T. M. Shen

Dispersion penalty for 1.3 mu m lightwave systems with multimode semiconductor lasers

The effect of fiber dispersion on the performance of lightwave systems is analyzed for the case where multimode semiconductor lasers operating near the zero-dispersion wavelength of the single-mode fiber are used as sources. Both the intersymbol interference and the mode-partition noise are considered in the discussion of dispersion-induced power penalties. The theory is in agreement with an experiment in which the bit error rate is measured for lasers at various bit rates. The tolerable limits on the deviation of the laser wavelength from the zero-dispersion wavelength are obtained for a 1.3- mu m system operating at 1.7 Gb/s. Monte Carlo simulations are used to predict the effect of mode-partition noise on the performance of such high-speed lightwave communication systems. >

Timing jitter in semiconductor lasers under pseudorandom word modulation

Timing jitter of semiconductor lasers under pseudorandom word modulation was studied at 1 Gb/s. For lasers biased above threshold the timing jitter was Gaussian in distribution, with a turn-on timing jitter amplitude tau =2 sigma =+or-6 ps. For lasers biased below threshold, the turn-on timing jitter showed two peaks of Gaussian distribution, with an amplitude tau =60+or-10 ps and separated by about 80 ps. This phenomenon is due to a pattern-dependent effect of the laser turn-on delay. Theoretical calculations show that for data transmission above 2.2 Gb/s, the laser has to be biased above threshold to avoid excessive (>0.5 dB) jitter-induced power penalty. >

Effect of fiber-far-end reflections on the bit error rate in optical communication with single-frequency semiconductor lasers

For a semiconductor laser oscillating predominantly in a single longitudinal mode, the reflection feedback from the far end of an optical fiber generates closely spaced side modes corresponding to the external-cavity longitudinal modes. Using a simple model, it is shown that power fluctuations associated with these feedback-induced side modes can decrease the receiver sensitivity over its shot-noise-limited value for a given bit error rate. Since fiber dispersion plays a minor role, the reflection-induced power penalty is likely to occur for all fiber lengths commonly employed in optical communication systems.

Conversion gain in millimeter wave quasi-particle heterodyne mixers

The physical mechanisms which give rise to conversion gain in SIS quasi-particle mixers are studied. It is shown that the S -shape tunneling structure at the gap voltage of the I- V curve is essential in achieving conversion gain. In the development of SIS quasi-particle mixers, a new approach is used to analyze the embedding network of the mixing experiment. The method described in this paper has the advantage over conventional methods that no separate measurements are necessary. In order to obtain a complete picture of the performance of SIS quasi-particle mixers, the photon-assisted tunneling theory used by Tucker to describe quasi-particle mixing is extended here to include pair current contribution. Based on this complete quantum theory, the effects of the Josephson noise on SIS quasi-particle mixing is discussed and an upper frequency limit of SIS quasi-particle mixing is estimated.

InGaAsP laser with semi‐insulating current confining layers

The fabrication and performance characteristics of a InGaAsP laser structure with semi‐insulating current confining layers are reported. The semi‐insulating layers are Fe‐doped InP and are grown using the metalorganic chemical vapor deposition growth technique. The lasers have threshold currents in the range 20–30 mA and external differential quantum efficiency ∼0.2 mW/mA/facet at 30 °C. The bandwidth for small‐signal response is ∼2 GHz which suggests that the laser structure is suitable for high bit rate lightwave transmission systems. Initial aging results yield an estimated operating lifetime of 10 years at 20 °C.

Computer simulation and noise analysis of the system performance of 1.55-µm single-frequency semiconductor lasers

A theoretical model for the noise analysis of the system performance of 1.55-μm single-frequency semiconductor lasers is presented. Computer simulations are used to analyze the role of various noise sources in a 1.7-Gbit/s transmission experiment where the data was transmitted over 69 km using a 1.56-μm distributed-feedback laser. The bit-error-rate curves generated from numerical simulations agree well with the results of the transmission experiment. The relative contributions of various noise sources in limiting the system performance are discussed and compared. In particular, we consider circuit noise, shot noise, laser intensity noise, mode-partition noise, parasitic reflections, and the frequency chirp.

Long wavelength InGaAsP (λ∼1.3 μm) modified multiquantum well laser

The fabrication and performance characteristics of InGaAsP (λ∼1.3 μm) double channel planar buried heterostructure lasers with multiquantum well (MQW) active layers are reported. The MQW structure has λg∼1.3 μm InGaAsP active wells and λg∼1.03‐μm InGaAsP barrier layers. The lasers have threshold current of ∼20 mA at 30 °C and external differential quantum efficiencies of ∼0.2 mW/mA/facet at 30 °C. The temperature dependence of threshold current is characterized by T0∼100 K both under electrical and optical pumping. The lasers have been operated to 110 °C and up to ∼30 mW/facet at 25 °C. The measured dynamic linewidth under modulation is ∼2 smaller than that for conventional double heterostructure lasers. The lower temperature dependence of threshold current and smaller dynamic linewidth makes real index‐guided InGaAsP MQW active layer lasers potentially attractive for many system applications.

Linewidth characteristics of fiber‐extended‐cavity distributed‐feedback lasers

A 1.55‐μm wavelength InGaAsP distributed feedback (DFB) laser with an antireflection coated facet has been coupled to a single‐mode fiber with a reflective end to construct a compact narrow‐linewidth source. The linewidth of the DFB laser is reduced by a factor of 1000 to 70 kHz at 1 mW output with a 5.5‐cm fiber length. The fiber mirror to DFB laser coupling parameters required for narrow‐linewidth operation are presented.

Pulse-shape effects on frequency chirping in single-frequency semiconductor lasers under current modulation

Directly modulated semiconductor lasers exhibit dynamic frequency shifts (chirping) due to gain-induced variations of the refractive index. Using the small-signal analysis of the single-mode rate equations, the effect of current-pulse shape on frequency chirping is analyzed, and the results are compared for the cases of sinusoidal and square-wave modulations. The chirp is generally larger for the square-wave case. However, its magnitude depends on the pulse rise and fall times, decreasing for a pulse with slower turn-on and turn-off characteristics. Chirp analysis presented here includes the effect of power-dependent gain changes arising from the processes such as spectral hole-burning.

Fabrication and performance characteristics of InGaAsP multiquantum well double channel planar buried heterostructure lasers

We report the fabrication and performance characteristics of InGaAsP double channel planar buried heterostructure (DCPBH) lasers with multiquantum well active layers emitting at 1.3 μm. These lasers have threshold currents in the range 40–50 mA at 30 °C, external differential quantum efficiencies of ∼50% at 30 °C, and T0 values ∼160–180 K in the temperature range 10–60 °C. Under optical pumping the measured T0 are in the range 100–150 K. The lasers operate in a single transverse mode up to high powers (>10 mW/facet), can be modulated at ∼2 Gb/s, and exhibit less frequency chirping than similar lasers with conventional active layers. The observed high T0 and smaller chirp make DCPBH multiquantum well lasers potentially attractive for system applications.