Martin M. Strasser

Optimum filter bandwidths for optically preamplified NRZ receivers

We present a comprehensive treatment of optically preamplified direct detection receivers for non-return-to-zero (NRZ) and return-to-zero (RZ) on/off keying modulation, taking into account the influence of different (N)RZ optical pulse shapes, specified at the receiver input, and filter transfer functions; optical Fabry-Perot filters (FPFs) and Bragg gratings as well as electrical fifth-order Bessel and first-order RC low-pass filters are considered. We determine optimum optical and electrical filter bandwidths and analyze the impact of bandwidth deviations on receiver sensitivity. Optimum receiver performance relies on a balance between noise and intersymbol interference (ISI) for NRZ transmission, while for RZ reception detection noise has to be traded against filter-induced signal energy rejection. Both for NRZ and 33% duty cycle RZ, optical filter bandwidths of around twice the data rate are found to be optimum. Receivers using RZ coding are shown to closely approach the quantum limit, and thus to outperform NRZ-based systems by several decibels. We further analyze the impact of important degrading effects on receiver sensitivity and optimum receiver bandwidths, including receiver noise, finite extinction ratio, chirp, and optical carrier frequency (or optical filter center frequency) fluctuations.

Dependence of optically preamplified receiver sensitivity on optical and electrical filter bandwidths-measurement and simulation

In this letter, we investigate both experimentally and by means of simulations, the dependence of receiver sensitivity on the optical filter bandwidth as well as on the bandwidth of the detection electronics for the optical noise limited direct detection case. The experiment is in good agreement with simulations employing advanced Gaussian noise statistics. Bandwidth optimization is performed both for nonreturn-to-zero and return-to-zero coded signals, yielding a measured sensitivity only 1.4 dB off the quantum limit at a data rate of 10 Gb/s.

Noise and intersymbol-interference properties of OTDM and ETDM receivers

We experimentally and theoretically confirm the similar performance of optical time-division demultiplexing (OTDM) and electrical time-division demultiplexing (ETDM) receivers for beat-noise limited return-to-zero signal detection. OTDM receivers show a slight sensitivity advantage, largely independent of parameters like the receivers optical filter bandwidth or center frequency. The OTDM sensitivity benefit is caused by reduced intersymbol-interference properties of the OTDM receiver, as well as by the suppression of beat noise by the temporal demultiplexing window. The latter effect can be captured by extended expressions for the beat-noise variances presented in this letter.

Experimental verification of optimum filter bandwidths in direct-detection (N)RZ receivers limited by optical noise

We present measurements performed at a data rate of 10 Gb/s to experimentally verify these theoretical results. In the case of RZ coding we reached a sensitivity of 52 photons per bit (ppb) at a bit-error ratio BER = 10/sup -9/, which is about 1.4dB above the quantum limit. To the best of our knowledge, at 10Gb/s, this result is the best previously reported sensitivity for optically preamplified on-off-keying (OOK). The (N)RZ transmitter consisted of a DFB laser with an integrated electroabsorption modulator.

Significance of chirp parameter for direct-detection free-space laser communication

We investigate the influence of chirp on free-space laser communication systems employing optically preamplified direct-detection receivers. We analyze the changes of receiver sensitivities in the presence of chirp and optimize both the optical and electrical receiver bandwidth with regard to sensitivity. For return-to-zero (RZ) coding with 33% duty cycle the receiver sensitivity decreases due to spectral broadening of the received signal by typically 3 dB when the chirp parameter amounts to alphaequals5. For optimized bandwidths, the penalty can be kept as low as 1 dB. In contrast, in the case of non-return-to-zero coding, chirp can reduce inter-symbol interference (ISI), thus improving receiver performance due to a pulse compression effect. The gain in sensitivity can be as large as to 1.8 dB when the magnitude of the chirp is alphaequals4. A larger chirp decreases the receiver sensitivity. In the case of NRZ, the optimum receiver bandwidths are quite insensitive with regard to chirp.

