An optical amplifying apparatus which includes an optical amplifier, an optical attenuator and a controller. The optical amplifier amplifies a light signal having a variable number of channels. The optical attenuator passes the amplified light signal and has a variable light transmissivity. Prior to varying the number of channels in the light signal, the controller varies the light transmissivity of the optical attenuator so that a power level of the amplified light signal is maintained at an approximately constant level that depends on the number of channels in the light signal prior to the varying the number of channels. While the number of channels in the light signal is being varied, the controller maintains the light transmissivity of the optical attenuator to be constant. Subsequent to varying the number of channels in the light signal, the controller varies the light transmissivity of the optical attenuator so that a power level of the amplified light signal is maintained at an approximately constant level that depends on the number of channels in the light signal subsequent to the varying the number of channels.

Controller which controls a variable optical attenuator to control the power level of a wavelength-multiplexed optical signal when the number of channels are varied

Thermooptics of solid-state lasers

An optical amplification device which includes first and second optical amplifiers, and a controller. The first optical amplifier receives a light and amplifies the received light. The second optical amplifier receives the light amplified by the first optical amplifier, and amplifies the received light. When a level of the light received by the first optical amplifier changes by Δ, the controller controls a level of the light received by the second optical amplifier to change by approximately −Δ. In various embodiments, the controller causes the sum of the gains of the first and second optical amplifiers to be constant. In other embodiments, the optical amplification device includes first and second optical amplifier and a gain adjustor. The gain adjustor detects a deviation in gain of the first optical amplifier from a target gain, and adjusts the gain of the second optical amplifier to compensate for the detected deviation.

Gain and signal level adjustments of cascaded optical amplifiers

Output of 1.25 J per pulse (1.064 mu m) with an absolute optical efficiency of 28% and corresponding electrical efficiency of 10% was demonstrated in a diode-array-pumped Nd:YAG laser using a side-pumped rod geometry in a master-oscillator/power-amplifier configuration. In Q-switched operation, an output of 0.75 J in a 17-ns pulse was obtained. The fundamental laser output was frequency doubled in KTP with 60% conversion efficiency to obtain 0.45 J in a 16-ns pulse at 532 nm. The output beam had high spatial quality with pointing stability better than 40 mu rad and a shot-to-shot pulse energy fluctuation >

One joule output from a diode-array-pumped Nd:YAG laser with side-pumped rod geometry

The key importance of the sign of the stress-optic anisotropy ratio for reducing thermally induced depolarization in cubic crystals with 432, 4¯3m and m3m symmetry is addressed. A simple method for measuring the stress-optic anisotropy ratio (including its sign) was proposed and verified in CaF2 and TGG crystals by experiment. The ratio at room temperature for the wavelength 1076 nm was measured to be −0.47 and + 2.25, respectively. In crystals with a negative value of this parameter thermally induced depolarization may be reduced significantly by choosing crystal orientation. In a CaF2 crystal with the [111] orientation a 20-fold reduction of thermally induced depolarization as compared to the [001] orientation was obtained in experiment, which is very promising for using CaF2 as an active element in high-average-power lasers.

Drastic reduction of thermally induced depolarization in CaF_2 crystals with [111] orientation

When an internal Brewster plate is used to polarize a Nd: YAlG laser, its output power generally decreases. This is shown to be largely due to the thermally induced birefringence in the laser rod, which causes a polarization rotation inside the optical cavity and hence a reflection loss at the polarizer. Calculation of this loss for a typical laser has been made and is compared to observed decreases in output power.

Nd:YAIG Laser Cavity Loss due to an Internal Brewster Polarizer.

This paper describes a device which samples the optical signal in an optical fiber for use in source stabilization via feedback control. This device incorporates a beam splitter formed on the fiber ends to deflect a portion of the light traveling down the fiber into a P-I-N photodetector. Advantages of this device include low insertion loss, an output that is relatively independent of the modal distribution within the fiber, and is a rugged small package.

Lightwave fiber tap

Several laser transmitters assembled for 44.736-Mb/sec lightwave field experiments exhibited large, unexpected changes in output power. This problem is now identified with laser front-to-back mirror mistracking and its effect on the back-mirror-monitoring scheme used for feedback control of the laser. To correct this problem, an optical-fiber tap was developed to monitor the actual output power for feedback control. This paper describes the output power stability of five GaAlAs laser transmitters operated using this optical tap for feedback control. Results show that tap-controlled transmitters have output-power stability within ±2% (±0.1 dB) over a 1000-h test period, this being an order of magnitude improvement compared with the earlier scheme of control.

Output power stability of GaAlAs laser transmitters using an optical tap for feedback control.

We have performed experiments to evaluate the sensitivity of the optical output power in lightwave transmitters to possible changes in coupling between the injection laser and the optical fiber. Specifically, coupling changes attributable to beam wander and moding in the laser as well as mechanical motion between the laser and fiber were considered. Since complete transmitters use feedback control circuitry to maintain the output power constant, the effects of beam wander and moding on the feedback sensors (photodiodes, either near the laser or in an optical-fiber tap) were also measured. Based on our results, a comparison of back-mirror monitoring vs fiber-tap monitoring for feedback control is presented. The effects of front/back-mirror mistracking often observed in injection lasers were also considered when comparing these two monitoring schemes. We conclude that fiber-tap monitoring is preferable from a system point of view.

M. A. Karr

Papers

Nd:YAIG Laser Cavity Loss due to an Internal Brewster Polarizer.

Lightwave fiber tap

Output power stability of GaAlAs laser transmitters using an optical tap for feedback control.

Effects of beam displacement and front and back mistracking of junction lasers on lightwave transmitter output stability.