Note: Descriptions are shown in the official language in which they were submitted.
'5
OPTICAL COMMUNICATIONS LASER
INCLUDING A RESONATOR FILTER
Technical Field
This invention relates generally to the field of optical devices and
particularly to optical filters as well as to devices which incorporate such optical
filters.
Backoround of the Invention
Optical filters are important in numerous applications including both
optical signal processing and optical communications applications. Likely uses for
optical filters include, but are not limited to, wavelength division multiplexing,
wavelength discrimination in frequency shift keying (FSK) coherent detection
schemes as well as spontaneous emission noise filtering for optical amplifiers. As
might be expected, several approaches have been taken in attempts to fabricate
optical filters.
Waveguide reflection gratings provide one means of obtaining narrow
band wavelength discrimination. This type of filter has been demonstrated in both
glass and semiconductor waveguides. See, for example, Applied Physics Letters,
24, pp. 194-196, 1974, and Applied Physics Letters, 45, pp. 1278-1280, 1984.
However, there are drawbacks to this type of grating. It is difficult to obtain the
desired narrow bandwidths. If the filter is made with strong coupling between the
light and the grating, the light is reflected before seeing the entire grating, and the
bandwidth is relatively large. If it is made with weak coupling between the light
and grating, it must also be made long. In this case, the problems of obtaining
both uniform grating and waveguide effective index become very difficult. In
spite of these difficulties, filter bandwidth of approximately 6 Angstroms has been
obtained for a center wavelength of ~ = 1.55 llm. It is often desired, however, for
many applications that a filter bandwidth less than 1 Angstrom be obtained.
Additionally, for some applications, a filter that works in transmission, rather than
reflection, is desired.
Summary of the Invention
A grating resonator filter comprising a substrate and, on said substrate,
first and second grating sections and a section of changed effective refractive
index, said latter section being between said first and second grating sections
yields a grating resonator. The two sections are effectively phase shifted with
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respect to each other by the reduced effective refractive
index section which yields the desired ~/2 phase shift. The
changed effective refractive index section may be termed a
phase shift section~ In other words, the required ~/2 phase
shift is obtained by using a nongrating section having a
different refractive index than the grating sections have. In
one preferred embodiment, the filter is fabricated with
semiconductor materials such as Group III-V compound
semiconductors including InGaAsP. In yet another embodiment,
an electro-optic material is used and an electrode contacts
the phase shift section to permit frequency tuning of the
optical filter. In still another preferred embodiment, the
phase shift section is formed by etching. This produces a
reduced effective refractive index. More generally, the phase
shift section may have a different refractive index or
dimension relative to the grating sections. The filter is
useful in devices such as modulators, optical amplifiers,
wavelength division multiplexing, as well as other device
applications.
In one such application, a resonator filter is optically
coupled to a semiconductor laser cavity, and a resulting
narrow-linewidth laser is suitable as a light source, e.g., in
coherent-lightwave communications systems.
In one embodiment of the present invention there is
provided an optical communications laser, said laser
comprising a substrate-supported layer which is capable of
emitting light and which is situated between first and second
means for providing optical feedback in said layer, means for
supplying an electrical current to said layer, and a resonator
filter element in optical coupling relationship with said
layer.
In another embodiment of the present invention there is
provided a method for optical communications, said method
comprising operating a communications laser at or near a
specified wavelength, the wavelength of said laser being tuned
to said specified wavelength by means of a resonator filter
which is optically coupled to a laser-active layer bet~een
first and second feedback-producing means.
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Brief Description of the Drawinqs
FIG. 1 is a sectional view of an embodiment according to
this invention;
FIG. 2 plots the wavelength horizontally versus the
relative transmission vertically for a filter according to
this invention;
FIG. 3 is a sectional view of another embodiment
according to this invention; and
FIG. 4 is a sectional view of a semiconductor laser
including a resonator filter, representing a preferred
embodiment of the invention.
For reasons of clarity, the elements of the devices
depicted are not drawn to scale.
