Etching a substrate: processes – Etching of semiconductor material to produce an article...
Reexamination Certificate
2002-03-06
2003-12-16
Powell, William A. (Department: 1765)
Etching a substrate: processes
Etching of semiconductor material to produce an article...
C216S024000, C372S023000, C372S046012, C372S050121, C372S069000, C438S689000
Reexamination Certificate
active
06663785
ABSTRACT:
TECHNICAL FIELD
This invention relates in general to lasers, and in specific to methods for fabricating a broad spectrum emitter array and systems for using a broad spectrum emitter array.
BACKGROUND OF THE INVENTION
Known incoherently beam combined (IBC) lasers combine the output from an array of gain elements or emitters (typically consisting of semiconductor material, such as GaAlAs, GaAs, InGaAs, InGaAsP, AlGaInAs, and/or the like, which is capable of lasing at particular wavelengths) into a single output beam that may be coupled into, for example, an optical fiber. The gain elements may be discrete devices or may be included on an integrated device. Due to the geometry of IBC lasers, each gain element tends to lase at a unique wavelength. Exemplary arrangements of IBC lasers are described in U.S. patent application Ser. No. 6,052,394 and U.S. patent application Ser. No. 6,192,062.
FIG. 1
depicts a prior art arrangement of components in IBC laser
10
. IBC laser includes emitters
12
-
1
through
12
-N associated with fully reflective surface
11
. The optical power emitted by emitters
12
-
1
through
12
-N is generated in their quantum wells (not shown) which are surrounded by waveguide layers (not shown) and cladding layers (not shown). The cladding layers confine the light produced by the laser in the waveguide layers and the gain region in a single mode. Semiconductor lasers that use quantum wells offer dramatically lower threshold current densities compared to bulk heterostructures and are therefore advantageous due to their higher efficiency.
In known IBC laser devices, each emitter is exactly the same, i.e., emitters
12
-
1
through
12
-N are grown via a single fabrication process and, hence, possess identical characteristics. Moreover, each emitter in known IBC laser technology only possesses identical quantum wells in the active region of the respective emitter. Accordingly, the intrinsic bandwidth of each emitter is limited to the bandwidth of the identical quantum wells defined by the selected fabrication process.
Emitters
12
-
1
through
12
-N are disposed in a substantially linear configuration that is perpendicular to the optical axis of collimator
15
(e.g., a lens). Collimator
15
causes the plurality of beams produced by emitters
12
-
1
through
12
-N to be substantially collimated and spatially overlapped on a single spot on diffraction grating
16
. Additionally, collimator
15
directs feedback from partially reflective
17
via diffraction grating
16
to emitters
12
-
1
through
12
-N.
Diffraction grating
16
is disposed from collimator
15
at a distance approximately equal to the focal length of collimator
15
. Furthermore, diffraction grating
16
is oriented to cause the output beams from emitters
12
-
1
through
12
-N to be diffracted on the first order toward partially reflective component
17
, thereby multiplexing the output beams. Partially reflective component
17
causes a portion of optical energy to be reflected. The reflected optical energy is redirected by diffraction grating
16
and collimator
15
to the respective emitters
12
-
1
through
12
-N. Diffraction grating
16
angularly separates the reflected optical beams causing the same wavelengths generated by each emitter
12
-
1
through
12
-N to return to each respective emitter
12
-
1
through
12
-N. Accordingly, diffraction grating
16
is operable to demultiplex the reflected beams from reflective component
17
.
It shall be appreciated that the geometry of external cavity
13
of IBC laser
10
defines the resonant wavelengths of emitters
12
-
1
through
12
-N. The center wavelength (&lgr;
i
) of the wavelengths fed back to the i
th
emitter
12
-i is given by the following equation: &lgr;
i
=A[sin(&agr;
i
) +sin(&bgr;)]. In this equation, A is the spacing between rulings on diffraction grating
16
, &agr;
i
is the angle of incidence of the light from the i
th
emitter on diffraction grating
16
, and &bgr; is the output angle which is common to all emitters
12
-
1
through
12
-N. The overall bandwidth of IBC laser
10
is &lgr;
1
-&lgr;
N
, or &Dgr;&lgr;
laser
. As further examples, similar types of laser configurations are also discussed in U.S. patent application Ser. No. 6,208,679.
As previously discussed, in known IBC laser technology, each laser diode is the same as the others, and each quantum well in a particular device is the same as the other quantum wells of the device. The quantum wells provide a peak gain at a particular wavelength, &lgr;
C
, or center wavelength, and have a bandwidth of &Dgr;&lgr;
QW
. The quantum well bandwidth is the range of wavelengths over which the quantum wells can provide a gain. Thus, the laser array is constrained by the bandwidth of the quantum wells, such that the bandwidth of the laser array, &Dgr;&lgr;
laser
, must be less than the bandwidth of the quantum wells, &Dgr;&lgr;
QW
.
Additionally, Raman amplifiers have been developed to amplify optical signals. A Raman amplifier relies upon the Raman scattering effect. The Raman scattering effect is a process in which light is frequency downshifted in a material. The frequency downshift results from a nonlinear interaction between light and the material. The difference in frequency between the input light and the frequency downshifted light is referred to as the Stokes shift which in silica fibers is of the order 13 THz.
When photons of two different wavelengths are present in an optical fiber, Raman scattering effect can be stimulated. This process is referred to as stimulated Raman scattering (SRS). In the SRS process, longer wavelength photons stimulate shorter wavelength photons to experience a Raman scattering event. The shorter wavelength photons are destroyed and longer wavelength photons, identical to the longer wavelength photons present initially, are created. The excess energy is released as an optical phonon (a lattice vibration). This process results in an increase in the number of longer wavelength photons and is referred to as Raman gain.
As is well understood in the art, SRS is useful for generating optical gain. Optical amplifiers based on Raman gain are viewed as promising technology for amplification of WDM and DWDM telecommunication signals transmitted on optical fibers. Until recently, Raman amplifiers have not attracted much commercial interest because significant optical gain requires approximately one watt of optical pump power. Lasers capable of producing these powers at the wavelengths appropriate for Raman amplifiers have only come into existence over the past few years. These advances have renewed interest in Raman amplifiers.
Single cavity IBC lasers have typically been considered inappropriate to stimulate Raman gain for many telecommunication networks, because known IBC laser technology suffers from limited bandwidth. Specifically, Raman amplifiers based on IBC laser technology will operate over a bandwidth that is limited by the intrinsic gain bandwidth (as defined by the quantum well characteristics) of the semi-conductor material from which the device is made. The intrinsic gain bandwidth is due to the limitations of the emitters used in the known IBC laser designs. Known amplifiers used in telecommunication networks typically have bandwidths of about 40 nanometers (nm) at the wavelengths of interest, namely the C (1530 to 1565 nm) or L (1570 to 1610 nm) bands. However, known IBC technology cannot generate gain over the entire wavelength range. In particular, known IBC laser technology is not sufficient for the current systems operating at both the C and L bands, and is unsatisfactory for future systems operating at the S (1430 to 1530 nm), C, L, and XL (1615 to 1660 nm) telecommunication bands.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to systems and methods for generating Raman gain utilizing an IBC laser that possesses heterogeneous emitter structures. Specifically, an emitter array may be fabricated according to embodiments of the present invention such that the quantum well cha
Crump Paul A.
Devito Mark A.
Farmer Jason N.
Grimshaw Mike P.
Huang Zhe
Fulbright & Jaworski L.L.P.
Nlight Photonics Corporation
Powell William A.
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