Optical Power Fluctuations Research Articles

Contribution of shot noise to laser diode linewidth

Yamamoto and others have shown that when the pump (or injected current) fluctuations of a laser diode are suppressed, the optical power fluctuations are much reduced, but the laser linewidth remains essentially unchanged. We find that for a realistic extended laser model, the linewidth may in fact be importantly reduced. Our finding is based on a semiclassical theory. With full shot-noise in the injected current, the linewidth is proportional to (1+α 2) av, where α( z) denotes the phase-amplitude factor, and the average is evaluated along the diode z-axis. Without injected current fluctuations, one must subtract from this expression half the variance (α 2) av−(α av) 2 of α. The linewidth reduction thus occurs only if α varies significantly along the diode.

Optical output power fluctuation due to reflected lightwaves in laser diode modules

The optical output power fluctuation caused by axial displacement of optical components in an actual laser diode module subject to the influence of reflected light waves has been theoretically investigated. Results are based on the comparison of data from two analytical methods. One takes the emission spectrum shift of a laser diode into consideration while the other ignores wavelength shift. It has been clarified that the differences between these two results are relatively small, as long as it can be assumed that the laser diode oscillates in the initial longitudinal mode. Suppression methods for the reflected lightwaves are also discussed. Practical and efficient antire-flection techniques for the laser diode modules are presented.

Master-slice monolithic integration design and characteristics of LD/LED transmitters for 100-400 mbit/s optical transmission systems

Monolithic integration of LD and LED transmitters operating at 100-400 Mbit/s is investigated using a 3-μm Si-bipolar process as a first step in the establishment of a master-slice IC technology for optical communication systems. An LD transmitter IC is developed as a master IC, which includes a differential current switch and an input buffer circuit for high-speed modulation and a current stabilizer and an APC circuit for the stabilization of the output optical power, An LED transmitter is fabricated by the minor change of wiring of the master IC. The former exhibits 0.5-ns rise and 0.7-ns fall times at the signal current of 40 mA. Also the latter exhibits 0.4-ns rise and 0.6-ns fall times at the signal current of 100 mA. Characteristics of LD and LED transmitters are estimated in order to prove the usefulness of the fabricated IC's. As a result, it is confirmed that both transmitters using the fabricated IC's operate at the modulation speed of 300-400 Mbit/s and output optical power fluctuations of less than ±0.5 dB over the ranges of 5V ±5 percent supply voltage and 0-50°C temperature. From these results, it is proven that the master-slice monolithic integration is feasible and will be very useful for reduction of the turn-around time in IC development.

The effects of linear-accelerator noise on the Los Alamos free-electron laser

The optical power produced by the Los Alamos free-electron laser has been flawed by a peristent instability. This instability appears as fluctuations in output power in a frequency band centered around 100 kHz. We have found that the fluctuations are dependent on the strength of the lasing and on variations in the electron micropulse's arrival time. When the lasing is strong, the fluctuations are small; when the lasing is weak, the fluctuations are large. The variations in the electron micropulse's arrival time primarily are due to fluctuations in the accelerator gradients. The primary noise sources in the accelerator are the electron gun and the klystron amplifier chain. In addition, this noise is uncontrolled because of a lack of bandwidth in the feedback controls for the RF system. Appropriate improvements are being made to eliminate these fluctuations in optical power.

Carrier-induced phase noise in angle-modulated optical-fiber systems

Phase noise in angle-modulated optical-fiber communication systems arising from optical power fluctuations is analyzed. The nonlinear refractive index of silica is the physical mechanism which converts power fluctuations into phase fluctuations. The effects of self-phase modulation (an optical wave acting on itself) and cross-phase modulation in wavelength-division multiplexed (WDM) systems (one optical wave modulating a channel at a different wavelength) have been calculated. The phase noise generated in single-channel systems is negligible for laser fluctuations less than 1-mW rms. In WDM systems containing as few as four channels the phase noise exceeds tolerable levels (0.15 rad) for power fluctuation of 1 mW in each channel.