David K. Donald

Coherent FMCW reflectometry using a temperature tuned Nd:YAG ring laser

A long-coherence-length, diode-pumped, monolithic Nd:YAG laser operating at 1.32 mu m is used to perform coherent FMCW (frequency-modulated continuous wave) reflectometry measurements. Compared with semiconductor-based lasers, the low phase noise of the Nd:YAG source offers greatly increased distance range combined with increased sensitivity to optical reflections. Measurement of Rayleigh backscatter from 50 km of fiber is demonstrated with over 60 dB of two-way optical dynamic range. Two-point spatial resolution is demonstrated to be better than 10 cm. This source and coherent measurement technique may have potential for both short and long-haul reflectometry applications.<<ETX>>

Efficient, Simple Optical Heterodyne Receiver: DC TO 80 GHz

An optical receiver front end was built which detects optical heterodyne signals up to 80 GHz. The unit combines optical detection in a GaAs photodiode with RF mixing of that same detected signal in the same photodiode. At low frequencies the diode is biased with DC, working as a conventional detector. Above 2 GHz the photodiode in the unit can be excited simultaneously with both light and a local oscillator voltage. The unit then produces intermedite frequency mixer products. When the signal mixes with the fundamental of the local oscillator (LO), these products are only 5 dB weaker than the original heterodyne signal. The LO voltage was 1.4 VRMS and its frequency 2-6 GHz. Above 6 GHz higher harmonics of the LO voltage mix with the RF photosignal. They mix unusually well in this receiver, and deliver significant power to the IF amplifier. Near 80 GHz, the receivers IF power of -88 dBm was generated when the 14th harmonic of the LO mixed with the signal.

Optimizing polarization states in a figure-8 laser using a nonreciprocal phase shifter

The polarization states in a figure and laser are analyzed using Jones matrices. The analysis shows that a figure-8 laser in CW operation does not give enough information to allow the optimization of the birefringence for mode-locking. A switchable nonreciprocal phase shifter is proposed to provide a simple way to optimize the birefringence for a mode-locked figure-8 laser. Using this device, the mode-locking of a diode-pumped figure-8 laser was started, with a probability better than 99%, by following a sequence of predetermined mechanical adjustment. With the proposed nonreciprocal phase shifter, the optimization and starting of a figure-8 laser can easily be automated. >

Photocurrent amplification in Schottky photodiodes

Up to 20 dB of photocurrent amplification with a frequency response of 2 GHz has been observed in GaAs Schottky photodiodes. The amplification may be caused by a reduction in Schottky built‐in potential under illumination. The device may find application in improving the sensitivity of optical receivers.

Adjustment-free femtosecond polarization-maintaining fiber lasers

This paper presents a pulsed Er-fiber laser constructed with all-polarization-maintaining (PM) components that meet all needed requirements. It is adjustment-free, and operates in a turn-key fashion. The output pulse duration is 540 fs, and the timing jitter is 150 fs.