N.A. Olsson

Self-phase modulation and spectral broadening of optical pulses in semiconductor laser amplifiers

Amplification of ultrashort optical pulses in semiconductor laser amplifiers is shown to result in considerable spectral broadening and distortion as a result of the nonlinear phenomenon of self-phase modulation (SPM). The physical mechanism behind SPM is gain saturation, which leads to intensity-dependent changes in the refractive index in response to variations in the carrier density. The effect of the shape and the initial frequency chirp of input pulses on the shape and the spectrum of amplified pulses is discussed in detail. Particular attention is paid to the case in which the input pulsewidth is comparable to the carrier lifetime so that the saturated gain has time to recover partially before the trailing edge of the pulse arrives. The experimental results, performed by using picosecond input pulses from a 1.52- mu m mode-locked semiconductor laser, are in agreement with the theory. When the amplified pulse is passed through a fiber, it is initially compressed because of the frequency chirp imposed on it by the amplifier. This feature can be used to compensate for fiber dispersion in optical communication systems. >

Lightwave systems with optical amplifiers

Fiber-optic communication systems using semiconductor laser amplifiers are investigated theoretically and experimentally. The noise and bit-error-rate characteristics of lightwave systems with optical amplifiers are calculated and the dependence of system performance on amplifier characteristics such as optical bandwidth, noise figure, gain, etc., is shown. Experimental results for both a 4-Gb/s optical preamplifier as well as coherent and direct detection systems with four inline amplifiers are presented. >

High‐speed direct single‐frequency modulation with large tuning rate and frequency excursion in cleaved‐coupled‐cavity semiconductor lasers

We report a new mechanism of direct frequency modulation, the cavity‐mode enhanced frequency modulation (CME‐FM), using the newly developed cleaved‐coupled‐cavity (C3) semiconductor laser. In this operation, one of the diode of the C3 laser was operated as a laser, while the other diode was operated as a frequency modulator. It was shown that a very large frequency excursion of 150 A and frequency tuning rate of 10 A/mA have been obtained with a C3 GaInAsP crescent laser operating at 1.3 μm. Time‐resolved spectral and spectral‐resolved pulse response measurements also showed that such C3 lasers operated in highly stable single‐longitudinal mode at all times even under high‐speed direct frequency modulation. In addition to the important application as the optical source in FM optical communication systems, the present CME‐FM C3 laser can also be used as the optical source in wavelength‐division multiplexing systems. Further, it opens the possibility of ultrahigh capacity multilevel optical FM information t...

Integrated four-channel Mach-Zehnder multi/demultiplexer fabricated with phosphorous doped SiO/sub 2/ waveguides on Si

An analysis and the device performance of an integrated optical 4-channel multi/demultiplexer based on Mach-Zehnder interferometers elements are presented. The channel spacing is 77 AA. Single-mode channel waveguides are made from P-doped SiO/sub 2/ core layers. Waveguide losses as low as 0.05 dB/cm and fiber-to-waveguide butt coupling as low as 0.5 dB for lambda =1.5 mu m have been obtained. The multiplexer transfers power from four input channels into a common output channel with an average fiber-to-fiber loss of 2.6 dB per channel. The average crosstalk of the demultiplexer is -16 dB. >

Locking range and stability of injection locked 1.54 µm InGaAsp semiconductor lasers

Measurements of the intensity of an injection locked 1.54 μm InGaAsP laser are reported. The change in intensity of the locked laser across the locking range is quite asymmetric, with a shape that agrees well with the theory of Lang. A linewidth parameter of \alpha = 6 \pm 1 was determined from the magnitude of the locking range. The injection locked laser was found to be unstable on the high frequency end of the locking range. The physical origin of this instability is explained in terms of a laser intensity change altering the phase of the laser field relative to that of the injected field.

Reflection noise in index-guided InGaAsP lasers

We report a detailed study of the excess noise induced in index-guided InGaAsP laser structures by reflection feedback. The phenomena of high-frequency noise (1-5 GHz), low-frequency noise (< 100 MHz), and intensity fluctuations are shown to have a common physical origin in the unusual instability of the coupled laser-external cavity system. After a randomly occurring light intensity drop, the light output recovers in 10-15 steps, each corresponding to an external cavity roundtrip (high-frequency noise); the total recovery time corresponds to the low-frequency noise. The instability, and the subsequent noise, can be suppressed under conditions of very strong feedback such as obtained for lasers with anti-reflection-coated facets. The reflection noise characteristics are shown to be largely independent of the laser structure and structure modifications such as distributed feedback.

Amplification and compression of weak picosecond optical pulses by using semiconductor-laser amplifiers

A novel pulse-compression scheme is proposed and demonstrated for compressing weak picosecond pulses having energies as small as 0.1 pJ. A semiconductor-laser amplifier is shown to impose a nearlylinear frequency chirp on the pulse as a result of self-phase modulation occurring because of gain-saturation-induced index changes. The amplified chirped pulse can be compressed by passing it through a dispersive delay line. The theory shows that, depending on the operating conditions, the pulse width can be reduced by a factor of ~2-4 and the peak power can be enhanced by a factor of ~10-100. The preliminary experimental results are in agreement with theory.

Ge0.6Si0.4 rib waveguide avalanche photodetectors for 1.3 μm operation

Performance of strained‐layer superlattice Ge0.6Si0.4 waveguide avalanche photodetectors is evaluated for optical fiber applications at 1.3 μm. These devices are grown on Si substrates by molecular beam epitaxy. Waveguiding is accomplished by means of a 1.5–1.8‐μm‐thick Si rib waveguide which provides an effective index step of δn=8×10−3. The detector response bandwidth exceeds 8 GHz at a gain of 6. A receiver sensitivity of ηP=−29.4 dBm has been obtained at the data rate of 800 Mb/s with the corresponding error‐free transmission over 45 km of single mode fiber.

Room‐temperature continuous‐wave vertical‐cavity surface‐emitting GaAs injection lasers

Room‐temperature continuous‐wave oscillation with an emission power in excess of 1 mW was achieved in vertical‐cavity surface‐emitting lasers containing a 0.5‐μm‐thick GaAs active layer sandwiched between a distributed Bragg reflector (DBR) and a hybrid metal DBR. The devices have a cw threshold current of 40 mA in 15‐μm‐diam size and a T0 of 115 K. Fiber butt coupling and pseudorandom data modulation of these lasers with open eyes up to 500 Mbit/s were demonstrated.

Characteristics of a semiconductor laser pumped brillouin amplifier with electronically controlled bandwidth

We demonstrate the operation of a semiconductor laser pumped fiber Brillouin amplifier. An optical gain of 5.5 dB/mW of pump power is obtained. The 15-MHz intrinsic bandwidth of the amplifier was extended more than one order of magnitude by adding frequency modulation to the pump laser. Bit-error-rate receiver sensitivity measurements with an in-line Brillouin amplifier demonstrate a 16-dB improvement in the system gain at 10 Mbit/s and a 8.5-dB improvement at 90 Mbit/s. Noise calculations show that the amplifier has an excess noise factor of 50-500 depending on the amplifier length and pump power.