Short coupled cavity laser

Single longitudinal mode operation is achieved and maintained under CW and high speed (Gbps) current modulation conditions by a short coupled cavity laser including a short cavity semiconductor laser having two parallel mirror facets and a reflective surface spaced apart from and in predetermined relationship with one of the mirror facets. A short external cavity resonator is formed between the one mirror facet and the reflective surface. In general, the laser cavity length is related to the external cavity resonator length by the equation, nL=md, where nL is the effective optical length of the injection laser, d is the length of the external cavity resonator, and m is a positive number preferably between 2 and 10.

TECHNICAL FIELD 
This invention relates to the field of semiconductor lasers and, more 
particularly, to CW and high speed, single longitudinal mode operation of 
a short coupled cavity laser. 
BACKGROUND OF THE INVENTION 
Operational characteristics of injection lasers under high speed modulation 
are of interest in optical fiber communication systems. Characteristics 
which are of particular interest for wideband single longitudinal mode 
fiber transmission are dynamic spectral behaviors such as frequency 
chirping and transient gain peak shifting in the transient regime and 
spectral envelope broadening at the occurrence of multiple longitudinal 
modes. Control of these dynamic spectral characteristics and others are 
important to achieving sufficient mode selection for single longitudinal 
mode operation under high speed operation. 
Several approaches are known for achieving longitudinal mode selection in 
lasers. The approaches, excluding the use of a built-in grating for 
feedback, are as follows: short cavity laser, external cavity laser, and 
two-section (coupled cavity) laser. Each approach is described in more 
detail below. 
Short cavity lasers employ an optical cavity which has a cavity length of 
approximately 30 to 80 microns. This cavity length is at least five or six 
times shorter than conventional optical cavity lengths. Mode selectivity 
of the short cavity laser arises from a much larger longitudinal mode 
separation and a larger gain difference between adjacent modes than in 
conventional lasers. Short cavity lasers are described in articles by T. 
P. Lee et al., IEEE J. Quantum Electron., QE-18, p. 1101 (1982), and C. A. 
Burrus et al., Electron. Lett., Vol. 17, p. 954 (1981). 
External cavity lasers are comprised of a combination of a long optical 
cavity, cleaved laser and an external reflector. The reflector and a 
cleaved facet of the laser form an external cavity resonator which is, in 
general, approximately as long as the optical cavity of the laser. 
Diffraction losses occur in the external cavity resonator because the 
propagation medium is air. Mode selectivity of this combination arises 
from modulation of the loss in the coupled resonator including the laser 
and the external cavity resonator as a function of frequency. External 
cavity lasers have been described in articles by K. R. Preston et al., 
Electron. Lett., Vol. 17, p. 931 (1981); D. Renner et al., Electron. 
Lett., Vol. 15, p. 73 (1979); C. Voumard et al., Opt. Commun., Vol. 13, p. 
130 (1975); and D. A. Kleinman et al., BSTJ, Vol. 41, p. 453 (1962). 
Two section and other multiple section lasers employ a corresponding number 
of monolithic laser cavities abutting each other. In this type of laser, 
the cavities are waveguiding regions which are controllable via current 
biasing. In general for two-section lasers, the sections are comprised of 
a long section and a short section. Mode selectivity results from 
modulation of the loss of the laser cavities as a function of frequency. 
Multiple section lasers have been described in U.S Pat. No. 3,303,431 
issured to A. B. Fowler on Feb. 7, 1967 and in articles by L. A. Coldren 
et al., Appl. Phys. Lett., Vol. 38, p. 315 (1981); K. J. Ebeling et al., 
Electron. Lett., Vol. 18, p. 901 (1982); Coldren et al., IEEE J. Quantum 
Elect., QE-18, p. 1679 (1982). 
In all of the lasers categorized above, there exist problems in achieving 
efficient single longitudinal mode operation under high speed modulation 
conditions because of dynamic spectral characteristics of the lasers. 
SUMMARY OF THE INVENTION 
Single longitudinal mode operation is achieved and maintained under CW and 
high speed (gigahertz) modulation conditions by a short coupled cavity 
laser including a short cavity semiconductor laser having two parallel 
mirror facets and a reflective surface spaced apart from and in 
predetermined relationship with one of the mirror facets. An external 
cavity resonator is formed between the one mirror facet and the reflective 
surface. 
In one embodiment of the invention a III-V heterostructure injection laser 
having a cavity length between 50 and 80 microns is coupled to a short 
external cavity resonator having a length between 30 and 80 microns. The 
short external cavity resonator includes one cleaved facet of the 
injection laser and reflective surface spaced apart from and facing the 
cleaved facet. In general, the laser cavity length is related to the 
external cavity resonator length by the equation, nL=md, where nL is the 
effective optical length of the injection laser, n is the index of 
refraction of the guiding region of the injection laser at the wavelength 
of interest, L is the physical length of the injection laser, d is the 
length of the external cavity resonator, and m is a positive number 
preferably between 2 and 10.

