Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-04-12
2003-02-04
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S026000, C372S034000, C372S043010, C372S020000
Reexamination Certificate
active
06516017
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an integrated external-modulator semiconductor laser device capable of operating at high speed for use in a transmitter for a wavelength multiplexed transmission system, and relates more specifically to such a semiconductor laser device selecting a plurality of wavelengths, and to a drive method for the semiconductor laser device.
2. Description of the Related Art
Wavelength-division multiplexing (WDM) techniques have been recently developed as a means of increasing the transmission capacity of fiber optic transmission paths. A particular benefit of WDM techniques is that the transmission capacity of existing fiber optic cables can be readily increased by several factors of ten. Semiconductor laser devices using an integrated modulator semiconductor laser, i.e., a semiconductor laser integrated with a modulator, have been developed as a light source for systems using WDM techniques. This type of semiconductor laser device dc drives a distributed feedback (DFB) laser diode and uses an electroabsorption modulator for high speed modulation of light emitted from the semiconductor laser.
The operating principle of an electroabsorption modulator is described briefly below.
The absorption layer of the electroabsorption modulator has either a multiquantum well or a strained multiquantum well structure. When a reverse field is applied to the absorption layer, the energy level of excitons (an electron and hole pair in a bound state) trapped inside the quantum well changes. This phenomenon is known as the quantum trapping Stark effect, and is illustrated in FIG.
31
. The exciton causes a change in the absorption wavelength, which produces a change in light transmission.
FIG. 32
illustrates exciton-induced change in absorption wavelength. As shown in
FIG. 32
, when there is a change in exciton energy level, the absorption wavelength of the exciton shifts to a longer wavelength. This effect is used for light modulation.
Referring to
FIG. 32
, if the oscillating wavelength of the laser is &lgr;, absorption coefficient a of wavelength &lgr; changes &Dgr;a depending on whether or not a reverse field is applied to the modulator. When a field is not applied, light passes the modulator, but when a reverse field is applied the modulator absorbs light from the laser. Considering exciton energy shift, it is also necessary to set the equivalent band gap wavelength of the modulator absorption layer to a shorter wavelength. A typical laser oscillation wavelength is approximately 1550 nm, and the corresponding band gap wavelength of the modulator absorption layer is approximately 1500 nm. The difference between these wavelengths, &Dgr;&lgr;, is thus approximately 50 nm, and the equivalent band gap wavelength of the modulator absorption layer must therefore be set to a wavelength that is &Dgr;&lgr; shorter than the laser oscillation wavelength.
FIG. 33
is a typical view of an exemplary integrated modulator semiconductor laser device according to the related art.
FIG. 34A
shows the offset bias input to an optical modulator
102
, and
FIG. 34B
shows the light output from the optical modulator
102
. Referring to
FIGS. 33
,
34
A, and
34
B, it is to be noted that the optical modulator
102
and semiconductor laser
103
are monolithically integrated on semiconductor substrate
101
, forming semiconductor laser chip
104
. This chip
104
is affixed to metallic block
106
by way of intervening submount
105
of SiC or other material, and to metal package
108
by way of an intervening temperature control Peltier element
107
affixed to the opposite side of metallic block
106
.
The Peltier element
107
is a solid cooling element that uses the Peltier effect commonly used for temperature control. The Peltier element
107
can be controlled to either heat or cool by changing the polarity of current flowing to the element
107
. A thermistor (not shown in the figure) is used to detect the temperature of the semiconductor laser chip
104
. Based on the temperature detected by this thermistor, the current flowing to the Peltier element
107
is controlled to maintain the semiconductor laser chip
104
at a desired temperature. The semiconductor laser
103
is driven to maintain a constant output by applying voltage to the semiconductor laser
103
in the forward direction.
The optical modulator
102
is driven by a high frequency square wave signal voltage. Light input to the optical modulator
102
from the semiconductor laser
103
is then modulated by the optical modulator
102
to a light wave output corresponding to the signal voltage. For example, when a normally 2- to 3-V reverse voltage is applied to the optical modulator
102
, light is absorbed and laser beam emissions from the modulator
102
stop. A desirable offset voltage Voffset of typically less than 1 V, for example, may be applied in some cases to the optical modulator
102
when in a light transmitting state.
As the technology has progressed and the degree of wavelength multiplexing has increased, a new type of light source that does not use a single integrated modulator semiconductor laser to provide all wavelengths used by the WDM system has been needed. Examples of such light sources include a variable wavelength light source that uses a single semiconductor laser chip to output at multiple wavelengths, and an array of multiple wavelength semiconductor lasers integrated on a single chip. In addition to achieving a light source that could be used in a variety of systems, significant cost benefits could be obtained by providing many light sources in a single chip for use as a backup light source for a WDM transmission device. Such multiple wavelength light sources can also be used to route signals through a network using a wavelength routing technique by tuning laser output to a specific wavelength at different points in the network, and will be needed to build all types of optical networks in the future.
As a result, there has been much research into variable wavelength light sources using a single chip to output multiple wavelengths of light, as well as devices that integrate an array containing a plurality of semiconductor lasers of different wavelengths into a single light source chip, and integrate a modulator for each of the multiple light sources on this chip. For example, “Six-Channel WDM Transmitter Module with Ultra-Low Chirp and Stable &lgr; Selection,” ECOC 1995 Th.B.3.4, from Lucent Technologies describes a device shown in FIG.
35
. This device integrates an array of six semiconductor lasers, each producing a different wavelength at a 1.6 nm wavelength interval, with a coupler, semiconductor amplifier, and modulator. The semiconductor laser chip shown in
FIG. 35
operates by selecting any one of the six output wavelengths.
As described above, the difference &Dgr;&lgr; between the oscillation wavelength of the semiconductor laser and the band gap wavelength of the optical modulator has a significant effect on the absorption characteristic of the optical modulator, and by extension has a significant effect on transmission characteristics at the oscillation wavelength of the semiconductor laser. The performance of the above-noted Lucent device when transmitting at 2.5 GHz over a 600 km path (Voffset=−1.5 V, 3.5 Vp-p) was evaluated using two wavelengths offset 3.2 nm. Minimum reception sensitivity at a bit error rate (BER) of 1e-9 was −31.2 dBm at 1559.71 nm and −30.4 dBm at 1556.49 nm, and its wavelength dependency is expected to be high.
The paper “Widely Tunable Sample Grating DBR Laser with Integrated Electroabsorption Modulator,” published by UCSB (University of California, Santa Barbara) (PTL Vol. 11 No. 6, 1999, pp. 638-640) describes a device having a tunable sampled grating DBR laser capable of producing 51 different wavelengths at 0.8 nm intervals integrated with a modulator. As in the above device, the modulator uses a bulk active layer (band gap wavelength &Dgr;g=1.43 &mgr;m), and was ev
Flores Ruiz Delma R.
Leung Quyen
Mitsubishi Denki & Kabushiki Kaisha
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