Single mode injection laser structure

The present invention pertains to an injection laser which advantageously suppresses mode-partition-noise. The laser comprises a laser cavity with a gain or active material and means for providing a small amplitude wavelength selective loss. In one embodiment of the present invention the means for providing a small amplitude loss is a wavelength selective reflector.

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 
wavelength region between 1.3 and 1.6 microns 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 lasing threshold is reached. If the main mode photon density is not 
the largest at that instant, another mode will build up first. In a 
communication system employing such a laser, mode-partition-noise can 
combine with dispersion in the transmission medium to produce random 
distortion of the received signal, thereby degrading system performance. 
Since the distribution of mode partition fluctuations is exponential 
rather than Gaussian, fluctuations large enough to cause bit error to 
occur result in intolerably high rates. For example, the main mode 
intensity drop-out due to the mode partition fluctuations can be related 
to the error rate in the following way. If the laser is modulated with a 
bit rate equal to the main mode intensity drop-out duration (approximately 
1 nanosecond), about half of the drop-out events will cause error in a 
system with high dispersion. Thus, a drop-out rate of 1 per second will 
cause an error rate of 10-.sup.9 at 1 Gbit/s. Furthermore, because mode 
partition fluctuations are a low-frequency phenomena, lasting a few 
nanoseconds, they cannot be reduced by averaging in high-bit-rate 
(1000-MB/s) systems. 
Mode-partition-noise is inherent in any laser with more than one resonant 
mode, e.g., the typical InGaAsP injection laser described above, and can 
be expected to some degree in all conventional Fabry-Perot lasers. 
However, the mode-partition-noise impairment does not exist when an ideal 
single-longitudinal-mode (SLM) laser is used, because all the power 
produced by the SLM laser exists in only one mode. However, in practice, 
there are always unwanted vestigial side modes in any laser and SLM lasers 
can only be approximate. 
Several structures, aimed at providing single-frequency operation, have 
been proposed and demonstrated. These include distributed feedback (DFB) 
and distributed Bragg reflector (DBR) lasers, lasers with an external 
cavity, injection-locking lasers, short-cavity lasers and coupled-cavity 
lasers. Unfortunately, these laser structures are either difficult to 
fabricate, difficult to operate or require external elements which are 
sensitive to mechanical vibration. 
SUMMARY OF THE INVENTION 
Advantageously, the present invention substantially eliminates 
mode-partition-noise in injection lasers by preferentially suppressing 
laser action in modes other than the main longitudinal mode of the laser 
cavity. 
An injection laser fabricated in accordance with the teachings of the 
present invention comprises a laser cavity with a gain or active material 
and means for providing small amplitude wavelength selective loss. In one 
embodiment of the present invention, the means for providing small 
amplitude loss is a wavelength selective reflector having a value of 
reflectivity at the wavelength of the main mode that is less than 10% 
higher than its reflectivity values at the wavelengths of modes other than 
the main mode. 
In further embodiments of the present invention, the loss is provided by 
selectively coupling energy out of the active material. In a first 
structure, the selective coupling is provided by lightwave coupling 
between the active material and an active waveguide having lossy 
terminations; in a second structure, the selective coupling is provided by 
lightwave coupling between a portion of the active material and a portion 
of an active waveguide, both the active material and the active waveguide 
having a lossy termination; and in a third structure, the selective 
coupling is provided by lightwave coupling between a waveguide comprising 
the active material, a passive section and a lossy termination and a 
passive waveguide having a lossy termination.

DETAILED DESCRIPTION 
Embodiments of the present invention comprise injection lasers having means 
for providing wavelength selective loss. The loss is greater at the 
wavelength of the side modes as compared to that at the wavelength of the 
main mode. More importantly, I have discovered, contrary to the present 
understanding in the art, that very small loss differences can have a 
dramatic effect in reducing side mode levels and thereby reducing 
fluctuations which cause mode-partition-noise. 
