Krzysztof Szczerba

4-PAM for high-speed short-range optical communications

In this work, we compare 4-pulse amplitude modulation and on-off keying modulation formats at high speed for short-range optical communication systems. The transmission system comprised a directly modulated vertical-cavity surface-emitting laser operating at a wavelength of 850 nm, an OM3+ multimode fiber link, and a photodetector detecting the intensity at the receiver end. The modulation formats were compared both at the same bit-rate and at the same symbol rate. The maximum bit-rate used was 25 Gbps. Propagation distances up to 600 m were investigated at 12.5 Gbps. All measurements were done in real time and without any equalization.

Optimizing Constellations for Single-Subcarrier Intensity-Modulated Optical Systems

We optimize modulation formats for the additive white Gaussian noise channel with nonnegative input, also known as the intensity-modulated direct-detection channel, with and without confining them to a lattice structure. Our optimization criteria are the average electrical, average optical, and peak power. The nonnegative constraint on the input to the channel is translated into a conical constraint in signal space, and modulation formats are designed by sphere packing inside this cone. Some dense packings are found, which yield more power-efficient modulation formats than previously known. For example, at a spectral efficiency of 1.5 bit/s/Hz, the modulation format optimized for average electrical power has a 2.55 dB average electrical power gain over the best known format to achieve a symbol error rate of 10-6 . The corresponding gains for formats optimized for average and peak optical power are 1.35 and 1.72 dB, respectively. Using modulation formats optimized for peak power in average-power limited systems results in a smaller power penalty than when using formats optimized for average power in peak-power limited systems. We also evaluate the modulation formats in terms of their mutual information to predict their performance in the presence of capacity-achieving error-correcting codes, and finally show numerically and analytically that the optimal modulation formats for reliable transmission in the wideband regime have only one nonzero point.

37 Gbps transmission over 200 m of MMF using single cycle subcarrier modulation and a VCSEL with 20 GHz modulation bandwidth

We report transmission at 37.2 Gb/s over 200 m of multimode fibre using a directly modulated VCSEL operating at 850 nm, using 20 GHz modulation bandwidth.

Comparison of Intersymbol Interference Power Penalties for OOK and 4-PAM in Short-Range Optical Links

We present results of experimental and theoretical investigations of intersymbol interference in 4-PAM transmission in short-range optical communications links based on the power penalty. A test link comprised of a directly modulated 850 nm VCSEL with up to 200 m of multimode fiber and direct detection was used. The link bandwidth was below 10 GHz and the maximum achieved data rate with 4-PAM was 44 Gbps over 100 m of fiber. In the same case and at similar sensitivity, only 32 Gbps could be achieved with OOK. If typical forward error correction could be applied, the sensitivity of the 4-PAM system was improved by up to 4 dB, reaching -10 dBm at 25 Gbps.

48.7-Gb/s 4-PAM Transmission Over 200 m of High Bandwidth MMF Using an 850-nm VCSEL

We demonstrate 4-PAM 48.67-Gb/s transmission, using a directly modulated VCSEL, over 50, 100, and 200 m of multimode fiber with partial dispersion compensation properties. The results of our experiments indicate that 200 m reach is achievable with forward error correcting codes with small overhead.

70 Gbps 4-PAM and 56 Gbps 8-PAM using an 850 nm VCSEL

We present 56 Gbps unequalized 8-pulse-amplitude-modulation (PAM) real-time transmission over 50 m of multimode fiber and 70 Gbps 4-PAM operation with offline equalization. FEC would be necessary in both schemes. The bit-rates are given taking into account the FEC overhead needed to reach a BER below 10-12. The experiments were performed with an 850 nm vertical cavity surface emitting laser with 20 GHz bandwidth and a 22 GHz photoreceiver. We show that 8-PAM modulation requires channel bandwidth comparable with the symbol rate to avoid excessive ISI penalty and that equalization of 4-PAM signal can significantly improve achievable bit-rates.

94-Gb/s 4-PAM Using an 850-nm VCSEL, Pre-Emphasis, and Receiver Equalization

We present the results from an experimental demonstration of a 100-Gb/s 4-PAM transmission over a few meter MMFs using an 850-nm VCSEL with both analog pre-emphasis and a digital offline equalizer. The effective data rate is 94 Gb/s after accounting for the forward error correction overhead. Digital equalizers of lengths 11 to 71 taps were investigated.

High-speed 850 nm quasi-single-mode VCSELs for extended-reach optical interconnects

This paper presents recent results on high-speed, quasi-single-mode, 850 nm vertical-cavity surface-emitting lasers (VCSELs) with a narrow spectral width for extended-reach optical interconnects. The top mirror reflectivity is adjusted for high output power, slope efficiency, and small signal modulation bandwidth. An oxide confined VCSEL with an ~3 μm aperture diameter delivers 2 mW of output power and reaches a resonance frequency as high as 25 GHz and a modulation bandwidth exceeding 20 GHz. A small K-factor of 0.17 ns and a large D-factor of 17.3 GHz/mA1/2, extracted from the VCSEL modulation response, along with the improved DC and modal properties enable energy-efficient data transmission at high bit rates over long-distance multimode fiber. Error-free transmission at bit rates exceeding 20 Gbits/s over 1.1 km of OM4 fiber is demonstrated and shown to be limited mainly by the photoreceiver bandwidth. A theoretical investigation of the dependence of link performance on photoreceiver bandwidth is also presented.

Intersymbol interference penalties for OOK and 4-PAM in short-range optical communications

Intersymbol interference penalties of OOK and 4-PAM were compared in the context of short-range optical links with directly modulated VCSELs. The maximum achieved bit rate with 4-PAM was 38 Gbps and 32 Gbps with OOK.

Experimental comparison of modulation formats in IM/DD links

We present an experimental comparison of modulation formats for optical intensity modulated links with direct detection. Specifically, we compare OOK, QPSK on an electrical subcarrier and a new modulation format named OOPSK. The OOPSK modulation format is shown to have better sensitivity than the other modulation formats, in agreement with theoretical predictions. The impact of propagation in multimode fiber is also studied and the results show that all modulation formats have similar sensitivity penalties, with respect to the fibre length.