Detailed Descri~tion
An exemplary embodiment of a waveguide grating
resonator filter according to this invention is schemati-
cally depicted in FIG. 1. It comprises substrate 1 and first
grating section 3 and second grating section 5 which are
disposed on substrate 1. There is also a section 7
between the first and second grating sections and which is
termed the phase shift or changed effective refractive
index section. The phase shift section has a refractive
index which is different from the refractive indices of the
grating sections. The first and second grating
:~L2~ 5
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sections are geometrically in phase with each other. That is, the distance between
a grating peak in the first section and any one in the second grating section is an
integer number of grating periods. These gratings are first order gratings with a
period 1~. The phase shift section has a length l. The substrate has a lower
refractive index than do the first and second grating sections. Also depicted are
tlle input radiation 11 and filtered output radiation 13.
Conventional fabrication techniques may be used to fabricate the
grating resonator filter. That is, well-known and conventional lithographic and
epitaxial crystal growth techniques may be used. For example, InGaAsP
waveguide layers of approximately 0.7 ~m thickness and having a bandgap
wavelength of approximately 1.1 ~m may be grown on n-type, less than
1018/cm3, InP substrates. Other thicknesses and bandgaps may be used.
Photoresist is then deposited on the epitaxial layer. First-order gratings having a
period 1~ of .2340 llm are interferometrically written in the photoresist, and then
transferred to the quaternary InGaAsP layer by etching with a saturated bromine
and phosphoric acid solution. Typical grating depths are between 700 and
1,000 Angstroms with the precise depth depending in well-known manner upon
the etching time. The photoresist grating mask is now removed and a section
having a length, 1, is then chemically etched in the planar grating using a
photolithographically delineated mask. The etched space or section is typically
approximately S00 Angstroms deeper than the grating valleys. The length of the
grating reflector is approximately 500 ,um. No residual grating was observed by
using a scanning electron microscope. The etched region ~orms a region of
reduced refractive index, i.e., a phase shift section, between the first and second
grating sections. This fabrication method insures that with respect to light
propagating in the waveguide, the first and second sections are not optically inphase.
The reduced effective refractive index section produces an ef~ective
phase shift as it has a section of reduced waveguide thickness and, therefore, of
reduced effective refractive index. To obtain the desired relative phase betweenthe two gratings at the resonant wavelength, it is required that
(N1-N2) 1 = ~o/4 (1)
where l is the length of the etched section and N1 and N2 are the effective
refractive indices of the grating sections and the etched sections, respectively.
The resonant wavelength is ~O. The same requirement must be satisfied if the
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phase shift section has an increased refractive index.
Other embodiments are contemplated. FOI example, a channel
waveguide could be fabricated with the width of the phase shift section differing
from the widths of the grating sections. Different fabrication techniques offer
possibilities of changing the actual refractive index of the phase shift section with
respect to the refractive index of the grating sections. For example, different
metals could be indiffused in the grating and phase shift sections to form
waveguides of different refractive indices. If semiconductor materials are used,the etching step previously described could be followed by selective regrowth inthe etched area of a semiconductor material having a material composition and
refractive index different from the grating sections.
Each grating reflector has an identical stopband wavelength region
centered about the desired resonant wavelength where it is strongly reflecting.
When the above equation (1) is satisfied, strong reflections from the two grating
sections are out of phase and result in strong transmission at the resonant
wavelength.
A variety of materials for the waveguide may be used. For example,
semiconductors such as Group III-V compound semiconductors including InGaAsP
may be used. Additionally, lithium niobate may be used. The latter material doesnot appear as desirable as do the compound semiconductors as there is a relatively
small refractive index difference between the waveguide and the substrate and the
known techniques for etching lithium niobate do not etch lithium niobate as easily
as do the comparable techniques for the etching of semiconductors. However, it
has very low loss which is advantageous for very narrow-band resonator filters
with low insertion loss. The Group III-V compound semiconductors appear
desirable because a large refractiYe index difference between epitaxial layer and
the substrate may be obtained. Additionally, silica may be used.
The response ~or an exemplary grating resonator according to this
invention is depicted in FIG. 2 with the wavelength in ,um plotted horizontally
versus the relative transmission plotted vertically. The measured response has the
shape expected, i.e., an approximately 30 Angstroms wide stopband characteristicof grating reflectors with a single transmission resonance in the center. The
resonance width, that is, the full width at half maximum, is approximately
4 Angstroms. The excess resonator loss is approximately 0.9dB. This permits the
waveguide losses to be estimated at SdB/cm, which is a value consistent with
~8~
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losses in similar uncorrugated waveguides. Assuming this loss coefficient is
accurate, the filter bandwidth is not loss-limited but rather is limited by leakage
through the grating mirrors.
