power-consumption chips. A homodyne
receiver using an analog approach for phase locking would allow for increased system simplicity at a lower cost. Use of
commercial-off-the-shelf (COTS) DFB lasers embedded within the receiver would also increase system feasibility for
defense applications. We demonstrate synchronous demodulation of a 42.8 Gbaud signal using an analog optical phase-locked
loop. The homodyne system was optimized to use COTS DFB lasers having an aggregate linewidth of ~2 MHz.
We also analyze the impact of uncompensated phase noise on receiver performance.
Time is the most precisely measured physical quantity. Such precision is achieved by optically probing hyperfine atomic
transitions. These high Q-factor resonances demonstrate frequency instability of ~10-18 over 1 s observation time.
Conversion of such a stable optical clock signal to an electrical clock through photodetection introduces additional phase
noise, thereby resulting in a significant degradation in the frequency stability. This excess phase noise is primarily
caused by the conversion of optical intensity noise into electrical phase noise by the phase non-linearity of the
photodetector, characterized by its power-to-phase conversion factor. It is necessary to minimize this phase nonlinearity
in order to develop the next generation of ultra-high precision electronic clocks.
Reduction in excess phase noise must be achieved while ensuring a large output RF signal generated by the
photodetector. The phase linearity in traditional system designs that employ a photoreceiver, namely a photodiode
followed by a microwave amplifier, is limited by the phase non-linearity of the amplifier. Utilizing high-power handling
photodiodes eliminates the need of microwave amplifiers.
In this work, we present InGaAs p-i-n photodiodes that display a power-to-phase conversion factor <6 rad/W at a
peak-to-peak RF output amplitude of 2 V. In comparison, the photodiode coupled to a transimpedance amplifier
demonstrates >44 rad/W at a peak-to-peak RF output amplitude of 0.5 V. These results are supported by impulse
response measurements at 1550 nm wavelength at 1 GHz repetition rate. These photodiodes are suitable of applications
such as optical clock distribution networks, photonic analog-to-digital converters, and phased array radars.
Greenhouse gases, such as carbon dioxide, carbon monoxide, and methane, can be remotely monitored through optical
spectroscopy at ~2 micron wavelength. Space based LIDAR sensors have become increasingly effective for greenhouse
gas detection to study global warming. The functionality of these LIDAR sensors can be enhanced to track global wind
patterns and to monitor polar ice caps. Such space based applications require sensors with very low sensitivity in order
to detect weak backscattered signals from an altitude of ~1000km. Coherent detection allows shot noise limited
operation at such optical power levels. In this context, p-i-n photoreceivers are of specific interest due to their ability to
handle large optical power, thereby enabling high coherent gain. Balanced detection further improves the system
performance by cancelling common mode noise, such as laser relative intensity noise (RIN).
We demonstrate a low-noise InGaAs balanced p-i-n photoreceiver at 2μm wavelength. The photoreceiver is
comprised of a matched pair of p-i-n photodiodes having a responsivity of 1.34A/W that is coupled to transimpedance
amplifier (TIA) having an RF gain of 24dB (transimpedance = 800Ω) and input equivalent noise of 19pA/√Hz at 300K.
The photoreceiver demonstrates a 3dB bandwidth of 200MHz. Such bandwidth is suitable for LIDAR sensors having 20
to 30m resolution. The photoreceiver exhibits a common mode rejection ratio of 30dB and optical power handling of
3dBm per photodiode.
Active optical remote sensing has numerous applications including battlefield target recognition and tracking,
atmospheric monitoring, structural monitoring, collision avoidance systems, and terrestrial mapping. The maximum
propagation distance in LIDAR sensors is limited by the signal attenuation. Sensor range could be improved by
increasing the transmitted pulse energy, at the expense of reduced resolution and information bandwidth. Coherent
detection can operate at low optical power levels without sacrificing sensor bandwidth.
Utilizing a high power LO laser to increase the receiver gain, coherent systems provide shot noise-limited gain thereby
increasing the sensing range. To fully exploit high LO powers without incurring performance penalties due to the RIN
of the LO, high power handling balanced photodiodes are used. The coherent system has superior dynamic range,
bandwidth, and noise performance than small-signal APD-based systems.
Coherent detection is a linear process that is sensitive to the amplitude, phase and polarization of the received signal.
Therefore, Doppler shifts and vibration signatures can be easily recovered. RF adaptive filtering following
photodetection enables channel equalization, atmospheric turbulence compensation, and efficient background light
filtering.
We demonstrate a coherent optical transmission system using 15mA high power handling balanced photodetectors. This
system has an IF linewidth <1Hz, employing a proprietary phase locked loop design. Data is presented for 100ps pulsed
transmission. We have demonstrated amplitude and phase modulated 10Gb/s communication links with sensitivities of
132 and 72 photons per bit respectively. Investigations into system performance in the presence of laboratory induced
atmospheric turbulence are shown.
