Injection laser

Details Injection laser Injection laser

2001-02-09

2004-06-08

Tran, Minh Loan (Department: 2826)

Coherent light generators

Particular active media

Semiconductor

C372S049010, C372S050121

Reexamination Certificate

active

06748002

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to quantum electronic technology, and more specifically to efficient semiconductor sources of radiation with a narrow radiation patterns.
BACKGROUND OF INVENTION
The injection laser (hereinafter referred to as “the laser”) is a device that converts electrical energy into the light possessing a narrow spectral composition and high directivity.
Different types of lasers are known: lasers with a strip-type active lasing region and with radiation output through the mirror of an optical resonator (S. S. Ou et. al.,
Electronics Letters
(1992), Vol. 28, No. 25, pp. 2345-2346), distributed-feedback lasers (
Handbook of Semiconductor Lasers and Photonic Integrated Circuits,
edited by Y. Suematsu and A. R. Adams, Chapman-Hill, London, 1994, pp. 44-45 and 393-417), laser amplifiers, including a master oscillator power amplifier (MOPA) (
IEEE J. of Quantum Electronics
(1993), Vol. 29, No. 6, pp. 2052-2057), and lasers with curved resonators and radiation output through a surface (
Electronics Letters
(1992), Vol. 28, No. 21, pp. 3011-3012). Further expansion of the applications of such lasers is impeded by insufficiently high output power, efficiency, operating life, and reliability, including situations with monomode lasing.
The laser described in U.S. Pat. No. 4,063,189, issued to D. R. Scifres et al. in 1977 includes a laser heterostructure (hereinafter referred to as a “heterostructure”) that contains an active layer of GaAs positioned between two optically homogeneous cladding layers. The gain region of the operating laser, of length L
GR
, in practice coincides with the thick active layer into which non-equilibrium carriers are injected by means of ohmic contacts. As the term is conventionally employed, a gain region is that part of the heterostructure which includes the active layer and from which radiation is spreading in a activated laser. The gain region, hereinafter referred to as “the GR”, is the medium of the optical resonator. The length of the GR along the longitudinal gain axis is bounded by flat end surfaces that act as reflectors. The length L
OR
of the optical resonator (Fabry-Perot) coincides with the length L
GR
, so that the ratio
&mgr;=
L
OR
/L
GR
  (1)
is equal to one. Reflective coatings with a coefficient of reflection close to one (hereinafter referred to as “reflective coating”) are applied to the reflectors of the optical resonator. The radiation inflow region (hereinafter referred to as “RIR”), as which a substrate of GaAs is used, borders on a surface of one of the cladding layers that is distant from the active layer. The inner surface of the RIR, whose area is equal to the area of the GR, is located on the cladding layer adjacent to the RIR. The flat optical facets of the RIR are a continuation of the planes of the reflectors of the optical resonator and are perpendicular to the longitudinal gain axis of the GR. A coating with a reflection coefficient close to zero (hereinafter referred to as “antireflective coating”) is applied to one of the optical facets (hereinafter referred to as “the facet”), while a reflective coating is applied to the other facet. The facet with the applied anti-reflective coating is the output surface. The RIR is made electrically conductive, and an ohmic contact is made with its outer surface, which is opposite the inner surface. Another ohmic contact is made from the direction of the heterostructure.
When direct current is supplied to the laser, nonequilibrium carriers are injected into the active layer, and induce the generation of radiation of a specified wavelength &lgr; and mode composition in the medium of the optical resonator. Part of the laser radiation from the GR exits the laser through the RIR. The angle of outflow of radiation is define by the following equation:
&phgr;=arccos(
n
eff


RIR
)  (2)
(see J. K. Buttler et al.,
IEEE Journ. of Quant. Electron.
(1975), Vol. QE-11, p. 402). Note that the use of identical compositions for the active layer and RIR (both made of GaAs) restricted the range of ratios n
eff


RIR
from more than 0.9986 to 1, and the outflow angle &phgr; to the range from 3° to 0°, respectively.
The following basic parameters were obtained for the fabricated laser (see D. R. Scifres et al., U.S. Pat. No. 4,063,189, 1977, as well as D. R. Scifres et al.,
Applied Physics Letters
(1976), Vol. 29, No. 1, pp. 23-25): a threshold current density j
thr
of 7.7 kA/cm
2
, a threshold current J
thr
of 7.0 A for a length L
OR
of 400 &mgr;m, a short-pulse output power of 3 W, a differential efficiency on the order of 35-40%, and an angle of divergence &THgr; of 2°
1
in the vertical plane for laser radiation output through the face. The vertical plane referred to is the plane that passes through the longitudinal gain axis and that is perpendicular to the active layer. The corresponding horizontal plane is perpendicular to the vertical plane.
SUMMARY OF THE INVENTION
An injection laser comprising at least one gain region having a longitudinal gain axis and outputting laser radiation at an outflow angle &phgr; comprises a laser heterostructure, at least one radiation inflow region adjoining the laser heterostructure, and reflectors that together form an optical resonator. The laser heterostructure comprises an active layer, which forms at least one of the gain regions, cladding layers comprising at least one layer having a refractive index, and ohmic contacts. The radiation inflow region adjoining the laser heterostructure is transparent to the laser radiation, has a refractive index n
RIR
, and is located on at least one side of the active layer. The radiation inflow region additionally includes at least one optical facet, an outer surface, and an inner surface, the optical facet being oriented at an angle of inclination &psgr; with respect to a plane perpendicular to the longitudinal gain axis. At least part of the optical resonator coincides with at least part of the radiation inflow region and at least part of the gain region. The laser heterostructure and the adjoining radiation inflow region together have an effective refractive index n
eff
such that
n
RIR
exceeds n
eff
,
arccos(
n
eff


RIR
)≦arccos(
n
eff-min


RJR
),
and
 n
eff-min
is greater than n
min
,
where n
eff-min
is the minimum value of n
eff
for laser heterostructures with radiation inflow regions that produce outflow of radiation from the active layer into the radiation inflow region, and n
min
is the smallest of the refractive indices in the cladding layers of the heterostructure.


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