Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-07-14
2004-01-13
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S044010
Reexamination Certificate
active
06678301
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an integrated circuit device and more particularly, to a semiconductor laser having an isolation region for reducing wavelength chirp.
BACKGROUND OF THE INVENTION
Distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers are light sources of choice in many optical communications systems. See, e.g., Loewen et al., DIFFRACTION GRATINGS AND APPLICATIONS (M. Dekker, NY 1997). Such lasers are designed to restrict laser oscillation to a single longitudinal mode with use of a diffraction grating provided in a semiconductor crystal.
For example, a typical DFB semiconductor laser includes a semiconductor substrate having a series of corrugated ridges formed thereon to define a grating; the grating reflects select wavelengths of light into the laser, and light at only one resonant wavelength is amplified. With the capability of oscillating light in a single, stable longitudinal mode, such devices are useful as light sources for coherent transmissions and in optical communications systems. Chromatic dispersion is minimized, and a signal emitted from the laser can be transported in an optical fiber over long distances.
In the distributed Bragg reflection (DBR) laser, the grating is etched outside the part of the laser that is pumped with electrical current. Current is driven into the laser at one region providing optical gain (external to the grating), and then pumped into the grating region to provide tuning. By separating the gain region from the tuning region in a DBR laser, increased tuning range can be provided from a DFB laser. However, the DBR works in much the same way as the DFB laser in that optical gain and grating feedback is needed for each.
For example,
FIG. 1
is a schematic illustration of one configuration for a DBR laser. This laser comprises a semiconductor substrate
10
which can be viewed as comprising three regions, e.g., a modulation or phase control region
20
, a tuning or distributed feedback (DFB) region
30
, and an amplifier or gain region
40
. An optical waveguide layer
12
is grown on the entire surface of the substrate
10
, and thus, is shared by all sections. A diffraction grating
13
is provided in that part of the waveguide layer corresponding to the DBR region
30
. An active layer
15
fabricated with semiconducting material may be formed contiguous the waveguide layer
12
for amplifying light. The active layer
15
typically is confined to the gain region
40
of the DBR laser, as shown, although it may be deposited adjacent the entire surface of the waveguide layer
12
.
A cladding layer
14
is grown over the entire surface of the optical waveguide layer
12
. Then, provided above the cladding layer
14
in each of the modulation
20
, DBR
30
, and gain regions
40
are top electrodes
22
,
32
, and
42
, respectively. The cladding layer
14
is partially removed between the electrodes to define grooves
18
,
19
for electrically isolating the top electrodes. A bottom electrode
16
is provided on the underside of the substrate and connected to common ground. An anti-reflection coating
24
is deposited at the modulation region and a reflecting facet coating
44
at the gain region. Typically, the anti-reflection coating
24
will allow for less than 1% reflection, more preferably less than 0.1%, and the reflection coating allows for reflection of from 30% to 99% or more, usually between 70% and 90% reflection.
In operation, current is injected into the laser from gain electrode
42
to excite electrons and stimulate the emission of light in waveguide layer
12
. Since the waveguide layer has a relatively high index of refraction relative to the substrate
10
and cladding layer
14
, light is guided along the high-index waveguide layer and reflected within the laser. Due to the presence of diffraction grating
13
, only certain wavelengths of light will be reflected back into the laser, e.g., to gain region
40
. This wavelength selectivity will depend upon the period of the diffraction grating
13
. The wavelength response can be modulated by injecting current into the waveguide layer with tuning (DBR) electrode
32
. The modulator electrode
22
may be activated to impress data upon the light beam. Various other configurations for DFB and DBR lasers are known and described in the field, see, e.g., U.S. Pat. No. 5,581,572 to Delorme et al., “Wavelength-Tunable Distributed Bragg Reflector Laser Having Selectively Activated, Virtual Diffraction Gratings”; U.S. Pat. No. 5,177,758 to Oka et al., “Semiconductor Laser Device with Plural Active Layers and Changing Optical Properties”; U.S. Pat. No. 4,719,636 to Yamaguchi, “Wavelength Tunable Semiconductor Laser Device Provided with Control Regions”; and U.S. Pat. No. 4,833,684 to Krekels et al., “Distributed Feedback Laser with Anti-Reflection Layer,” each of which is incorporated herein by reference.
A drawback with such lasers relates to wavelength chirp. As described above, the light emitted from the laser may be directly modulated by changing the current passing through it, because a change in current alters the density of electrons in the laser. However, as the density of electrons changes, so does the refractive index of the materials comprising the laser, which effectively may change the optical length of the waveguide layer. Such fluctuations in wavelength due to changes in the refractive index of the materials comprising the laser is known as wavelength chirping. Wavelength chirping causes distortions in the pulse waveform of the emitted signal which can result in interference and/or limit the signal transmission distance.
As may be appreciated, those in the field of optical communications systems continue to seek new designs and components that increase efficiency, modulate more quickly, and exhibit higher performance than previous counterparts. There is particularly a need for a laser that minimizes wavelength chirping. This and further advantages of the instant invention may appear more fully upon considering the detailed description set forth below.
SUMMARY OF THE INVENTION
Summarily described, the invention embraces an improved semiconductor laser having a substrate, an optical waveguide layer disposed on the substrate with a grating region, and a cladding region disposed on the optical waveguide layer, wherein an isolation region is disposed in at least one of the substrate, optical waveguide layer, and cladding layer. The isolation region is comprised of a material adapted to increase the resistivity of the one or more layers in which it is placed for reducing electrical cross-talk and wavelength chirp. The isolation region may be fabricated with at least one of hydrogen, deuterium, helium, oxygen, and iron. Preferably, the isolation region is positioned in at least the cladding layer, which the inventors have found to be the greatest contributor of cross-talk and wavelength chirping.
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Eng Lars E.
Hartman Robert Louis
Shtengel Gleb E.
Wilt Daniel Paul
Ip Paul
Lowenstein & Sandler PC
Nguyen Dung T
TriQuint Technology Holding Co.
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