Previous workers in this field have not been aware of the surprisingly 
large effect such small loss-differentials can have. For example, the 
equilibrium side mode level in a 500 micron long simple Fabry-Perot laser 
is about 8.5 to 1 at the l.6 mW power level when both facet reflectivities 
are 0.3. This side mode level is raised to 139 to 1 by increasing the 
reflectivity at one end of the laser cavity to 0.31 at the main mode 
wavelength. Table 1 shows the results of further calculations, considering 
17 total modes, for a structure having a cavity length of 500 microns and 
being pumped with a current density, J, equal to 1.03 ka/cm.sup.2. 
TABLE 1 
______________________________________ 
Main Mode Main Mode 
Main Mode 
Side Mode to Power Output 
Reflectivity 
Reflectivity 
Side Mode (mW) 
______________________________________ 
.30 .30 8.5 1.418 
.31 .30 139 1.845 
.32 .30 270 1.904 
.35 .30 658 1.987 
.40 .30 1286 2.124 
.90 .30 6621 2.783 
______________________________________ 
As the table readily shows, main mode output increases sharply for 
reflectivity differences of less than 10% between the main mode and the 
side modes. In particular, a small increase in main mode reflectivity from 
0.30 to 0.31 increases main mode output from 1/418 mW to 1.845 mW. This 
represents a transfer of laser output from side modes to the main mode. 
This transfer has further advantages. First, embodiments of the present 
invention reduce laser output fluctuations when operating in a continuous 
mode, i.e., constant injection current. This reduces mode-partition-noise 
in lightwave communications systems using fibers having wavelength 
dispersion. 
Second, embodiments of the present invention also improve the laser 
transient response to the leading edge of pulses in a pulse code 
modulation (PCM) system. Specifically, as the current density, J, in an 
injection laser is increased to its value at threshold, a ringing occurs 
in the output which results from an oscillatory interaction between photon 
density and carrier density. The mean of the oscillatory output at turn-on 
grows with an exponential-like waveform. I have discovered that a small 
wavelength selective loss dramatically reduces the turn-on-time, i.e., the 
time for the laser output to reach steady state in the main mode. For 
example, in the case of a 500 micron laser, the turn-on time can be 
reduced by a factor of ten, i.e., from 10 nanoseconds to less than one 
nanosecond, for a mere 2% change in reflectance. As a result, a lightwave 
communications system employing a laser embodying the principles of the 
present invention can advantageously permit the speed of the transmitted 
pulse stream to be increased by a factor of ten. 
Specifically, embodiments of the present invention comprise an active 
material disposed in a laser cavity and means for providing wavelength 
selective loss. The loss element can be a separate structure that is 
appended to the active material. Furthermore, one laser can provide a 
multiplicity of output wavelengths while using the same active material. 
In this instance, the response of the wavelength selective loss element 
must be changed in order to suppress the unwanted wavelengths--either 
electrically, through an effect such as the electro-optical effect, 
acoustically, through the acousto-optic effect or physically, through 
appending a different element to the device. 
FIG. 1 shows an embodiment of the present invention where reflection 
grating 10 provides small amplitude wavelength selective loss by means of 
wavelength selective reflections. Active laser section 50 is known in the 
art as a ridge-type laser structure. As shown, it has simple cleaved ends 
and smooth internal layers. The laser output from the structure is 
transmitted along the direction shown by arrow 20. Layer 5 is a heat sink. 
Contacts 6 and 7 serve as electrical contacts for applying injection 
current to active laser section 50. Gap 9 between the end of active laser 
section 50 and the beginning of reflector 51 can be non-existent, i.e., 
the two structures may be butted together. Reflector 51, containing 
reflection grating 10, can be fabricated separately from active laser 
section 50. Reflector 51 may be fabricated from a number of different 
materials. For example, reflector 51 can be a LiNb0.sub.3 layer. When 
reflector 51 is fabricated from LiNbo.sub.3, metallic contacts may be 
deposited thereon in order to apply a voltage therebetween and create an 
electric field in the reflector. This field, in turn, permits the index of 
refraction to be varied by means of the electro-optic effect. 
Alternatively, reflector 51 can be a fused silica layer. Such a layer has 
a low temperature coefficient and thereby provides enhanced stability of 
the structure. Since the reflection vs. wavelength pattern of the grating 
depends on the index of refraction of the material, stabilization of the 
index of refraction stabilizes the oscillation frequency of the laser. 
