Christoph Wree

High spectral efficiency 1.6-b/s/Hz transmission (8 x 40 Gb/s with a 25-GHz grid) over 200-km SSMF using RZ-DQPSK and polarization multiplexing

With relatively simple standard transmission equipment, we achieved a wavelength-division-multiplexing transmission with a record-high spectral efficiency of 1.6 b/s/Hz using return-to-zero differential quadrature phase-shift keying and polarization multiplexing over 200-km standard single-mode fiber with a 40-Gb/s capacity per wavelength but 10-GSymbol/s rate.

CF-RZ-DPSK for suppression of XPM on dispersion-managed long-haul optical WDM transmission on standard single-mode fiber

We present a new optical modulation format chirp-free return-to-zero differential phase shift keying (CF-RZ-DPSK), which enables wavelength-division-multiplexing (WDM) transmission at 10 Gb/s/ch at a channel spacing of 100 GHz over 3000 km without significant impairments due to cross-phase modulation (XPM). A transmitter setup is presented, which allows a simple implementation of CF-RZ-DPSK with two Mach-Zehnder modulators in push-pull operation. The robustness toward XPM is shown theoretically with the help of a simple analytical model for the XPM-induced phase modulation. The superior performance of CF-RZ-DPSK over other modulation formats [RZ-ampfitude shift keying (ASK), nonreturn-to-zero (NRZ)-DPSK, and NRZ-ASK, respectively] is clarified. Finally, simulation results for CF-RZ-DPSK in comparison to RZ-ASK show the superior performance of the newly proposed modulation format in a dense WDM setup.

Experimental investigation of receiver sensitivity of RZ-DQPSK modulation using balanced detection

We investigate experimentally the power penalty obtained by doubling the data rate from 10 Gb/s using RZ-DPSK modulation format to 20 Gb/s using RZ-DQPSK modulation format. The measurement results confirm theoretical limits and compare balanced and single-ended detection.

Balanced photoreceivers for analog and digital fiber optic communications (Invited Paper)

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.

Repeaterless 10.7-Gb/s DPSK Transmission Over 304 km of SSMF Using a Coherent Receiver and Electronic Dispersion Compensation

Record repeaterless transmission of differential phase-shift keying (DPSK) at 10.7 Gb/s over 304 km of standard single-mode fiber (SSMF) is demonstrated using a coherent optical receiver and electronic dispersion compensation. This is the longest repeaterless 10-Gb/s transmission over SSMF in the absence of Raman amplifiers. The high receiver sensitivity and the high tolerance to nonlinearities of DPSK allow us to overcome a total link loss of 58 dB with a 3-dB system margin. Coherent detection enables linear electrical dispersion compensation and avoids the use of optical dispersion compensation.

Coherent optical receiver system with balanced photodetection

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.

Measured Noise Performance for Heterodyne Detection of 10-Gb/s OOK and DPSK

Heterodyne detection of 10-Gb/s nonreturn-to-zero (NRZ) on-off keying and NRZ differential phase-shift keying is demonstrated. The noise performance of the heterodyne receiver with and without the use of an optical preamplifier is experimentally investigated

Ten Gb/s optically pre-amplified RZ-DPSK for FSO communications systems with very large link losses

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 cm3, 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.

Coherent Receivers for Phase-Shift Keyed Transmission

We review coherent receiver implementations and their performance for phase-shift keyed transmission. Experimental results of a heterodyne receiver for 10 Gb/s DPSK are presented as well.

Multimode fibered photodetectors for high-power high-speed applications beyond 10 Gb/s

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 fibers 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.