Low noise injection laser structure

The present invention pertains to an injection laser which advantageously reduces main mode output power fluctuation. The laser comprises a laser cavity having an active material joined at one end to a passive waveguide. The passive waveguide has a length which is equal to or greater than the length of the active material and an index of refraction which is substantially equal to that of the active material to prevent multicavity interference. In preferred embodiments of the present invention, the passive material is fabricated from a semiconductor material having a bandgap which is larger in energy than the energy of a photon in the laser radiation.

BACKGROUND OF THE INVENTION 
This invention relates to the field of lasers and more specifically to the 
field of injection lasers. 
Present silica-based optical fibers can be fabricated to have a loss in the 
1.3-1.6 micron wavelength region which is an order of magnitude lower than 
the loss occurring at the 0.85 micron wavelength of present lightwave 
communications systems, e.g., 1/4 db/km versus 2-3 db/km. Furthermore, 
these fibers can be fabricated to have a transmission delay distortion in 
the 1.3-1.6 micron wavelength region which is two orders of magnitude 
lower than the transmission delay distortion at 0.85 microns, e.g., 1-2 
ps/km nm versus 100+ ps/km nm. Thus, the dispersion-limited transmission 
distance for high bit rate lightwave communications systems can be 
maximized by using a single-frequency, i.e., single-longitudinal-mode, 
injection laser generating output at the 1.55 micron wavelength where the 
fibers have minimum loss. For these reasons, present efforts in the 
development of lightwave communications systems are aimed at the 1.3-1.6 
micron wavelength region instead of at the wavelength region surrounding 
0.85 microns. 
InGaAsP injection lasers produce output in the desired 1.3-1.6 micron 
wavelength region. However, typical single-resonator InGaAsP injection 
lasers have a laser cavity length in the 250-300 micron range. This 
results in mode spacing between 6 and 9 Angstroms. Since the gain spectral 
width of InGaAsP injection lasers is approximately 250-300 Angstroms, 
there are more than 30 longitudinal modes under the gain spectral width of 
a 250 micron long laser. Thus, the gain difference between modes is small 
and mode discrimination between the main mode and side modes is poor in 
InGaAsP injection lasers. 
The injection laser, like all other oscillators, is perturbed by internal 
random processes which cause its output to fluctuate. One example of this, 
known as mode-partition-noise, is the fluctuation at turn-on in the 
relative intensities of various laser modes while the total output power 
of the laser remains fixed. Mode-partition-noise is a consequence of 
random fluctuations in the photon densities of the various modes at the 
moment threshold is reached. 
A great deal of time has been expended in an effort aimed at decreasing 
mode-partition-noise. These efforts generally involve the use of lasers 
with improved mode selectivity. However, even if the mode-partition-noise 
is substantially decreased, main mode power fluctuations in the laser 
output (even in a dc operated laser) impose an ultimate limit on the 
performance of any lightwave communications system. Experimental 
observations indicate that power dropouts frequently occur in the main 
spectral line of dc-biased injection lasers having nearly 
single-longitudinal-mode behavior. These dropouts can cause errors in a 
communications system when the ratio of the main mode to side mode power 
is less than 50:1. 
Main mode power fluctuations generally result from at least two 
significantly different physical mechanisms: (1) as a direct result of 
main mode photons interacting with the carrier density; i.e., a given 
photon may stimulate another photon or be absorbed and (2) as a result of 
gain fluctuations driven by carrier-density fluctuations, the latter 
resulting from photon-electron interactions of all modes--i.e., the main 
mode and all side modes--interacting with the carrier density. The side 
mode photon density interacts with the main mode photon density only 
through the carrier density; however, because the variance of the side 
mode photon density is far larger than that of the main mode (a 
consequence of the very small number of photons in the side mode), the 
side mode influence on the carrier density can be significant even though 
the mean value of the side mode power is low. The second mechanism can 
bring about mode dropouts when the mean value of the side mode power is 
large enough so that its fluctuating value can be larger than the 
fluctuating value of the main mode power, with enough joint probability to 
be significant. 
