Modulated DFB Research Articles

Long wavelength InGaAsP/InP distributed feedback lasers grown by chemical beam epitaxy

We have demonstrated successful operation of long wavelength InGaAsP low threshold-current gain-coupled DFB lasers. This is accomplished by using a InGaAsP quarternary grating or quantum well grating that absorbs the DFB emission. The use of a quantum well grating, in particular, greatly facilitates the reproducible regrowth (defect-free) over grating and the control of the coupling coefficient. CW threshold currents were in the range of 10–15 mA for 250 μm and 13–18 mA for 250 and 500 μm cavities, respectively. Slope efficiencies were high, ∼0.4 mW/mA (both facets). SMSR was as high as 52 kB and remained in the same DFB mode with SMSR staying ∼50 dB throughout the entire current range. Linewidth x power products of 1.9-4.0 were measured with minimum linewidths of 1.8-2.2 MHz. No detectable chirp was measured under 2.5 Gb/s modulation. Unlike index-coupled DFB lasers in which mode partition events decrease slowly even when biased above threshold, these lasers have mode partition events shut off sharply as bias approaches threshold (≿0.95 I th). A very small dispersion penalty of 1.0 dB was measured at 10 -11 BER in transmission experiments using these lasers as sources at 1.7 Gb/s over an amplified fiber system of 239 km. No self-pulsation was observed in these gain-coupled DFB lasers.

High performance InGaAsP/InP semiconductor quantum well lasers realized by gas source molecular beam epitaxy

A wide variety of lasers with multiquantum well structures have been sucessfully realized: 1.55 μm single longitudinal mode DFB laser (50 mW output power); 1.48 μm Fabry-Perot pump source (190 mW output power); multiphase shift DFB laser with narrow linewidth (<330 kHz); multielectrode laser usuable as FM transmitter (tunability 100 GHz with linewidth <1 MHz); multisection distributed Bragg reflector laser (tunability 375 GHz with 10 MHz linewidth). All these devices have been fabricated using Gas Source Molecular Beam Epitaxy (GSMBE) for the double heterostructure growth and Liquid Phase Epitaxy (LPE) for blocking layer regrowth. This process of fabrication leads to a good reproducibility and reliability

Generation of soliton pulse train in optical fibre using two CW singlemode diode lasers

A train of fundamental solitons at 1.53 μm has been generated, with pulse durations of 1.5–3.0 ps, at a repetition rate in the range 80–130GHz from the output of two CW single mode DFB lasers amplified in an Er-doped fibre. The transformation of the beat signal into a soliton train resulted from nonlinear propagation of the signal in a special fibre with dispersion decreasing along the length.

High performance DFB-MQW lasers at 1.5 μm grown by GSMBE

1.5 μm GaInAs multiquantum well distributed feedback lasers have been successfully fabricated on InP grating substrate by GSMBE. DFB mode oscillation at high output power (50 mW) and narrow linewidth (1.3 MHz) have been obtained.

Linewidth dependence on side-mode suppression in three-electrode DFB laser

A 1000μm-long three-electrode DFB laser with a large sidetuning and with a minimum linewidth of 4 MHz is demonstrated. The correlation between the main mode linewidth and DFB submode suppression is large. Side-modes on the opposite sides of the stop band are seen to broaden the linewidth to different extents. A Fabry-Perot mode 50 nm away has no observable effect on the linewidth.

Wavelength discriminator method for measuring dynamic chirp in DFB lasers

Wavelength fluctuations in a modulated DFB laser are transformed into intensity fluctuations using a Fabry-Perot interferometer as a wavelength discriminator. Using this method we have measured chirp in several DFB lasers. The results show a high correlation between chirp and regenerator sensitivity degradation when transmitting through dispersive spans.

Measurement of a modulated DFB laser spectrum using gated delayed self-homodyne technique

The mixing of a modulated DFB laser with a CW local oscillator can be achieved with a single laser by using a gated modulation technique combined with an appropriate optical delay. Frequency chirp measurements up to ±22 GHz are demonstrated using this technique.