With a resonator having increased grating reflectivity, both the width
and depth of the stopband can be increased and the resonance bandwidth
decreased. Filter bandwidths as small as 1 Angstrom have been obtained. For the
waveguide losses mentioned, a filter bandwidth less than 0.25 Angstrom should beachievable.
The filter is useful in numerous devices. For example, it may be used
with an optical amplifier as shown in FIG. 3. In addition to the elements
described with respect to FIG. 1 and designated by identical numerals, the filter
also comprises an electrode 15 to the reduced effective refractive index section. A
source 19 of input light is also shown. The source is typically a laser; for
example, a semiconductor laser, and is optically coupled to the first grating
section, i.e., its emitted light enters the first grating section. This source has a
spectnlm broader than that desired. The electrode permits tuning of the passbandto the desired frequency.
The filter may also be used to modulate light from a laser. This is
easily done by using the embodiment depicted in FIG. 3. The electrode, upon
application of an appropriate voltage, modulates the electro-optic effect, and
thereby changes the position of the resonant wavelength. This permits the
effective transmission of the filter to be varied between two values such as 0.0 and
1Ø
The phase shift section need not be positioned symmetrically with
respect to the first and second grating sections. For some situations, it is desirable
that a grating length on the input side be of a different length than on the output
side.
Other embodiments are contemplated. For example, it will be readily
appreciated that the filters may be cascaded. That is, more than two grating
sections may be used with a corresponding increase in the number of reduced
effective refractive index sections. Additionally, second order gratings may be
used for some applications.
Contemplated further is the inclusion of a resonator filter as an intra-
cavity feature in a semiconductor laser. For example, as shown in FIG. 4, a laser
may comprise substrate 41, active layer 42, cladding layer 43, partially reflecting
~ Z~36'7~5
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mirror 44, totally re~lecting mirror 45, contact layers 46 and 47 for application of
(forward-biased) driving voltage, resonator filter layer 48, and contact 49 to aphase-shift section of resonator filter layer 48. For the sake of low-loss optical
coupling between active layer 4~ and (passive) resonator filter layer 48, the
material of the latter is preferably chosen to have a bandgap which is less than the
bandgap of the material o~ the former, desired equality of effective refractive
index in the two layers being achieved geometrically, e.g., by suitable choice of
layer thickness. Thus, typically, the thickness of resonator filter layer 48 is greater
than the thickness of active layer 42. Resulting relatively broad-band coupling
between active and resonator filter layers, combined with filter characteristics as
exemplified in FIG. 2, results in preferred narrow-linewidth output from the
device.
Preferred laser structure in accordance with FIG. 4 is such that laser
operation does not involve pumping of the portion of the active layer along the
length of the resonator filter layer. As a result, light traveling straight through the
active layer is lost - as is advantageous in the interest of narrowness of laserlinewidth.
Manufacture of the structure shown in FIG. 4 may involve deposition
of layer 48 in the presence of a mask; alternatively, the material of layer 48 may
be deposited on all of layer 43, followed by removal of portions of layer 48
material and deposition of contacts 47, e.g., by evaporation. Another convenientapproach does not involve selective deposition or removal of layer 48 material,
with contacts 47 deposited on portions of layer 48 away from the grating
resonator sections. Among suitable specific materials and fabrication proceduresare those mentioned above with respect to resonator filters in general.
~ hen suitably tuned or adjusted, a laser in accordance with this
embodiment of the invention is capable of single-frequency, narrow-linewidth
operation. Preferred tuning is such that the resonant wavelength of the filter at
least approximately coincides with a Fabry-Perot peak of the resonant cavity.
Typically, tuning capability is over a wavelength range of 10 to ~0 Angstroms,
permitting adjustment of laser operating wavelength to one of several nominal
values as desired, e.g, in wavelength-multiplexed systems. Not precluded is the
use of external feedback, e.g., for laser stabilization.
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Tuning may be effected with low loss, e.g., via the electro-optic effect
in a phase-shift section of filter 48 upon application of a reverse-biased voltage
between electrodes 46 and 49. To achieve larger changes in refractive index as
compared with the relatively small changes achievable electro-optically, tuning
may be effected by current injection under forward-biased conditions; this
approach, however, entails larger loss as compared with electro-optic tuning.
A resulting laser is considered to be particularly suited, e.g., for use as
an optical source in wavelength-multiplexed and coherent-lightwave
communications systems.