SUMMARY OF THE INVENTION 
Advantageously, embodiments of the present invention substantially reduce 
main mode power fluctuations in nearly single-longitudinal-mode injection 
lasers by raising the internal Q of the resulting structure over that of a 
conventional Fabry-Perot injection laser without adding photon-carrier 
density interactions. 
Specifically, an injection laser fabricated in accordance with the 
teachings of the present invention comprises a laser cavity having a gain 
or active material joined at one end to a passive waveguide. The passive 
waveguide has an index of refraction which is substantially equal to that 
of the active material in order to prevent multicavity interferences. The 
passive waveguide has a length which is greater than or equal to the 
length of the active material. 
Since the passive waveguide has essentially no loss or gain, it serves as a 
noise-free reservoir of photons. This increases the Q of the laser and 
reduces the main mode power fluctuations. 
In accordance with an illustrative embodiment of the present invention the 
passive material is a semiconductor material having a larger bandgap than 
the active semiconductor material.

DETAILED DESCRIPTION 
FIG. 1 shows, in block form, the general structure of injection lasers that 
are constructed in accordance with the present invention. Relatively 
lowloss passive waveguide 51 is disposed adjacent to active gain region 50 
in a laser cavity formed by mirrors 52 and 53. Passive waveguide 51 is an 
index-guided region in which an electromagnetic wave is carried between 
mirror 53 and the interface between regions 50 and 51, i.e., junction 54. 
The index of refraction of passive waveguide 51 should be substantially 
equal to that of active region 50 in order to reduce multicavity 
interference effects. 
Active region 50 may either be gain-guided or index-guided according to the 
particular embodiment, as is more fully described hereinbelow. The length 
of active region 50 is LA and the total length of the laser cavity is LT. 
Embodiments of the present invention are constructed so that LT is 
preferably greater than or equal to 2LA. 
The combination of active and passive waveguide regions constructed 
according to the present invention has a higher internal Q than the active 
region alone. Since this occurs without adding photon-carrier density 
interactions, main mode fluctuations in the laser output are significantly 
reduced. 
In an illustrative embodiment of the present invention, using 
semiconductors, passive waveguide region 51 is a semiconductor material 
having a larger bandgap than active region 50. Thus, photons generated in 
active region 50 do not interact in passive waveguide 51. Passive 
waveguide 51 is formed from an alloy which has a different composition 
from that of active region 50. Because the alloy providing a larger 
bandgap for passive waveguide 51 results in an index of refraction which 
is very close to that of active region 50, there is no appreciable 
reflection at junction 54. In other embodiments passive waveguide 51 could 
be a different material from that of active region 50. For example, active 
region 50 could be a III-V compound or an alloy of III-V materials and 
passive waveguide 51 could be SiN or glasses of various kinds. Mirrors 52 
and 53, at the ends of the laser cavity, can either be simple cleaved 
facets or more complex structures having wavelength selectivity, as more 
fully described in my co-pending patent application Ser. No. 655,257, 
entitled "Single Mode Injection Laser Structure", which has been filed 
simultaneously herewith and which is incorporated by reference herein. 
FIG. 2 shows an "all-semiconductor" embodiment of the present invention and 
more particularly shows a slice through the structure taken substantially 
perpendicular to the top and bottom surfaces and along its longitudinal 
axis, i.e., the latter being the direction of laser beam propagation in 
the laser cavity. Layers 1, 2, 3, 4, 5 and 6 are epitaxially grown on InP 
substrate 60 by liquid phase epitaxy (LPE) or other techniques well known 
to those skilled in the art. For clarity, FIG. 2 only shows those layers 
directly involved in lightwave generation and guidance. It is well known 
in the art that other layers may be situated on top of layer 1 both to 
facilitate electrical contact and also to confine the injection current, 
which provides carriers to active region 50, to a narrow stripe in order 
to provide transverse guidance of the laser beam. Layer 2 is an active 
InGaAsP semiconductor alloy layer having a thickness in the range of 0.1 
to 0.2 microns. Layers 1 and 3 are light guidance and carrier-confinement 
layers which have a larger bandgap than layer 2 and may illustratively be 
InGaAsP alloys or InP. At junction 54 the compositions of the layers 
change in order to form passive waveguide 51. Layers 4, 5 and 6 comprise 
passive waveguide 51. Layer 5 has a larger index of refraction than layers 
4 and 6. Illustratively, layer 2 can have a bandgap with an emission peak 
at 1.5 microns and layer 5 can have a bandgap with an emission peak at 1.3 
microns. Thus, photons generated in layer 2 will not have enough energy to 
be absorbed in layer 5. 
