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
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.
For free-space optical (FSO) communications systems, sensitive optical receivers are the key to
closing the link over long distances in inter-satellite transmission scenarios, or to overcome large
atmospheric attenuation in terrestrial FSO systems. We present a 10.7 Gb/s optical transmitterreceiver
pair operating at 1550-nm, based on return-to-zero, differential phase-shift keying (RZDPSK).
The receiver is pre-amplified and uses an optical delay interferometer and a balanced
photo-receiver. The outer dimensions, the weight, and power consumption are 44×44×18 cm<sup>3</sup>,
14.1 kg, and 35 W, respectively. This optical receiver is single-mode fiber coupled. At 10.7
Gb/s, a receiver sensitivity of 27 photons/bit was measured, which yields a bit error rate of 1e-9.
This is less than 1 dB from the quantum limit (22 photons/bit). Coupled with a commercial
optical booster amplifier having an output power of about +37 dBm, a link loss of more than 80
dB can be bridged. In an inter-satellite communications scenario, this corresponds to several
tens of thousands of kilometers. Additionally, high link losses can also be experienced in
terrestrial systems as the result of atmospheric scintillation. To study this effect, the transmitter
and receiver combination were tested with simulated turbulence (scintillation). A turbulence box
was used to emulate different levels of scintillation under which the pre-amplified RZ-DPSK
system was investigated. Results of these tests are presented.
We present an optical heterodyne receiver for data rates up to 10 Gb/s. Its outer dimensions are 44x44x18 cm<sup>3</sup>, 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<sup>-9</sup>) 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.
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.
In order to increase the transmittable data rate and to enlarge the transmission distance, the spacing between WDM channels has to be decreased while the optical transparent length must be increased both giving rise to interchannel crosstalk induced by fiber nonlinearities like cross-phase modulation (XPM). Thus, the development of modulation techniques being robust towards these effects is necessary. Recently, phase shift keying (PSK) techniques have attracted remarkable interest. For PSK-techniques, the optical power as a function of time is approximately constant (for nonreturn- to-zero (NRZ) signaling) or periodic (for return-to-zero (RZ) signaling). This is an advantageous property for the reduction of nonlinear phase modulation (PM) induced by the effect of XPM. On the other hand, since for PSKtechniques the information is carried by the phase of the optical carrier, the sensitivity to the nonlinear PM is high. In our contribution, we present an analytical model for the XPM-induced PM. With the help of this model and the visualization of XPM in the complex plane, we prove that the differential self-homodyne implementation of PSK is robust towards the nonlinear PM while PSK-techniques using a local oscillator in the receiver are extremely sensitive.