Although active laser section 50 is shown as a ridge-type laser, any other 
laser type could be used. The active laser section merely supplies gain 
over the total line width of the semiconductor used. The frequency of 
operation, within that line width, is then selected, varied and stabilized 
in accordance with the properties of reflector 51. 
Laser output face 12 may be a simple cleaved facet. Alternatively, it may 
be coated to decrease its reflection coefficient. A moderate decrease or 
increase in reflectivity could be used to optimize the laser output power. 
An accurate antireflection coating could alternatively be used to produce 
a superluminescent diode wherein the spectral width and peak transmission 
wavelength are both determined by the characteristics of reflector 51. 
Another embodiment of the present invention, not shown in the drawings, is 
obtained when a series of layers of material having alternating value of 
index of refraction is substituted for reflector 51 in the embodiment 
shown in FIG. 1. The values of the indexes of refraction are chosen to 
give a reflection peak at one wavelength. The fabrication of multilayer 
coatings for forming laser mirrors is well known in the art. Usually, in 
the art, a very large ratio of peak to side level reflectance is required, 
but embodiments of the present invention utilize my discovery that 
substantial advantage may be obtained from small differences in 
reflectivity. Although the multilayer reflector needs as sharp a 
reflectance peak as can be conveniently produced to suppress side modes in 
a long laser, it need not have large peak-to-side mode reflectance values. 
Further embodiments of the present invention are obtained by utilizing 
alternative structures to obtain wavelength selective loss. As discussed 
hereinabove, the selectivity need only to be on the order of a few percent 
at the wavelengths corresponding to the longitudinal mode spacing to 
provide advantageous results. FIG. 2 shows the general structure of 
various embodiments of the present invention using a novel means to 
provide wavelength selective loss. For sake of clarity, only those parts 
of the structure are shown which enable one skilled in the art to 
understand the operation of the embodiment. In particular, FIG. 2 shows a 
view looking down at the top of the injection laser structure. Active 
laser waveguide section 105 is disposed between broadband mirrors 100 and 
101. Mirrors 100 and 101 form a laser cavity. As indicated by arrows 108, 
active waveguide section 105 and auxiliary light guiding section 115 are 
situated so that light is continuously coupled from active waveguide 
section 105 to auxiliary light guiding section 115. The coupling between 
these sections causes notable transfer of power therebetween. If the phase 
constants of the two sections differ, the power transfer is wavelength 
selective. Furthermore, by making the gain of auxiliary light guiding 
section 115 greater than the gain of active laser waveguide section 105, 
the effect can be enhanced. This is shown symbolically in FIG. 3. Here, 
curve 120 represents the field in section 105 and curve 121 represents the 
field in section 115 when there is equal gain between the two sections. 
Dotted curve 122 represents the field in section 105 when there is larger 
gain in section 115. With larger gain in auxiliary light guiding section 
115 than in active laser waveguide section 105, waves coupled over to 
auxiliary light guiding section 115 are amplified faster than waves in 
active laser waveguide section 105. Thus, waves subsequently coupled out 
of auxiliary light guiding section 115 and back into active laser 
waveguide section 105, out of phase with waves in active laser waveguide 
section 105, do a more effective job of canceling out lasing section waves 
than if the gain in auxiliary light guiding section 115 was lower. 
A larger gain in auxiliary light guiding section 115 can advantageously be 
obtained by pumping this section with a larger current density than that 
used to pump active laser waveguide section 105. The dimensions of the two 
waveguides, i.e., sections 105 and 115, are chosen to obtain the desired 
phase constant difference between them. In FIG. 2, ends 109 and 110 of 
auxiliary light guide section 115 are terminated in lossy absorbing 
regions. Hence, the photon density does not build up in auxiliary light 
guide section 115 to the same extent it does in active laser waveguide 
section 105, even for the same current density. Thus, high gain in 
auxiliary light guiding section 115 can be advantageously obtained without 
using a higher current density. 
In a further embodiment of the present invention the coupling between 
active laser waveguide section 105 and auxiliary light guiding section 115 
need not be continuous, as shown in FIG. 2. As such, this coupling can be 
discontinuous and only occur near the ends of auxiliary light guiding 
section 115. 
