Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-02-10
2001-12-04
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C359S199200
Reexamination Certificate
active
06327401
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to multifrequency laser apparatus and, more particularly, to such apparatus that employs a novel resonant structure.
BACKGROUND OF THE INVENTION
The growing trend to optical wavelength division multiplexed (WDM) systems has created a need for laser apparatus that can be a source of a plurality of optical signals of well separated and stable frequencies. To this end, it is important that multilongitudinal mode operation at the chosen wavelength be suppressed. Additionally, it is important that a high bitrate modulation of the chosen wavelength be possible. It is also desirable that the laser apparatus be of small size, and preferably largely integrated on a single semiconductive chip.
Generally laser apparatus to be of small size involves the use of a semiconductive chip that is enclosed within an appropriate resonant structure. In one particular form, a waveguide grating router (WGR) is used to provide filter control of the operating wavelength. In such a structure it is desirable to have the filter bandwidth of the WGR as narrow as possible so that at most only a few longitudinal modes of the laser chip are lying in the WGR passband. Ordinarily a relatively large-sized WGR is needed for generating a narrow passband filter bandwidth.
However, the geometrical size of the laser cavity generally should be as small as possible because, to a first order approximation, the laser cavity length determines the maximum speed for data modulation in a long-cavity laser. Additionally the smaller the size of the laser chip, generally the smaller its cost because less semiconductive material is involved and more laser chips can be obtained from a wafer of a given size.
A resonant structure that has been developed for use in gas laser systems is the so-called DiDomenico laser resonator described in a paper in the
IEEE Journal of Quantun Electronics
, August 1966, pp. 331-322 entitled “Characteristics of a Single-Frequency Michelson-Type He-Ne Gas Laser.” This structure is an open-ended resonator that provides longitudinal mode selection and amplitude stabilization in gas lasers.
It comprises a pair of coupled cavities, each enclosing an independently tunable amplifier whose outputs are combined. By choosing slightly different lengths for the two cavities, filter functions similar to those of a Fabry-Perot filter can be realized and the passband bandwidth of the resulting structure can be much narrower than that of either one of the two cavities. The conventional DiDomenico resonant structure includes a set of three mirrors and a beam splitter to form the pair of coupled cavities.
SUMMARY OF THE INVENTION
The present invention employs a modified form of DiDomenico resonant structure for the suppression of undesired multilongitudinal modes so that a plurality of well-separated stable laser wavelengths can be obtained from a structure of small size, advantageously largely integrated on a common substrate. To this end, the prior art MFL WGR-type lase is modified to provide two sets of resonant structures of different lengths at each frequency selected for operation.
In particular, in one embodiment of the invention, the multifrequency laser (MFL) comprises a WGR with an input coupling section and an output coupling section, each section of the free-space planar form, such as a star coupler, between which is connected a grating comprising planar waveguide arms of appropriate lengths. The ports of the coupling sections between which the waveguides are connected shall be described as the interior ports and the other ports of the coupling sections shall be described as the exterior ports. For a MFL to provide N separate laser outputs of different frequencies, in this embodiment there will be 2N semiconductive narrowband amplifiers forming two sets, each of N members, each tuned to a different one of the N output frequencies and one broadband semiconductive amplifier that will amplify the range of frequencies provided by the laser. Each of the 2N amplifiers has its output supplied to a different one of 2N exterior ports of the input coupling section of a WGR. Each of the interior ports of the input coupling section is connected to a separate interior port of the output coupling section. A single exterior port of the output coupling section of the WGR is connected, preferably by way of a broadband optical amplifier, to an optical fiber by which the desired optical beam is supplied to utilizing apparatus, such as a wavelength division multiplexing (WDM) transmission system. Reflecting surfaces are provided at opposite ends of the structure to enclose the amplifiers and the WGR.
In an alternative embodiment that employs only N narrowband optical amplifiers as the basic signal sources but a pair of broadband optical amplifiers, a portion of the output light that becomes available at a port in the output coupling section of the WGR is fed back into a second exterior port of the output coupling section of the WGR and is used as the second laser signal in a DiDomenico type laser. In another embodiment, the output from a single exterior port of the output coupling section of the WGR is passed through a suitable power divider, such as a beam splitter, and the two portions are supplied to separate amplifiers. One amplified portion is reflected back through the power divider back into the exterior port of the output coupling section of the WGR and the other amplified portion is partially transmitted via an optical fiber for utilization and the remainder reflected back into the exterior port of the output coupling section.
The invention will be better understood from the following more detailed description taken in conjunction with the accompanying drawing.
REFERENCES:
patent: 5488500 (1996-01-01), Glance
patent: 5889906 (1999-03-01), Chen
Doerr Christopher Richard
Josef Moeller Lothar Benedict Erhard
Joyner Charles H
Zirngibl Martin
Agere Systems Optoelectronics Guardian Corp.
Ullah Akm E.
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