Wavelength tuning and switching of a coupled distributed feedback and Fabry–Perot cavity laser

We present a theoretical analysis of a newly demonstrated semiconductor laser with coupled distributed feedback and Fabry–Perot (DFB-FP) cavities and show that three modes of operation are possible for such a laser. In mode-switched DFB mode, the wavelength can be switched between longitudinal modes on either side of the stopband. In coupled-cavity laser mode, there are successive mode hops inside the stopband. Finally, in continuously tunable distributed Bragg reflector mode, a wide wavelength tuning range (4.8 Å) without mode hopping can be obtained. The analysis is general enough to be applied to any laser with a periodic waveguide section, and provides an understanding of the mechanisms and the limits of wavelength tuning in such lasers. This type of laser has very important applications in coherent optical communications.

Mode stability of a two-wavelength Fabry-Perot/distributed-feedback laser

A theory based on a Fokker-Planck equation has been developed to analyze the two-mode operation of a Fabry-Perot/distributed-feedback (FP/DFB) laser. Simultaneous oscillation of the DFB mode and the FP mode near the gain peak with negligible mode power fluctuation can be achieved if the DFB mode is detuned sufficiently from the gain peak. Calculated results for the design of stable two-wavelength lasers are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

AlGaAs/GaAs visible distributed feedback laser operating at 770 nm

A visible-light AlGaAs/GaAs DFB laser with practical characteristics oscillating at 770 nm has been fabricated by using two-step liquid phase epitaxy for the first time. Threshold current of 90 mA measured at room temperature is satisfactorily low for its simple oxide stripe structure. The same DFB mode oscillation without mode hopping has been maintained in a considerably wide range of temperature. The results show a possibility of realising a high-performance DFB laser in the visible-wavaelength region.

Wide temperature range of stable single-mode operation in weakly coupled asymmetric mirror reflectivity DFB lasers

A maximum difference between the peak gain wavelength and the lasing wavelength where stable DFB mode operation is achieved has been obtained for weakly coupled (KL̃; 0–7), asymmetric mirror structure (Rf = 5%, Rr = 31%) DFB lasers. DFB lasers optimally fabricated exhibit single-longitudinal-mode operation over a wide temperature range of 5–100°C.

Fabrication and performance characteristics of InGaAsP ridge-guide distributed-feedback multiquantum-well lasers

The fabrication and performance characteristics of InGaAsP ridge-guide distributed-feedback lasers with multiquantum-well active layers are reported. The lasers are 320 μm long and have four active wells (300 A thick). The lasers have threshold current 100 mA at 30°C, external differential quantum efficiency 0.1 mW/mA/facet at 30°C and To 60 K and can be operated in the same DFB mode up to 70°C. The measured frequency chirp is about a factor of 3 smaller than that for conventional double-heterostructure lasers. The smaller chirp should allow larger repeater spacing for high-bit-rate long-distance fibre transmission systems applications.

Longitudinal mode behaviors of 1.5 µm range GaInAsP/InP distributed feedback lasers

Longitudinal mode behaviors of asymmetric structure distributed feedback buried heterostructure (DFB-BH) lasers are examined theoretically and experimentally. A 1.5 μm range GaInAsP/InP DFB-BH laser was fabricated by a three-step LPE growth process. We measured the stopband in the spectrum of the DFB laser. It was found that no resonance mode emission occurred in the gain spectrum and its spectrum was asymmetric with respect to the Bragg wavelength. Most of the lasing power concentrated on the DFB mode adjacent to the stop-band which was determined by the Bragg condition. The measured spectrum was explained by the calculated results of the coupled wave theory with external reflectors. The asymmetric spectrum was caused by the relative position of the cleaved facet on the corrugation grating. It was shown that the asymmetric structure DFB laser, which consisted of two end facets with different reflection coefficients, gives a stable single longitudinal mode. There was no mode jump up to 2.3 times threshold. At a modulation depth of 100 percent, the ratio of the highest nonlasing mode intensity to the lasing DFB mode was estimated to be -16.0 dB.