Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-02-20
2002-09-03
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S096000
Reexamination Certificate
active
06445724
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to integrated semiconductor lasers.
2. Description of the Related Art
High-power, single-mode laser diode sources are employed in a wide variety of applications, such as medical sensing devices and high-speed, optical telecommunication network components. For example, Raman amplifier components for optical networks typically require pump powers of 1 Watt and above.
A variety of laser diode structures exist in the prior art, including edge-emitting stripe, tapered stripe, broad stripe, grating-surface-emitting, master oscillator power amplifier, grating-stabilized broad stripe, surface-emitting distributed feedback, and antiguided array structures. Important features of any given laser diode structure include: high output power, single near-Gaussian spatial mode, short- and long-term lasing mode stability over time, long-term reliability, compact packaging, manufacturability, and ease of coupling to an external waveguide such as an optical fiber. However, no single laser diode structure of the prior art adequately possesses all of these features.
Obtaining relatively high output power with single-mode emission by a laser diode is difficult. Structures having greater output power tend to produce either (a) multi-mode emission or (b) low long-term stability and reliability. For example, linear stripe lasers, typically including either buried heterostructure or ridge waveguide lateral index guiding, have attained emitted powers of up to 700 mW in reports dating to 1992. However, these devices typically suffer from a high optical power density of 10-20 MW/cm
2
at the diode's output facet, which tends to degrade performance.
Some high-power, semiconductor laser diodes utilize a device structure with a light source (termed “master oscillator”) and other components, all integrated onto a common semiconductor substrate. For example, a master oscillator power amplifier (MOPA) laser includes an oscillator and a high-gain optical amplifier that are monolithically integrated.
FIG. 1
shows an exemplary structure for a tapered-amplifier MOPA laser
100
of the prior art. MOPA laser
100
comprises single-mode laser diode oscillator
101
, optional pre-amplifier
102
, and optical power amplifier
103
that are formed on a common substrate
110
. Single-mode laser diode oscillator
101
includes active region
120
and a gain region
121
that, along with the adjacent layers above and below them, form a transverse waveguide (i.e., a waveguide with direction parallel to the plane of the active region
120
). However formed, MOPA laser
100
includes a light emitting region (e.g., active region
120
) near a p-n-junction. Pump current applied to electrodes
111
and
112
greater than the lasing threshold current causes lasing (i.e., generation of amplified lightwaves) in active region
120
and gain region
121
. MOPA laser
100
includes facets AR
1
and AR
2
that have anti-reflective coatings to minimize residual reflection of lightwaves within MOPA laser
100
.
If a distributed Bragg reflector (DBR) laser is employed for the single-mode laser diode oscillator
101
, gain region
121
is bounded by first- and second-order gratings
122
and
123
. Gain region
121
may be formed by a lateral real refractive index waveguide material structure. Optional pre-amplifier
102
may be employed to optimize signal level and adjust beam shape of the lightwave produced by single-mode laser diode oscillator
101
that is subsequently applied to the following optical power amplifier
103
. Pre-amplifier
102
typically includes a single-mode waveguide region that may be tapered. The single-mode waveguide region is formed from layers
115
adjacent to the active region
120
, may be electrically isolated from single-mode laser diode oscillator
101
, and is energized with pump current applied to electrodes
111
and
113
.
Optical power amplifier
103
is coupled to pre-amplifier
102
. Optical power amplifier
103
generally includes a transverse waveguide region about active region
120
. In the transverse waveguide region, active region
120
is sandwiched between adjacent higher-bandgap, lower-refractive-index layers. Optical power amplifier
103
is electrically isolated from optical preamplifier
102
and is energized with pump current applied to electrodes
111
and
114
.
A drawback of the MOPA laser structure of
FIG. 1
is that the amplification of the beam emitted from the oscillator occurs when the beam passes through a relatively high-gain amplifier (e.g., optical power amplifier
103
). The high-gain amplifier may have a typical single-pass gain in the neighborhood of 15 to 30 dB. In contrast, in solid-state lasing media supporting large, high-power optical modes (e.g., Nd:YAG rod external cavity lasers), the single-pass gain is relatively low (e.g., on the order of 0.1 dB per pass).
In a high-gain amplifier, a semiconductor region that supports multiple, propagating optical modes exhibits non-linearities associated with the optical amplification process. The non-linearities of the amplification process result from saturation of gain and cause beam distortions, including both those known as “self-focusing” which is related to the phenomenon known as “filamentation” that tends to distort the wavefront of the propagating radiation in an uncontrolled fashion. Self-focusing and filamentation arise in large part and are related to the Kramers-Kronig relationship between imaginary and real parts of the refractive index in the amplifier regions of the semiconductor. Self-focusing exists in many semiconductor laser structures, and is particularly pronounced in those structures that support more than one waveguide mode under pumped-cavity conditions. Unstable resonator lasers and surface-emitting, distributed-feedback lasers similarly exhibit distortion from self-focusing.
FIG. 2
a
illustrates the broad-area gain section optical intensity profile for a high-gain, high non-linearity gain section affected by self-focusing and filamentation. As shown in
FIG. 2
a
, a plot of optical intensity versus wavelength position indicates that the wavefront exhibits an irregular shape about the center position 75 &mgr;m, and is thus long-term unstable.
FIG. 2
b
illustrates the broad-area gain section optical intensity profile for a low-gain, low non-linearity gain section not affected by self-focusing. As shown in
FIG. 2
b
, a plot of optical intensity versus wavelength position indicates that the wavefront exhibits a smooth roll-off shape about the center position 75 &mgr;m, and is thus long-term stable.
Most laser diodes are edge emitting and are so called because the light beam emits from the cleaved edge of the processed laser diode semiconductor chip (e.g., through facet AR
2
of FIG.
1
). These types of laser diodes are commonly termed Fabry-Perot (FP) laser diodes since the laser diode cavity is similar to that of a conventional gas or solid state laser, but the cavity is formed inside the semiconductor laser diode chip itself. Mirrors may be formed by the cleaved edges of the chip, or one or both of the cleaved edges may be anti-reflection (AR) coated and external mirrors are added.
A vertical-cavity, surface-emitting laser (VCSEL), on the other hand, emits its beam from the top surface, and potentially the bottom surface, of the semiconductor chip. The cavity comprises a hundred or more layers of mirrors and active regions formed epitaxially on a bulk (inactive) substrate.
VCSEL devices exhibit the characteristics of low threshold current and low power when compared to other semiconductor laser diode devices that emit single-mode radiation. Lower lasing threshold and drive current results in lower electrical power requirements, potentially faster modulation, simpler drive circuitry, and reduced radio frequency interference (RFI) emission. VCSEL devices are also more tolerant of fluctuations in power supply drive. Directly controlling current for continuous operation is generally sufficient without requiring
Burke William J.
Leung Quyen
Sarnoff Corporation
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