FIG. 4 shows an embodiment of the present invention in accordance with the 
general structure shown in FIG. 2. For sake of clarity, only those parts 
of the structure are shown which are necessary to enable one skilled in 
the art to understand the operation of the embodiment. Specifically, FIG. 
4 shows a view looking down at the top of the injection laser structure. 
Active laser waveguide section 200 is disposed between broadband mirrors 
201 and 202. Mirrors 201 and 202 form a laser cavity. Active laser 
waveguide section 200 and auxiliary light guiding section 203 are 
fabricated from semiconductor materials and are situated so that light 
couples between them. Gap 220 between active laser waveguide section 200 
and auxiliary light guiding section 203 is filled with air or SiN.sub.2. 
In either case, the coupling strength between the waveguides is greater 
where the width of the gap is smallest since the indexes of refraction of 
the semiconductor waveguides are larger than that of either air or 
SiN.sub.2. In this embodiment, the phase constant, B1 and B2, of the 
waveguides are unequal and are no required to change with wavelength. The 
coupling strength between the waveguides periodically varies along the 
direction shown by arrow 210 in the manner shown by curve 211 in FIG. 5. 
This periodic variation is created by the periodic variation of auxiliary 
light guiding section 203. The period of the variation of auxiliary light 
guiding section 203 is G, as shown in FIG. 4. At wavelengths where G is 
related to the B1 and B2 of the waveguides by G=2.pi./(B1-B2), the 
effective loss in the laser cavity is minimized. Auxiliary light guiding 
section 203 has lossy terminations 204 and 205. 
FIG. 6 shows a top view and FIG. 7 shows a vertical slice along the 
direction shown by arrows 302 of an injection laser structure having the 
periodic variation in auxiliary light guiding section 203, as described 
above. Metal contacts 300 and 301 and bottom electrode 330 are all used to 
separately pump the active laser waveguide section and the auxiliary light 
guiding section, respectively. Layers 315, grown on substrate 320, are 
n-type active and confining layers well known in the art. Layer 323 is a 
p-type confining layer well known in the art and layer 321 is an SiN.sub.2 
insulating layer. Regions 311 and 312 are Zn-diffused regions in p-type 
layer 323 (SiN.sub.2 layer 321 also acts as a diffusion mask). FIG. 6 
merely shows one of many different ways of fabricating a laser having a 
periodic variation in its light guiding structure. For example other 
structures may be fabricated where ion implantation is used instead of the 
SiN.sub.2 insulation layer to confine current injection to the desired 
strips. Further, ion milling or selective chemical etching with masks can 
be used to form the periodic waveguide pattern. 
FIG. 8 shows the general structure of various embodiments of the present 
invention in which coupling between active portions of transmission lines 
139 and 140--having lossy terminations 135 and 136, respectively--provides 
wavelength selective loss. Here too, for sake of clarity, only sufficient 
detail is shown to enable one skilled in the art to understand the 
operation of the embodiment. Specifically, FIG. 8 shows a view looking 
down at the top of the injection laser structure. Current is injected into 
each transmission line along its entire length. Mirrors 130 and 131 are 
broadband mirrors and form a laser cavity. Transmission lines 139 and 140 
are fabricated to have phase constants which are equal at one wavelength 
and unequal at all others by using materials having different indexes of 
refraction and forming transmission line structures having different 
transverse dimension. The resulting wavelength dependent phase constants 
are graphically shown in FIG. 9. 
The relatively broadband coupling between transmission lines 139 and 140 
causes appreciable power transfer at the main mode wavelength, but little 
net power transfer at other wavelengths. In accordance with the present 
invention, there need not be a complete transfer of power between 
transmission lines 139 and 140 in order to obtain single mode behavior. 
FIG. 10 shows a top view and FIG. 11 shows a vertical slice along the 
direction shown by arrows 400 of an injection laser structure in which 
coupling between active portions of two transmission lines is provided. 
FIG. 12 is an enlargement of a portion of FIG. 11. Mirrors 401 and 402 in 
FIG. 10 are broadband mirrors and form a laser cavity. Arrows 403 and 404 
show the direction of laser radiation which emerges from the structure. 
