Self-collimating multiwavelength lasers

Details Self-collimating multiwavelength lasers Self-collimating multiwavelength lasers

1999-04-16

2001-09-11

Scott, Jr., Leon (Department: 2881)

Coherent light generators

Particular resonant cavity

Specified cavity component

C372S020000, C372S092000

Reexamination Certificate

active

06289032

ABSTRACT:

FIELD OF THE INVENTION
The present Invention relates generally to multiwavelength lasers and more particularly to self-collimating multiwavelength lasers constructed using superimposed gratings.
BACKGROUND OF THE INVENTION
Multiwavelength lasers (MWLs) have great potential in a variety of civilian and military applications, enabling the increased transmission rates of wavelength-division multiplexing (WDM) systems, and enhanced operation in free-space settings such as range-finding and beam guidance. Ideally, MWLs should have low inter-channel interference (crosstalk), high power, low beam divergence for optimum coupling or free-space propagation, and be compact. In addition, it is highly desirable that any associated tuning circuitry be as simple as possible for ease of packaging and control, and that device size and complexity scale well as the number of wavelengths increases. No existing MWL design achieves all these ideals. This is due in part to gaps in the understanding of gain cavity behaviour under multiwavelength lasing conditions, but also to limitations inherent in established laser designs.
A number of MWL schemes have been developed, and generally fall in two categories: array and shared-gain. Array MWLs consist of a row of single-wavelength laser (SWL) designs, along which some wavelength-selecting parameter is varied. They offer the advantages of being a relatively simple extension of SWLs, and allow straightforward individual modulation of each laser. However, such devices are prone to cross-talk from independent drifting of individual wavelengths; subject to channel deviations from fabrication imprecision; and suffer from low yield. For shared-gain MWLs, channels share a gain region integrated with multi-resonance feedback elements, yielding a wavelength comb whose spacing is maintained even in the event of overall drifting. However, gain-coupling cross-talk must be properly treated when the wavelength spacings are too small (<1 nm), and individual channel modulation can be more difficult. The performance characteristics of current MWL designs are summarized below in Table 1.
As can be seen, impressive individual characteristics have been achieved. However, no single device combines the virtues of high power, large channel density, and low divergence. In fact, all designs suffer from high divergence: near-field beam size is no more than a few &mgr;m, which (for &lgr;~1 &mgr;m) corresponds to a divergence of at least ~10°. The ideals of high power and low divergence are in contradiction due to the requirement of monomode operation, which for existing MWLs restricts both current density and beam width.
The ideals of low divergence and high-power have been realized concurrently at a single wavelength in a ring laser configuration as disclosed in V. A. Sychugov, A. V. Tishchenko, A. A. Khakimov, “Nonlocalized Bragg Mirror Of The Comer-Reflector Type”, Soviet Technical Physics Letters, 5 1270-1274, 1979., and refined by K. M. Dzurko et al, see K. M. Dzurko, D. R. Scifres, A. Hardy, D. F. Welch, R. G. Waarts, and S. O'Brien, “500 mW coherent large aperture ring oscillators”, Electronics Letters, 28 1477-1478, 1992. In both implementations, conventional single-pitch gratings were empilayed, and output was single-wavelength only.
To overcome the aforementioned shortcomings, there is a need for a lasers which simultaneously permit broad-beam collimation and monomode operation, with simultaneous emission of multiple wavelengths
SUMMARY OF THE INVENTION
It is an object of the present Invention to provide self-collimating multiwavelength laser.
It is also an object of the present invention to provide self-collimating laser whose output wavelength can be tuned quasi-continuously over a broad range.
In one aspect of the invention there is provided a self-collimating multiwavelength laser, comprising:
a gain medium:
at least two superimposed gratings formed in the gain medium, the at least two superimposed gratings being oriented at an effective angle to each other to define a resonance cavity; and
means for pumping the gain medium to produce a population inversion in the gain medium.
In this aspect of the invention the gain medium may be a substantially planar gain medium and the superimposed gratings may be binary superimposed gratings. The binary superimposed gratings include an integral number of single-pitch gratings.
In another aspect of the invention there is provided a tunable lasers comprising a gain medium; at least two superimposed gratings formed in the gain medium, the at least two superimposed gratings being oriented at an effective angle to each other to define a resonance cavity;, tuning means for independently tuning each of said at least two superimposed gratings to independently adjust an effective refractive index of each of said at least two superimposed grating thereby shifting diffraction spectra of the superimposed gratings for tuning of a resonant wavelength in said resonance cavity; and means for pumping the gain medium to produce a population inversion in the gain medium.
In this aspect of the invention the gain medium may be a substantially planar gain medium and the superimposed gratings may be binary superimposed gratings.
The present invention also provides a method of producing a self-collimating multiwavelength laser. The method comprises providing a gain medium and producing therein at least two superimposed gratings. The at least two superimposed gratings are oriented at an effective angle to each other to define a resonance cavity. The method includes pumping the gain medium to produce a population inversion in said gain medium.
In another aspect of the invention there is provided a imethod of tuning a self-collimated laser. The method comprises providing a gain medium and producing therein at least two superimposed gratings. The at least two superimposed gratings are oriented at an effective angle &thgr; to each other to define a resonance cavity. A first of the two superimposed gratings emulates a superposition of the set of pitches &Lgr;
A
and has an effective refractive index in a region of said first grating of (n
eff
)
A
. The second superimposed grating emulates the set of pitches &Lgr;
B
, and has an effective refractive index in a region of the second grating of (n
eff
)
B
. A set of wavelengths diffracted by the first grating is &lgr;
A
2(n
eff
)
A
&Lgr;
A
and a set of wavelengths diffracted by the second grating is &lgr;
B
=2(n
eff
)
B
&Lgr;
B
sin(&thgr;). The method includes adjusting an effective refractive index of at least one of the superimposed gratings to achieve a resonance condition in which &lgr;
A
=&lgr;
B
. The method includes pumping the gain medium to produce a population inversion in the gain medium.