Because of the above-described guidance properties of the structure, the 
peak photon density in the structure occurs in or near layers 2 and 5, 
with decreasing density occurring in the adjacent cladding layers. This is 
illustrated by curve 7 in FIG. 2. 
A method of fabricating the embodiment shown in FIG. 2 will now be 
described. FIG. 3 shows InP substrate 60. Epitaxial layers 4, 5 and 6 are 
grown over the entire surface of substrate 60. Then, as shown in FIG. 4, 
after photolithography and well known etching techniques are applied to 
the structure shown in FIG. 3, a portion of these layers is removed to 
expose InP substrate 60 in the region which is to become the active region 
of the final laser structure. 
FIG. 5 shows the structure resulting after layers 3, 2 and 1 are 
epitaxially grown on the structure shown in FIG. 4. Metal layer 12 has 
been deposited in areas where current flow is desired. Metal layer 13 acts 
both as an electrode for current injection along with layer 12 and as a 
heat sink for the structure. Note that junction 54, between active region 
50 and passive waveguide 51, need not be perpendicular to the direction of 
light propagation because the change in the index of refraction across the 
junction is small and therefore causes little reflection or deflection of 
the lightwaves in the laser cavity. 
In order to provide single moded laser output, the transverse structure of 
the laser must be designed to maintain a single transverse mode. Many 
methods are known in the art for fabricating structures having a single 
transverse mode, e.g., buried heterostructures, dual-channeled buried 
heterostructures, ridge or rib waveguides and stripe contact defined 
structures. Also, passive waveguide 51 need not have a single transverse 
mode as long as active region 50 does. 
Furthermore, any one of many standard laser structures may be used to guide 
the laser beam in the transverse plane perpendicular to the direction of 
propagation of the laser beam. For example, a version of a ridge waveguide 
is shown in FIG. 6 which depicts a slice taken through the active region 
of the structure in a plane perpendicular to the direction of propagation 
of the laser beam. Specifically, layer 1 has ridge 19 of width W and is 
partially covered by SiN insulating layer 15. In order to maintain a 
single transverse mode, in an illustrative embodiment of the invention, W 
is approximately 5 microns. Ridge 19 can be fabricated to extend across 
the entire length, LT, of the laser, i.e., covering both active region 50 
and passive waveguide 51. 
Another structure for providing transverse guidance in active region 50 is 
a dual-channel buried heterostructure. This is shown in FIG. 9. After 
growing layers 3, 2 and 1, a pair of channels 30 and 31 are etched to 
leave a mesa having width WA. Then, a lower index layer 21, often InP, 
which serves as a confining region, may be grown in the groove. Channels 
30 and 31 may extend the entire length LT of the laser structure. Layers 
12 and 13 are metallic contact layers; layer 60 is an n-InP substrate; 
layer 3 is an n-InGaAsP layer; layer 2 is a p-InGaAsP layer; layer 1 is a 
p-InP layer; layer 21 is a p-InP layer and layer 22 is a SiO.sub.2 layer. 
An example of a dual-channel buried heterostructure is shown in the 
Technical Digest OFC '84 0.S.A. meeting on Optical Fiber Communication 
Jan. 23-25, 1984, paper MF2, FIG. 1, page 15. 
A further well-known structure, not shown in the figures, for providing 
transverse guidance in active region 50 is a stripe-contact current 
injection structure. 