Optimum optical and electrical filter characteristics in optically preamplified direct detection (N)RZ receivers

We give optimum values for the bandwidths of realistic optical and electrical filters (optical Fabry-Perot filters and fiber Bragg gratings, electrical Bessel filters and first order RC low pass filters), as well as for realistic NRZ and (33% duty cycle) RZ input pulses for a free space laser communication system employing an optical booster and a direct detection receiver with optical preamplification. Different extinction ratios, the presence of background radiation, and the influence of the booster amplifiers amplified spontaneous emission (ASE) are emphasized. Our results show that the optimum optical filter bandwidth (both for NRZ and RZ) has to be sought in the range of 1.5 to 3 times the data rate. Using the optimum filter bandwidths, RZ coding yields a sensitivity improvement of up to 1.5 dB compared to NRZ transmission. For typical system parameters and link distances higher than several thousand kilometers the booster ASE becomes less important than strong background radiation, while it causes severe sensitivity degradations for shorter distances.

Optimum source concepts for optical intersatellite links with RZ coding

We discuss several potential methods of generating optical RZ data signals, distinguishing between direct RZ modulation and modulation of a primary pulse train which is either generated by using a modelocked laser, by sinusoidally driving of an external modulator, or by gainswitching of a laser diode. We analyze the properties of each method with regard to the most critical aspects for space-borne laser communication systems such as repetition rate, duty cycle, extinction ratio, frequency chirp, timing jitter, robustness, complexity, commercial availability, and lifetime. Most modelocked lasers are highly sensitive to ambient perturbations, necessitating accurate temperature control and mechanical stabilization. Also, they typically provide pulses with less than 10% duty cycle, which can result in a decreased sensitivity of optically preamplified receivers. Directly modulated semiconductor lasers are compact and robust but suffer from large frequency chirp, which deteriorates the receiver sensitivity. One reliable RZ source is a conventional DFB semiconductor laser with two intensity modulators, one for pulse generation and one for data modulation. Both Mach-Zehnder modulators co-packaged with a laser diode or monolithically integrated electroabsorption modulators should be considered. These modulators can provide almost transform-limited pulses at high repetition rates and with duty cycles of about 30%. Robustness and lifetime are highly promising.

Correction to "optimum filter bandwidths for optically preamplified NRZ receivers"

We present a comprehensive treatment of optically preamplified direct detection receivers for non-return-to-zero (NRZ) and return-to-zero (RZ) on/off keying modulation, taking into account the influence of different (N)RZ optical pulse shapes, specified at the receiver input, and filter transfer functions; optical Fabry–Pérot filters (FPFs) and Bragg gratings as well as electrical fifth-order Bessel and first-order low-pass filters are considered. We determine optimum optical and electrical filter bandwidths and analyze the impact of bandwidth deviations on receiver sensitivity. Optimum receiver performance relies on a balance between noise and intersymbol interference (ISI) for NRZ transmission, while for RZ reception detection noise has to be traded against filter-induced signal energy rejection. Both for NRZ and 33% duty cycle RZ, optical filter bandwidths of around twice the data rate are found to be optimum. Receivers using RZ coding are shown to closely approach the quantum limit, and thus to outperform NRZ-based systems by several decibels. We further analyze the impact of important degrading effects on receiver sensitivity and optimum receiver bandwidths, including receiver noise, finite extinction ratio, chirp, and optical carrier frequency (or optical filter center frequency) fluctuations.

Emulating ETDM receivers by OTDM receivers for high speed system measurements

Theory and experiment quantify to what extent OTDM receivers can emulate the performance of ETDM receivers. OTDM receivers can yield /spl sim/1 dB better absolute sensitivities than ETDM receivers, but perform similar for sub-optimum optical filter bandwidths.

Influence of chirp on optical free-space communication systems employing preamplified direct detection

Summary form only given. With the large-scale commercial deployment of Erbium-doped fiber amplifiers, optically preamplified direct detection (DD) receivers have become the technically most practicable way of achieving (nearly) quantum limited receiver performance in the 1.5 /spl mu/m wavelength range. In fiber communication systems, DD reception is affected by chirp due to fiber dispersion and nonlinear effects, but in free-space communications, narrow optical filtering changes receiver sensitivity if the input signal is chirped. We present a detailed investigation on the influence of chirp in free-space preamplified DD systems and analyze the (different!) impact in the case of non-return-to-zero (NRZ) and return-to-zero (RZ) coding. Optimizing the optical and electrical receiver bandwidths leads to improved sensitivity.