Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-08-03
2003-07-01
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Input/output coupler
Reexamination Certificate
active
06587619
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to optical functional devices, their manufacturing method and an optical communication system. More specifically, the invention relates to optical functional devices including distributed feedback (DFB) lasers with an optical waveguide structure having 2nd- or higher-order gratings, and other various optical functional devices having similar waveguide structures, and their manufacturing methods. The invention also relates to an optical communication system using these devices.
DFB lasers are often used in optical communication because single longitudinal mode oscillation is easily obtained. Single longitudinal mode oscillation is realized by a periodic structure formed along the waveguide of a laser, that is, diffraction gratings, because optical feedback for resonance becomes greatest in a specific longitudinal mode determined by the period of the gratings.
In optical communication using silica fibers, both the 1300 nm wavelength band and the 1550 nm wavelength band are used because these wavelength bands correspond to low-loss, low-dispersion regions of silica fibers. The InGaAsP/InP material system are most suitable for fabricating light emitting devices which emit in these bands. Therefore, InGaAsP/InP DFB lasers are widely used for optical communication.
FIG. 19
is a longitudinal cross-sectional view showing a structure of a conventional InGaAsP/InP-DFB laser. That is,
FIG. 19
shows a cross-sectional view taken along a plane parallel to the waveguide of the DFB laser. This laser has sing 1st-order Bragg gratings with a &lgr;/4 phase shift. The structure of the laser shown here is explained below, following to its manufacturing procedures.
First made on an n-type InP substrate
101
is an n-type InP buffer layer
101
′ by crystal growth. Next grown thereon are an active layer
102
having a multi-layered structure of InGaAsP quantum well layers and barrier layers, and a waveguide layer
103
having a lower refractive index than that of the active layer
102
. After these steps of growth, the wafer is taken out from the growth furnace.
After that, 1st-order gratings
110
are grooved on the waveguide layer
103
. In this process, a phase shift
115
by ¼ or −¼ of the wavelength &lgr; in the waveguide, is simultaneously made at a central position of the cavity. The same effect is also obtained by using a structure changing the effective refractive index of the waveguide instead of the actual phase shift. That is, even when the period of the gratings
110
is uniform, a region (not shown) where the waveguide structure changes in width, thickness or refractive index effectively functions as a phase shift.
After that, while keeping the configuration of the gratings
110
and the phase shift
115
, a p-type InP cladding layer
104
and InGaAs contact layer
105
are stacked on them by crystal growth.
Thereafter, a stripe structure (not shown) is made to extend in parallel to the wafer surface. Typical stripe structures are BH structure (buried heterostructure) and RWG (ridge waveguide) structure.
After that, a p-side electrode is formed on the p-type contact layer
105
, and an n-side electrode is formed on the bottom surface of the n-type substrate
101
(both not shown).
In the phase shift structure, the probability of the single longitudinal mode operation decreases if the reflectivity of both edges exceeds 1%. Therefore, both edges are coated by AR (anti-reflection) coating
150
. This can be realized by depositing dielectric thin films on the edges by the thickness of &lgr;/4 (&lgr;: oscillation wavelength).
Other than the structure shown in
FIG. 19
, there is a HR/AR (high reflectivity/anti-reflection) structure. A cross-sectional configuration of a laser having this structure is shown in
FIG. 20. A
difference from
FIG. 19
lies in having no phase shift
115
but having a HR coat
160
with the reflectivity greater than 90% on one of the edges. The HR coat
160
is a dielectric multi-layered film. It is discerned here that the relative phase between the HR coat edge and the gratings corresponds to the phase shift of
FIG. 190
, if it is folded back about the position of the HR coat
160
as the center of a mirror image. Usually, the phase of the gratings at the edge cannot be controlled. Therefore, the probability of obtaining preferable facet phases becomes lower. Taking account of it, the yield of the single longitudinal mode in the structure of
FIG. 20
is inferior to that of the &lgr;/4 for &pgr;/2 shift structure of FIG.
19
. Nevertheless, because of a large optical output from the AR edge, it is still useful as a high-output or high-efficiency structure.
These conventional DFB lasers, however, involved the problem that they were difficult to manufacture and often difficult to realize acceptable properties.
More specifically, the period of the gratings of a DFB laser utilizing 1st-order Bragg diffraction has to be approximately 200 nm to realize the wavelength of 1300 nm and approximately 240 nm to realize the wavelength of 1550 nm. When making the gratings, patterning must be as small as half the period, and an ultimate nano-process technique is required. Therefore, it is not easy to realize such gratings.
On the other hand, coupling efficiency
K
, which strongly affects the performance DFB laser, depends on the shape of the gratings. If the coupling efficiency
K
is excessively small, sufficient distribution feedback is not obtained, and the laser becomes difficult to oscillate in a single longitudinal mode. If it is excessively large, the threshold current of other longitudinal modes also become lower, and spatial hole burning phenomenon caused by longitudinal non uniformity of optical power makes single longitudinal mode operation unstable. That is, the coupling efficiency
K
must be within an optimum range. (Since the property of a DFB laser depends on its cavity length L as well, it is usually evaluated in terms of kL by multiplying L.) In order to realize an optimum value of
K
, the gratings must be precisely fabricated in depth and configuration. However, considering that 1st-order gratings are extremely fine as explained above, control of their configuration is very difficult. Additionally, optimum depth of 1st-order gratings is as very shallow as 20 to 30 nm approximately, its control is also difficult. As a result, there has been the problem that an optimum
K
value cannot be realized, and lasers satisfying desired properties cannot be obtained easily.
However, if utilizing 2nd- or higher-order Bragg gratings, their period is elongated to twice or more than 1st-order gratings, and the size of their patterning is enlarged sufficiently to make their fabrication easy. Additionally, depth of the gratings for obtaining the same value of
K
increases as well, and this makes it easy to control
K.
However, the use of 2nd- or higher-order gratings introduces lower-order diffraction light as radiation modes emitted from the waveguide. This is a loss for the DFB laser. This increases the threshold currents and deteriorates single longitudinal mode capability.
SUMMARY OF THE INVENTION
The present invention has been made from recognition of these issues. It is therefore an object of the invention to provide optical functional devices, such as low-threshold DFB laser, which are decreased in radiation mode loss even when using easily processed high-order gratings. It is a further object of the invention to improve their single longitudinal mode properties higher than those of using 1st-order gratings. It is another object of the invention to provide an optical functional device as surface-emitting laser (GCSEL: grating-coupled surface emitting laser) using 2nd-order gratings which can be optimized in threshold currents and emitted output or in light emitting pattern. Additionally, it is an object of the invention to provide their manufacturing method and an optical communication system using these devices.
According to the invention, there is provided an optical functional devic
Font Frank G.
Hogan & Hartson LLP
Mooney Michael P.
LandOfFree
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