The structures providing transverse guidance in active region 50 may 
optionally be fabricated by bombarding the areas adjacent to active region 
50 to make them highly resistive instead of using insulating layers as 
previously described. Furthermore, injection current top contact 12 can be 
extended over passive waveguide 51 to further reduce photon absorption in 
the waveguide. However, when the gain peak for passive waveguide 51 is 
chosen to be 1.3 microns, there will only be a small improvement in 
performance due to the extended top contact when active region 50 emits at 
1.5 microns. Note that the principal contribution towards reduction of the 
photoncarrier interaction in passive waveguide 51 occurs whenever the 
bandgap of passive waveguide 51 is larger than that of action region 50. 
Furthermore, the reduced photon absorption in the passive waveguide 
advantageously provides the significant reduction in main mode 
fluctuations in the laser output. 
A structure providing transverse guidance in passive waveguide 51, 
independent of active region 50, is fabricated as shown in FIGS. 7 and 8. 
A groove having width WP and depth d is etched in InP substrate 60. 
Illustratively, WP is approximately 10 microns and d is between 3 and 5 
microns. Layers 6, 5 and 4, which together form passive waveguide 51, are 
epitaxially grown on the structure shown in FIG. 7 to give the structure 
shown in FIG. 8. Illustratively, layer 6 is grown with a thickness of 
approximately 1 micron and layer 5 with a thickness of approximately 2 
microns and layer 4 with a thickness of approximately 1 micron. The bulge 
caused by the groove in substrate 60 will cause layer 5 to confine 
lightwave energy in the transverse direction. Then layer 3, situated in 
the active region, is grown sufficiently thick to isolate active layer 2 
from the effects of the groove in substrate 60 and to ensure that active 
layer 2 will mate with passive layer 5. The structure providing transverse 
guidance for the active region can be chosen to be any of the well-known 
structures, e.g., the structure shown in FIG. 6. 
Alternative embodiments of the present invention are fabricated by 
butt-joining a conventional injection laser to a passive waveguide 
structure. For example, FIG. 10 shows passive waveguide structure 51 lined 
up with active region 50 on heat sink 13. Active region 50 is disposed 
directly on heat sink 13. 
FIG. 11 shows a cross section of an illustrative passive waveguide 
structure 51 fabricated in an SiO.sub.2 glass system. The plane of FIG. 11 
is perpendicular to the direction of laser beam propagation in the 
structure. Waveguide layer 71 is germanium-doped SiO.sub.2. Waveguide 
layer 71 is fabricated by growing a germanium-doped SiO.sub.2 layer over 
the entire face of SiO.sub.2 substrate 72. Then the germanium-doped 
SiO.sub.2 layer is etched to form waveguide layer 71 having a 5 micron 
square cross section. Waveguide layer 71 is then covered with 2 micron 
thick SiO.sub.2 layer 70. 
FIG. 12 shows a slice through the passive waveguide structure 51, shown in 
FIG. 11, taken along its longitudinal axis, i.e., the direction of laser 
beam propagation. End 41 of passive waveguide 51, opposite the active 
region, could have a simple mirror coating, either metallic or dielectric, 
to form a broad-band reflector or a multi-layer dielectric coating in the 
form of a frequency selective reflector. Alternatively, passive waveguide 
51 could contain a grooved grating, as shown by grating 42 in FIG. 12, to 
provide a frequency-selective reflector. 
In a further embodiment of the present invention, passive waveguide 51 is a 
LiNbO.sub.3 waveguide formed by the diffusion of titanium. Of course, one 
could use any alternative technique to increase the index of refraction 
and form a waveguide in the LiNbO.sub.3. A cross section of this structure 
is shown in FIG. 13. Waveguide 101 is diffused into LiNbO.sub.3 substrate 
100. A grating can also be used in this embodiment at a position as shown 
by grating 42 in FIG. 12. The grating grooves may be air or a low index 
dielectric such as epoxy, SiO.sub.2 or SiN. In this embodiment, the 
electro-optic effect may be used to alter the frequency-selection property 
of the grating when metallic electrodes 15 and 16 are deposited on 
substrate 100. In an illustrative embodiment metallic electrodes 15 and 16 
are approximately 3 to 10 microns wide. 
