Wavelength stabilized light source

Details Wavelength stabilized light source Wavelength stabilized light source

1990-05-04

1991-11-05

Davie, James W.

Coherent light generators

Particular beam control device

Optical output stabilization

372 26, 359328, H01S 310

Patent

active

050635689

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention generally relates to a wavelength stabilized light source, and more particularly to a wavelength stabilized light source which is suitable for a light source for use in coherent optical communications.
2. Background Art
Currently, optical communications are widely used. In optical communications, it is important to use a light source which emits a light having a stabilized wavelength. Presently, two different kinds of wavelength stabilized light sources have been put to practical use.
FIG. 1A is a block diagram of a first kind of conventional wavelength stabilized light source. A laser diode (semiconductor laser) LD emits a light or laser beam having a wavelength of 1.55 .mu.m. The laser beam emitted from the laser diode LD is split into two beams by a beam splitter BS. One of the split beams enters a transmission line formed by an optical fiber OF, and the other beam passes through a filter FL and is supplied to an opto-electric (photoelectric) converter O/E. The filter FL is designed to pass a light having a wavelength .lambda..sub.0 equal to that of the laser beam to be penetrated in the optical filter OF. The opto-electric converter O/E converts the incident laser beam into a corresponding electric signal, which is supplied to a drive DV. The electric signal changes based on a deviation of the laser beam from the wavelength .lambda..sub.0 of the filter FL. Thus, the driver DV changes a driving current based on the electric signal derived from the opto-electric converter O/E so that the laser diode LD emits the predetermined wavelength of light equal to 1.55 .mu.m.
FIG. 1B is a block diagram of a second conventional wavelength stabilized light source. The driver DV outputs a constant driving current to the laser diode LD, which emits not only light to be entered into the optical fiber OF but also light to be entered into an etalon ET. The etalon ET is disposed at a position where n.lambda..sub.0 =.lambda..sub.0 is satisfied (n is an integer). A light having the wavelength .lambda..sub.0 resonates in the etalon ET and returns to the laser diode LD. Thus, only the laser beam having the wavelength .lambda..sub.0 is increased and emitted toward the optical fiber OF.
However, the wavelength of light provided by the prior art shown in FIG. 1A is based on precision of the filter FL. Similarly, the wavelength of light provided by the prior art shown in FIG. 1B is based on precision of the etalon ET. For these reasons, neither the prior art shown in FIG. 1A nor the prior art shown in FIG. 1B cannot provide light of the wavelength which is absolutely equal to the desired wavelength .lambda..sub.0. It is noted that recently there has been considerable activity in the development of phase modulation or frequency multiplexing in coherent optical communications. In such advanced optical communications, it is essential to use a light source which is capable of emitting light having an absolutely fixed (stabilized) wavelength.
FIG. 2 is a block diagram of a third conventional wavelength stabilized light source, which utilizes an absorption line of atoms (or molecules). The light source shown in FIG. 2 is superior to the prior art shown in FIG. 1A or 1B. Referring to FIG. 2, the illustrated light source is made up of a laser diode (semiconductor laser) 31, a beam splitter 32, a phase modulator 33, an absorption cell 34 in which an NH.sub.3 gas is filled, a photodetector 35, an amplifier 36, a synchronous rectifying circuit 37, an oscillator 38 outputting a frequency fm, a control circuit 39, a laser diode driver 40 and a temperature controller 41. A laser beam emitted from the laser diode 31 is split into two beams by the beam splitter 32. One of the split beams is drawn as a light output of the light source, and the other beam is phase-modulated by the frequency fm through the phase modulator 33. The phase-modulated beam enters the absorption cell 34 which has an NH.sub.3 gas having a resonance wavelength .lambda..sub.0 within a band in the or

REFERENCES:
English Abstract of 63-18820, Gas Cell Type Atomic Oscillator.
English Abstract of 62-213186, Semiconductor Laser Wavelength Stabilizer.

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