The performance of microwave photonic systems can be improved by utilizing high power handling photodetectors.
Operation at higher photocurrents enables larger output RF signals to be produced directly by the photodetector. This
reduces the requirement of signal amplification by RF amplifiers, thereby simultaneously improving the dynamic range
and the noise figure. In optical coherent systems, high power handling photodetectors enable operation at high local
oscillator power levels to boost the coherent gain and the detection sensitivity. Thus, techniques to enhance the power
handling capability of photodetectors are of interest for both free space and fiber based applications.
Photodetector current saturates at high optical power levels due to space-charge screening effect. The saturation effect is
maximized where the illumination intensity, and the resulting photocurrent density, is largest. In this work, we focus on
optimizing the optical field profile incident on top-illuminated InGaAs photodiodes to minimize the peak photocurrent
density. This was achieved by employing graded-index (GRIN) lens coupling to uniformly distribute the optical power
across the diode cross-section.
We demonstrate 5dB improvement in photodiode's power handling capability and linearity by employing GRIN lens
coupling as compared to single mode fiber (SMF) coupling. Our GRIN lens-coupled photodetectors have achieved
small-signal 1dB compression current of >50mA and 12.5dBm amplifier-free RF output. These devices also exhibit
linear behavior for a peak-to-peak RF pulse output of >2.5V, at ~30ps pulse width. This constitutes a 100%
improvement over SMF coupled devices. Further, the GRIN photodiodes demonstrate pulse broadening =0.65ps/mW,
as compared to 2ps/mW for SMF devices.
We present an optical heterodyne receiver for data rates up to 10 Gb/s. Its outer dimensions are 44x44x18 cm3, it
weighs 16.5 kg and consumes 70 W of power. This optical receiver is single-mode fiber coupled to distribute the
received signal from the outside of the spacecraft to the inside. This approach improves the ruggedness of the receiver
system against shocks and vibrations. Under an ESA funded project, the photodiodes of this receiver have passed space
qualification tests, such as Particle Impact Noise Detection tests, shock and vibration survivability, as well as proton and
gamma radiation exposure.
High receiver sensitivities (BER=1·10-9) of 390 photons/bit and 619 photons/bit were measured at 1550 nm for
differential phase-shift keying (DPSK) and on/off keying (OOK), respectively. No low noise optical preamplifier
(EDFA) was used in this case. These are one of the highest sensitivities reported for heterodyne detection of 10 Gb/s
signals without using optical amplification. Avoiding the use of an EDFA allows to adapt the coherent receiver to other
wavelengths such as 1064 nm. We also investigated the receiver sensitivity of the coherent receiver when combined
with a low noise optical preamplifier. For 10 Gb/s DPSK and OOK sensitivities of 74 photons/bit and 132 photons/bit
were measured, respectively.
We report the development of top illuminated InGaAs photodetectors pigtailed to 50 &mgr;m core multimode (MM) fibers. These PIN diodes, in conjunction with low dispersion graded index MM fibers, allow for low cost and rugged solutions for high speed digital and analog applications. Our PIN diodes have previously demonstrated high optical power handling capability at large signal bandwidths. Coupled with large collection efficiency of MM fibers, these devices are suitable for a diverse range of systems, including avionics, ultra-fast Ethernet, radio over fiber, optical backplanes and free space laser links. The effect of the MM fiber's transfer function and fiber misalignment on the photodetector response is addressed. The spatial and temporal filtering effects of the MM fiber and the photodiode are explored experimentally through a 40 Gb/s link. Enhancement in photodiode linearity due to MM fiber is also reported.
Recently, there has been a renewed interest in coherent optical detection. The reasons for this are: a) coherent optical
receivers achieve high receiver sensitivities; b) multilevel modulation formats can be detected very efficiently; c)
optical WDM systems with high spectral efficiency can be implemented; and d) preservation of the optical phase
allows electrical equalizers to efficiently compensate optical channel impairments. These advantages of coherent
optical detection over direct detection can be used to overcome some of the obstacles that limit the data capacity and
the reach of current direct detection systems, both fiber and
free-space based. The essential part of the coherent optical receiver is the optical local oscillator (LO) laser. It has to provide a high
optical output power with low linewidth and low relative intensity noise (RIN). With a widely tunable LO laser a
frequency-agile receiver can be constructed.
To determine the best candidates for tunable LO lasers, different laser technologies are discussed in terms of output
power, power variation, electrical power dissipation, switching time, control leads, package dimensions, tuning
range, linewidth and RIN.
A heterodyne receiver to detect 2.5 Gb/s and 10 Gb/s signals has been implemented with a standard distributed feed
back (DFB) laser. Upgrades of the coherent receiver with a widely tunable LO will be presented. Experimental
comparison of the LO lasers and their impact on the receiver sensitivity will be shown.