Metal contacts 405 and 406 and bottom electrode 410, all shown in FIG. 11, 
are used to separately inject current into the active laser waveguide 
section and the auxiliary light guiding section. Layers 412, 411 and 415 
are grown on substrate 420. Layer 411 is an active laser material and 
layers 412 and 415 are cladding layers. Selective etchings, by means of 
techniques well known in the art, creates the two mesas which are disposed 
under metal contacts 405 and 406. Active laser waveguide 139 and auxiliary 
light guiding section 140 shown in FIG. 8 are formed in these two mesas. 
The mesas have widths W2 and W1, respectively, as shown in FIG. 12. 
In the embodiment shown in FIG. 12, the index of refraction of the active 
region of both transmission lines, 411 and 430, is chosen to be the same, 
i.e., nA. This is not a necessary condition, but merely one example which 
results in a structure having the type of wavelength-variable phase 
constants shown by the curves in FIG. 9. Further, for illustrative 
purposes, layers 415, 412, 421 and 422 are all chosen to have the same 
index of refraction, nB, and the index of refraction of layer 420, nC, is 
chosen to be related to nA and nB in the following manner: 
EQU nC&lt;nB&lt;nA (1) 
In addition, width W2 is illustratively chosen to be greater than width W1. 
These choices result in the wavelength-variable phase constants shown in 
FIG. 13, as theoretically explained in an article entitled "Tunable 
Optical Waveguide Directional Coupler Filter" by R. C. Alferness and R. V. 
Schmidt in Applied Physics Letters, Vol. 33 No. 2, 15 July 1978, p. 161. 
Layers 412, 411, 430 and 415, all shown in FIG. 12, are fabricated from 
known alloys of InGaAsP and layer 420 may be SiN.sub.2 or any other 
suitable material having the appropriate value of index of refraction. 
Widths W2 and W1 are chosen to give appropriate cross-over points of the 
phase constants at the operating wavelength of the structure, as shown in 
FIG. 13. For example, in the wavelength region of interest, widths W1 and 
W2 should be approximately equal to 1.0 and 1.5 microns, respectively. 
Further, in accordance with the principle of a cutoff in asymmetric 
waveguides illustrated in pp. 19-25 of Integrated Optics, edited by T. 
Tamir and published by Springer-Verlag, there will be no guiding in the 
mesa under contact 406 after the cutoff wavelength shown in FIG. 13. 
FIG. 14 shows a further embodiment of the present invention which is 
similar to the embodiment shown in FIG. 8. Also, as in other figures, for 
the sake of clarity, only sufficient detail is shown to enable one skilled 
in the art to understand the operation of this embodiment. In particular, 
FIG. 14 shows a view looking down at the top of the injection laser 
structure. Here, distributed coupling is provided between the passive 
sections in the two waveguides. Specifically, transmission line 160 has 
active region 154 and passive region 155. Active region 154 is pumped by 
injection current as shown by arrow 156. Transmission line 153 is a 
passive region. Both transmission lines 153 and 160 have lossy 
terminations, 151 and 152 respectively. Broadband mirrors 157 and 158 form 
a laser cavity. Transmission lines 153 and 160 are coupled to transfer 
energy in passive coupling regions 170 and 171 over a length L. 
Illustratively, passive coupling regions 170 and 171 can be fabricated 
from LiNbO.sub.3. In a further aspect of this embodiment, passive coupling 
regions 170 and 171 can be subjected to a variable electric field, by 
means of voltages applied to electrodes deposited over passive regions 170 
and 171, in order to utilize the electro-optic effect to tune the value of 
the main mode oscillation frequency. By so doing, a single active region 
can provide a single-mode-laser having a multiplicity of different output 
wavelengths. Furthermore, a high speed frequency modulated (FM) laser 
output is produced when an FM signal is fed into the lithium niobate 
coupling structure. Coupling between passive waveguide sections using 
LiNbO.sub.3 has been disclosed to the art in the above-mentioned Alferness 
and Schmidt article. Furthermore, the wavelength selective passive regions 
153 and/or 155 may be fabricated from fused silica in order to provide 
excellent stability against temperature variations. 
Furthermore, lossy termination 151 in transmission line 160 may easily be 
achieved by not injecting current into region 155 in an otherwise 
continuous structure 160. Clearly, many other varied embodiments may be 
constructed by those skilled in the art without departing from the spirit 
and scope of the present invention.