DETAILED DESCRIPTION 
As shown in FIGS. 1 and 5, the present invention is a coupled short cavity 
laser for achieving and maintaining single longitudinal mode operation 
under both CW and high speed modulation conditions. While the following 
description shows the preferred use of current injection for exciting the 
laser, it should be obvious to those skilled in the art that optical 
sources are capable of being utilized for pumping the laser. 
The short coupled cavity laser is illustrated in simplified block diagram 
form in FIG. 1. In accordance with the present invention, the short 
coupled cavity laser includes current source 10, short cavity laser 20, 
and reflective surface 30. Current source 10 provides current to pump the 
active region of short cavity laser 20. Short cavity laser 20 is typically 
a short cavity, semiconductor laser having an effective optical length nL, 
where n is the index of refraction of the guiding region of short cavity 
laser 20 at the wavelength of interest and L is the physical length of 
short cavity laser 20. Light quanta 25 are generated by short cavity laser 
20 and exit through one of two parallel mirror facets of short cavity 
laser 20 toward reflective surface 30. Reflective surface 30 is spaced 
apart from and appropriately positioned with respect to one mirror facet 
of short cavity laser 20 so that at least a portion of light quanta 25 is 
reflected back toward short cavity laser 20. One facet of short cavity 
laser 20 and reflective surface 30 form an external cavity resonator of 
length d. External cavity resonator length d is related to the effective 
optical length of short cavity laser 20 by the following equation, nL=md, 
where m is a positive number. Optimization of a value and range of value 
for m is described below in more detail. It should be clear to those 
skilled in the art that reflective surface 30 and both facets of short 
cavity laser 20 form a coupled resonator. 
Cleaved facet, stripe geometry, InGaAsP/InP double heterostructure 
injection lasers are adaptable for use as short cavity laser 20. Other 
Group III-V semiconductor lasers having cleaved or etched facets are also 
suitable for use as short cavity laser 20. For purposes of illustration 
and not for purposes of limitation, exemplary types of short cavity lasers 
are stripe geometry, v-groove (buried crescent), ridge and various buried 
heterostructure lasers from the InP or GaAs alloys and their derivatives. 
Regardless of the type of laser selected as short cavity laser 20, it 
should be noted that the laser cavity length L is less than 100 microns 
and preferably between 50 and 80 microns. 
Reflective surface 30 is realized by forming a highly reflective material 
into or on a planar or curved shape. In one example, gold is evaporated 
onto a cleaved facet of semiconductor material to form a flat (planar) 
reflective surface 30. Other exemplary reflective surfaces are formed by 
coating one end of an optical fiber with reflective material or by 
fabricating spherical or parabolic or other concave surfaces with high 
reflectivity. Reflective surface 30 is aligned, in the case of a flat 
reflective surface, normal to the longitudinal axis of light quanta 25. 
That is, the flat reflective surface is substantially parallel to the 
external mirror facet of short cavity laser 20. It is desirable to 
permanently mount reflective surface 30 on the same platform or substrate 
as short cavity laser 20. 
In the short coupled cavity laser, good longitudinal mode selection is 
achieved by employing the largest and steepest modulation of the resonator 
loss as a function of frequency because this tends to maximize the loss 
difference between adjacent modes. If the gain peak of short cavity laser 
20 is positioned near a loss minimum, the corresponding mode oscillates 
strongly while nearby modes are suppressed. In general, these 
characteristics are controlled by choosing a suitable value for m or, 
alternatively, by properly designing the length of the external cavity 
resonator. 
A small value for m, such a 2 or 3, results in a loss modulation period of 
every two or three modes with a substantially large modulation slope. But, 
it should be noted that there may be several modulation periods under the 
gain curve for short cavity laser 20 which would cause oscillation of 
several distant modes. On the other hand, a large value for m, such as 10 
or 12, guarantees that there is only one modulation period under the gain 
curve for short cavity laser 20. In this case, the resulting modulation 
slope is small, thereby increasing the possibility of oscillation in 
adjacent modes along with the dominant, minimum loss mode. In light of the 
above considerations, the value of m is between 2 and 10 and preferably 
between 3 and 8 depending upon operating conditions and the envelope of 
the gain curve for short cavity laser 20. Hence, the desired length of the 
external cavity resonator is less than 100 microns and preferably between 
30 and 80 microns. While variations of either the external cavity 
resonator length d or the short cavity laser length L may be necessary for 
different applications, the combined lengths of the external cavity 
resonator and the short cavity laser, i.e., d+L, should be less than 200 
microns. 
FIGS. 2 through 5 illustrate progressive improvements in single 
longitudinal mode operation realized in going from a conventional long 
cavity laser (several hundred microns in length) in FIG. 2 to a short 
cavity laser in FIG. 3 and then to a conventional external cavity laser 
(conventional long cavity laser coupled to an external cavity resonator) 
in FIG. 4 and finally to the short coupled cavity laser in FIG. 5 of the 
present invention. As shown in these FIGURES, the laser gain curves are 
substantially identical regardless of the type of laser, i.e., long or 
short cavity. Moreover, in FIGS. 4 and 5, the coupled resonator loss 
curves have been drawn in position relative to a fixed reference line 
(dashed line in FIGS. 2-5) and the period of each coupled resonator loss 
curve is determined by selecting m equal to 6, for example. .DELTA.g 
represents the net gain or amplitude difference between the center mode 
and the adjacent mode. The longitudinal mode spectra as shown in FIGS. 2 
through 5 illustrate output amplitude versus optical frequency variations 
for each particular laser operating near threshold. 
Effective mode suppresion is dependent upon .DELTA.g. For the short coupled 
cavity laser, .DELTA.g is large thereby allowing strong oscillation of the 
dominant central mode while suppressing all other modes during 
above-threshold operation. From experimental practice, CW operation of the 
short coupled cavity laser at approximately 1.4 times threshold result in 
more than 20 dB mode suppression, i.e., single longitudinal mode 
operation.