Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-03-09
2002-08-20
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S092000, C372S099000
Reexamination Certificate
active
06438150
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to semiconductor lasers. In particular, the invention relates to mirrors defining the optical cavity of a semiconductor edge-emitting laser.
BACKGROUND ART
Semiconductor lasers are being increasingly used as the light source for telecommunication systems utilizing optical fibers as the transmission medium. The laser is positioned to irradiate one end of the fiber, and an electrical signal either directly modulates the laser or controls a modulator through which the laser output passes before it enters the fiber. Optical fiber has the capability of transmitting the optical signal for hundreds of kilometers without regeneration or amplification. An optical detector receives the optical output at the other end of the fiber, and the electrical signal output by the detector corresponds to the modulation signal used at the input end of the fiber.
Semiconductor lasers have several advantages. They operate at relatively high efficiency and are relatively rugged. A waveguide laser produces an optical output that is easily coupled into a single-mode optical fiber. Semiconductor lasers can be fabricated to emit in the two portions of the optical spectrum most favored for the transmission over silica fiber, the 1310 nm band and the 1550 nm band. These two bands are achieved by using semiconductor materials from the AlInGaAs and InGaAsP families grown on InP substrates.
The precise lasing frequency is determined by a number of factors. A typical edge-emitting semiconductor laser
10
is illustrated schematically in
FIG. 1. A
chip
11
of the semiconductor composition capable of emitting in the desired optical regime includes a p-type layer
12
and an n-type layer
14
separated by an intrinsic active layer
16
, which may include one or more quantum wells. The substrate under the lower layer
16
is not explicitly shown. Optical waveguiding means are included to confine light in both the vertical direction and in the horizontal direction perpendicular to the plane of the illustration. Viewed from above, the laser and associated electrode and waveguiding structure appear as a stripe extending between the chip facets. The vertical confinement structure is closely associated with the intrinsic layer
16
and preferably is implemented as a distributed Bragg grating or reflector (DBR)
17
having an optical period P generally corresponding to the desired emission at wavelength &lgr;
0
. Two mirrors
18
,
20
are formed on the chip facets and define the ends of an optical cavity of length L. The chip facets are usually fabricated by cleaving the crystalline substrate along a cleavage plane so that the facets are mirror smooth and perpendicular. As such, the facets themselves may be sufficient to acts as mirrors, at least on the output side, but additional metal coatings may be deposited. One mirror
18
is made as reflective as possible while the other, output mirror has a small but finite transmissivity to allow output of laser light
22
at wavelength &lgr;
0
. Electrodes
24
,
26
are contacted to the p-type and n-type layers
12
,
14
and a power supply
28
forward biases the p-n junction. For telecommunications, the power supply
28
can be electrically modulated with a data signal, or alternatively a separate optical modulator can receive CW laser light
22
and modulate it according to the data signal.
In an edge-emitting laser, the cavity length L is typically of the order of millimeters or greater and thus much longer than the lasing wavelength &lgr;
0
so that a large number of potential axial waveguide modes
30
exist, as illustrated in the power/gain spectral diagram of FIG.
2
. Lasing occurs for a particular mode
30
when, at the wavelength of the mode, the round-trip gain through the optical cavity exceeds the round-trip loss. If this condition is satisfied for more than one mode
30
, usually the mode with the largest excess gain draws power away from the less favored modes. An optical gain spectrum
32
in the amplifying portion of the laser is determined primarily by the precise composition of the ternary or higher-order compound semiconductor used to produce the desired optical emission wavelength, and in these materials the gain spectrum
32
is relatively wide. The gain is often expressed in terms of an absorption coefficient&agr; per unit length with the round-trip gain being equal to 2&agr;L. The gain bandwidth is usually greater than 10 nm and often much greater. The loss spectrum is composed of at least two components. The mirror loss
34
is small and essentially flat if the mirrors are metallic or rely on the semiconductor/air interface. On the other hand, the Bragg transmission spectrum
36
is relatively sharply peaked with a peak position at P determined by the Bragg grating
16
. Transmission bandwidths of 0.3 nm are typical. Any additional loss in the waveguide is included in the absorption coefficient a, which in a laser has a sign that denotes power growth. Typically the internal loss is relatively flat over the wavelength bands of interest. The mode
30
that experiences the largest value of gain less loss is the one that lases. The resultant lasing wavelength peak is very narrow.
The narrow laser bandwidth and the control of the emission frequency by the setting of the physical pitch of the feedback grating allows multiple lasers emitting at slightly different wavelengths to be integrated on a single semiconductor substrate. Such a multi-wavelength laser is particularly advantageous for a wavelength-division multiplexing (WDM) telecommunication system in which a single optical fiber carries multiple optical signals which have slightly different wavelengths and which are separately modulated by respective data signals. WDM can multiply the transmission capacity of a fiber by the number of wavelength, which in current systems may be from 4 to 40 or even higher. For the larger number of WDM channels, the WDM wavelengths are separated by approximately 1 nm. Zah describes an example of such an integrated multi-wavelength laser in U.S. Pat. 5,612,968, in which he references fabricational techniques described by Zah et al. in “Multiwavelength light source with integrated DFB laser array and star coupler for WDM lightwave communications,”
International Journal of High Speed electronics and Systems
, vol. 5, no. 1, 1994, and in “Monolithic integrated multiwavelength laser arrays for WDM lightwave system, ”
Optoelectronics—Devices and Technologies
, vol. 9, no. 2, 1994.
However, a useful WDM network includes some amount of optical switching so that an optical fiber may be transmitting and a receiver array may be receiving optical signals originating from multiple transmitting sites. Any overlap of nominally different wavelengths at any point in the network must be avoided so that the wavelengths of the different emitters must be tightly registered. But, supposedly identical laser arrays may be transmitting at somewhat different wavelengths. DBR lasers rely for their emission control on the e-beam writing of the grating, which must be controlled to 0.03 nm for a 240 nm pitch. Such fine lithography imposes a major problem in fabrication and reduces yield below an economically useful level. Furthermore, the semiconductor composition of the waveguide and the Bragg grating introduces a large temperature dependence, with a typical wavelength drift with temperature of 0.1 nm/° C. As a result, the temperature must be stabilized at all times and at all transmitting sites. Such control is possible, but expensive and operationally difficult in a distributed commercial environment.
An alternate laser structure is the vertical-cavity surface-emitting laser
40
schematically illustrated in
FIG. 3
following the original disclosure by Jewell et al. in U.S. Pat. No. 4,949,350. A tall semiconductor stack having a diameter of about 1 &mgr;m is etched from layers epitaxially grown on an n-type substrate
42
. From the bottom up, the stack includes an n-type semiconductor lower interface mirror
44
, a n-type lower spacer
Giordano Joseph
Ip Paul
Jackson Cornelius H
Schoneman William A.
Telecordia Technologies, Inc.
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