FIG. 14 shows an embodiment of the present invention which utilizes the 
electro-optic effect to form the transmitting end of a communications 
system. Active region 50 is butt-joined to passive waveguide 51. Passive 
waveguide 51 is fabricated as described hereinabove, and as specifically 
shown in FIG. 13. The laser output beam is generated in the direction 
shown by arrow 120. Current injection source 200 is applied across 
electrodes 150 and 151 to metallic contacts 112 and 113 of active region 
50 to provide lasing action thereof. Communications signal information is 
provided from signal source 117, through amplifier 116, to modulator 115. 
Modulator 115 provides a voltage, carrying the signal information, to 
metallic contacts 110 and 113 and thereby to passive waveguide 51. The 
voltage applied across metallic contacts 110 and 113 varies the index in 
grating 42. This changes the frequency of peak reflectance of grating 42 
and hence the frequency of the laser output. The structure shown in FIG. 
14 can be used as an analog frequency modulated (FM) or as a digital 
frequency-shift keyed (FSK) transmitter. 
FIG. 15 illustrates a method for fabricating injection lasers, according to 
the present invention, in a batch processing procedure analogous to batch 
processing procedures used in making injection lasers of various types. In 
particular, this figure shows the broad face of a wafer which is used to 
make many lasers simultaneously. Dotted lines 230-232, 240 and 241 
represent cleavage planes after the wafer is processed. Illustratively, 
the length, LT, of the laser is 500 microns and the length, LA, of the 
active region is 100 microns. Although, for ease of handling, a laser chip 
is chosen to be 250 microns wide, the transverse width of the active laser 
layer comprises only a few microns of the full 250 micron width. Layers 4, 
5 and 6, which together form passive waveguide 51, as described 
hereinabove, are grown over the entire wafer surface. Then layers 4, 5 and 
6 are etched away in strips 200-202 that are to become the active regions 
of the laser. Next layers 1, 2 and 3 are grown to form the active regions. 
The particular structure for providing transverse guidance is formed on 
the wafer using lithographic techniques in the appropriate regions as has 
been described hereinabove. Finally, using known scribe-and-cleave 
procedures, the wafer is then broken into individual lasers. Typically, a 
wafer fabricated in this manner yields hundreds of lasers. 
I have studied laser output parameters, which are described in my 
above-mentioned co-pending patent application, such as the main mode time 
constant and the main mode to side mode ratio for embodiments of the 
present invention described hereinabove. I have discovered that use of a 
passive waveguide region increases the main mode time constant and reduces 
the main mode to side-mode ratio. For example, a laser constructed in 
accordance with the principles of my invention and having an active 
length, LA, of 70 microns and a total cavity length, LT, of 250 microns, 
has a main mode time constant of 0.8 nanoseconds and a main mode to side 
mode ratio of 3.3 when running at about 3/4 mW in the main mode. This is 
to be compared with a laser having LA=LT=250 microns which has a main mode 
time constant of 1.6 nanoseconds and a main mode to side mode ratio equal 
to 10.8 when running at about 3/4 mW in the main mode. Both the time 
constant and the modal ratio change as the loss difference between the 
main and the side modes becomes smaller. This occurs when the low-loss 
passive waveguide section is present because the absolute level of loss 
and hence the absolute level of gain in the laser is reduced. With a lower 
absolute level of gain, the difference in the gain between the main mode 
and side mode wavelengths becomes smaller by reason of the shape of the 
gain curve. It is this difference which gives the equilibrium modal power 
ratio when there is no wavelength selective loss in the laser structure. 
Thus, it is advantageous to fabricate embodiments of the present invention 
to include means for preferentially suppressing laser action in modes 
other than the main longitudinal mode of the laser cavity. To accomplish 
this, these means should advantageously provide small amplitude loss as is 
more fully described in detail in my above-referenced copending patent 
application. 
It should also be clear to those skilled in the art that further 
embodiments of the present invention may be made by those skilled in the 
art without departing from the teachings of the present invention.