REFERENCES:
patent: 4505582 (1985-03-01), Zuleeg et al.
A. Hardy et al., “Design Considerations of Large Aperture Perpendicular Gratings Semiconductor Ring Lasers”, American Institute of Physics, Appl. Phys. Lett. 62(9), Mar. 1, 1993, pp. 931.
K.M. Dzurko, “1-W Single-Mode Edge-Emitting DBR Ring Oscillators”,IEEE Photonics Technology Letters, vol. 4, No. 4, Apr. 1993, pp. 369-371.
V.A., Sychugov et al., “Nonlocalized Bragg Mirror of the Corner-Reflector Type”, American Institute of Physics, Sov. Tech. Phys. Lett. 5(10), Oct. 1979, pp. 533-534.
K.M. Dzurko, “Single Mode Broad Area Distributed Bragg Reflector Ring Oscillators”, American Institute of Physics, Appl. Phys. Lett. 61(20), Nov. 16, 1992, pp. 2389-2391.
K.M. Dzurko, “Distributed Bragg Reflector Ring Oscillators”, (Sponsored by U.S. Air Force, Phillips, Laboratory, Pilot Program) Spectra Diode Labs, pp. 513-514.
K.M. Dzurko, “500 mW Coherent Large Aperture Ring Oscillators”,Electronic Letters, vol. 28, No. 16, Jul. 30, 1992, pp. 1477-1478.
K.M. Dzurko, “Distributed Bragg Reflector Ring Oscillators: A Large Aperture Source of High Single-Mode Optical Power”,IEEE Journal of Quantum Electronics, vol. 29, No. 6, Jun. 1993, pp. 1895-1905.
Sychugov et al; Nonlocalized Bragg mirror of the corner-reflector type; Sov. Tech. Phys. Lett. 5(10),

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