We have manufactured a miniaturized, light weight, high data rate, optical coherent receiver system with weight less
than 37 lbs and power consumption less than 70 W. By using a coherent heterodyne method, the bench-top receiver has
achieved a link rate of 2.5 Gb/s at a Bit Error Ratio of 1e-9 with a sensitivity of -40 dBm. This receiver could be used as
a critical component of a free-space optical link, where the large distances and power limitations necessitate a high sensitivity. Optical communications links provide tremendous bandwidth and can achieve data rates two orders of magnitude higher that an RF communications link. Potential mass and power savings that go with using an optical system over an RF, along with the significantly higher bandwidth and reduced susceptibility to interference make them very attractive in the further development of the space environment.
We report -31 dBm receiver sensitivity for heterodyne detection of 10 Gb/s OOK without using an optical pre-amplifier. These are the highest receiver sensitivities for unpreamplified heterodyne 10 Gb/s detection. We also show the development of a coherent heterodyne balanced fiber optic receiver. The receiver incorporates a DFB or a solid state laser local oscillator, balanced PIN photodiodes, RF post amplifier, automatic frequency control (AFC), phase locked loop (PLL), polarization control, and precision power supplies in a small instrument case. We will show shot noise limited detection of amplitude modulated signals, cancellation of laser RIN noise, performance improvement using balanced detection at 2.5 and 10 Gb/s, and IF linewidth reduction.
Optical communications systems are vital to allow high speed satellite-to-satellite and satellite-to-ground-based communication links with low power consumption and low weight. To predict the performance of such systems it is essential to have an accurate simulation model which allows to predict the experimental results. We have implemented a coherent optical communications system which can be used for ultra long free-space distances. It incorporates a challenging optical phase lock loop (PLL). We also developed a simulation model for this advanced optical telecommunication system. It is shown that the experimental and numerical results obtained are in excellent agreement. By changing the parameters of the simulation model we can predict which of those parameters are most important to achieve a reliable high speed intersatellite optical link over a long free-space distance. One of the key parameters is the performance of our optical PLL. This is most important for systems which use the highly sensitive phase-shift keying (PSK) modulation format. Our developed optical PLL with a linewidth of as low as 130Hz shows excellent results both in simulation and experiments.
We report the development of a coherent heterodyne balanced fiber optic receiver with a small laboratory footprint. The receiver incorporates a DFB or a solid state laser local oscillator, fiber optic combiner/splitter, adjustable fiber optic delay line, balanced PIN photodiodes, RF post amplifier, optical phase lock loop, polarization control, and precision power supplies in a small instrument case. We will show shot noise limited detection of amplitude modulated signals, cancellation of laser RIN noise, and line narrowing of the IF signal. Several examples of coherent balanced detection as enabling technology for high value applications in fiber optic communication and remote sensing will be presented.
KEYWORDS: Photodiodes, Analog electronics, Receivers, Signal to noise ratio, Photodetectors, Signal detection, Modulation, Phase shift keying, Modulators, Fiber optic communications
We have developed 10, 20, 30, and 40 Gb bandwidth balanced photoreceivers which have applications for both analog and digital fiber optic communications. The devices can operate at C and L optical bands as well as 1064 nm and 1310 nm wavelengths. The analog applications include low noise RF photonic links. The digital applications include 10 Gb and 40 Gb DPSK and DQPSK modulation formats for enhanced sensitivities. The advantages of balanced photoreceivers are: RIN noise cancellation, suppression of even order harmonics, doubling the optical power handling capacity of a photonic link, and better reliability.
We have developed 20 mA or higher photocurrent handling InGaAs photodiodes with 20 GHz bandwidth, and 10 mA or higher photocurrent handling InGaAs photodiodes with >40 GHz bandwidth. These photodiodes have been thoroughly tested for reliability including Bellcore GR 468 standard and are built to ISO 9001:2000 Quality Management System. These Dual-depletion InGaAs/InP photodiodes are surface illuminated and yet handle such large photocurrent due to advanced band-gap engineering. They have broad wavelength coverage from 800 nm to 1700 nm, and thus can be used at several wavelengths such as 850 nm, 1064 nm, 1310 nm, 1550 nm, and 1620 nm. Furthermore, they exhibit very low Polarization Dependence Loss of 0.05dB typical to 0.1dB maximum. Using above high current handling photodiodes, we have developed classical Push-Pull pair balanced photoreceivers for the 2 to 18 GHz EW system. These balanced photoreceivers boost the Spurious Free Dynamic Range (SFDR) by almost 3 dB by eliminating the laser RIN noise. Future research calls for designing an Avalanche Photodiode Balanced Pair to boost the SFDR even further by additional 3 dB. These devices are a key enabling technology in meeting the SFDR requirements for several DoD systems.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.