Optical communications laser including a resonator filter

A structure of first and second grating sections separated by a phase-shift section serves as a narrow-band resonator filter. Such structure may be optically coupled to a semiconductor laser cavity, and the resulting assembly can serve as a tunable narrow-linewidth laser, e.g., in wavelength-multiplexed and coherent-lightwave communications systems.

TECHNICAL FIELD 
This invention relates generally to the field of optical devices and 
particularly to optical fibers as well as to devices which incorporate 
such optical filters. 
BACKGROUND 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 fliter 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 
.lambda.=1.66 .mu.m. 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 respect to each other by the reduced 
effective refractive index section which yields the desired .pi./2 phase 
shift. The changed effective refractive index section may be termed a 
phase shift section. In other words, the required .pi./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 fiter. 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.

For reasons of clarity, the elements of the devices depicted are not drawn 
to scale. 
DETAILED DESCRIPTION 
An exemplary embodiment of a waveguide grating resonator filter according 
to this invention is schematically 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 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 A. 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 the 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 .mu.m thickness and having a bandgap 
wavelength of approximately 1.1 .mu.m may be grown on n-type, less than 
10.sup.18 /cm.sup.3 InP substrates. Other thicknesses and bandgaps may be 
used. Photoresist is then deposited on the epitaxial layer. First-order 
gratings having a period .LAMBDA. of 0.2340 .mu.m 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 ethcing time. the 
photoresist grating mask is now removed and a section having a length l, 
is then chemically etched in the planar grating using a 
photolithographically delineated mask. The etched space or section is 
typically approximately 500 Angstroms deeper than the grating valleys. The 
length of the grating reflector is approximately 500 .mu.m. No residual 
grating was observed by using a scanning electron microscope. The etched 
region forms 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 in phase. 
The reduced effective refractive index section produces an effective phase 
shift as it has a section of reduced waveguide thickness and, therefore, 
of reduced effective refractive index. To obtain the desired relative 
phase between the two gratings at the resonant wavelength it is required 
that 
EQU (N.sub.1 -N.sub.2)l=.lambda..sub.o /4 (1) 
where l is the length of the etched section and N.sub.1 and N.sub.2 are the 
effective refractive indices of the grating sections and the etched 
sections, respectively. The resonant wavelength is .lambda..sub.o. The 
same requirement must be satisfied if the phase shift section has an 
increased refractive index. 
Other embodiments are contemplated. For 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 in the 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 noibate may be used. The latter 
material does not 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 
noibate do not etch lithium noibate 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 smeiconductors appear desirable 
because a large refractive index difference between epitaxial layer and 
the substrate may be obtained. Additionally, silica may be used. 
The response for an exemplary grating resonator according to this invention 
is depicted in FIG. 2 with the wavelength in .mu.m plotted horizontally 
versus the relative transmission plotted vertically. The measured response 
has the shape expected, i.e., an approximately 30 Angstroms wide stopband 
characteristic of 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 0.9 dB. This permits the waveguide losses to be estimated at 
5dB/cm, which is a value consistent with 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 be achievable. 
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 spectrum broader than that 
desired. The electrode permits tuning of the passband to 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 1.0. 
The phase shift section used 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 resonate 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 mirror 44, totally reflecting mirror 45, contact 
layers 46 and 47 for application of (forward-biased) driving voltage, 
resonator filter layer 48, and contact 49 to a phase-shift section of 
resonator filter layer 48. For the sake of low-loss optical coupling 
between active layer 42 and (passive) resonator filter layer 48, the 
material of the latter is preferably chosen to ahve a bandcap which is 
less than the bandgap of the material of 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 laser linewidth. 
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 convenient approach does not involve selective deposition or 
removal of layer 48 material, with contacts 48 deposited on portions of 
layer 48 away from the grating resonator sections. Among suitable specific 
materials and fabrication procedures are those mentioned above with 
respect to resonator filters in general. 
When suitably tuned or adjusted, a laser in accordance with this embodiment 
of the invention is capable of single-frequency, narrowlinewidth 
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 20 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. 
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 reversebiased 
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.