Semiconductor laser having reduced temperature dependence

Abstract: In accordance with the present invention a semiconductor laser is provided with a cavity having decreasing loss with increasing wavelength in order to reduce temperature dependence. Decreasing loss can be provided, for example, by provision of appropriate multilayer reflection coatings (18-25) or by longitudinal gratings that favour longer wavelengths. The coatings advantageously have peak reflectivity centered at the lasing wavelength corresponding to maximum operating temperature. The result in long wavelength InGaAs lasers is an improvement in the threshold temperature coefficient from about 50K to about 85K. In addition, the improved lasers exhibited a decreased rate of quantum efficiency degradation with temperature.

Description: Field of the Invention This invention relates to semiconductor lasers and, in particular, to a semiconductor laser having reduced temperature dependence for threshold current and quantum efficiency. Background of the Invention Semiconductor lasers are attractive for a wide variety of applications including telecommunications, computing systems, optical recording systems and optical interconnection of integrated circuits. Semiconductor lasers provide a compact source of coherent, monochromatic light which can be modulated at high bit rates to transmit large amounts of information. One difficulty with semiconductor lasers is that their threshold current for lasing I th and their quantum efficiency is dependent on temperature. As a consequence thermal-electric coolers are typically required. However, for many applications, such as data links and fiber-to-the-home, thermal coolers are unduly expensive, and other means of reducing temperature dependence must be found. The temperature dependence of threshold current of a semiconductor laser is customarily described by the empirical expression: I th (T) α exp (T/To), where To is a curve-fitting constant characteristic of the laser material system and the laser design. For 1.3 - 1.55 micrometer wavelength InGaAsP/InP lasers, To is about 50K for both bulk active double heterostructure lasers and for multiquantum well lasers. While it was hoped that the multiquantum well lasers would exhibit a higher To and hence reduce temperature dependence, no significant increase in To has thus far been observed. Accordingly, there is a need for a semiconductor laser having reduced temperature dependence. Summary of the Invention The invention is disclosed in claim 1. A semiconductor laser is provided with a cavity having decreasing loss with increasing wavelength in order to reduce temperature dependence. Such decreasing loss can be obtained by providing the laser cavity with reflecting means which favor longer wavelengths. Decreasing loss can be provided, for example, by provision of appropriate multilayer reflection coatings or by longitudinal gratings. The coatings or gratings advantageously have peak reflectivity centered at the lasing wavelength corresponding to maximum operating temperature. The result in long wavelength InGaAs lasers is an improvement in the threshold temperature coefficient from about 50K to about 85K. In addition, the improved lasers exhibited a decreased rate of quantum efficiency degradation with temperature. Brief Description of the Drawings In the drawings: FIG. 1 is a schematic cross section of a first embodiment of a semiconductor laser, FIG. 2 is a plot of threshold current versus temperature for the laser of FIG. 1; FIG. 3 is a schematic cross section of a second embodiment of a semiconductor laser, FIG. 4 is a plot of threshold current versus temperature for the laser of FIG. 3; and FIGS. 5 and 6 are graphical plots useful in explaining the operation of the lasers of FIGS. 1 and 3. Detailed Description Referring to the drawing, FIG. 1 is a schematic cross section of a first embodiment of a semiconductor laser having a cavity loss which decreases with increasing wavelength. In essence the embodiment comprises a conventional semiconductor laser 9 modified by stack 10 of reflection coatings at one or both facets to produce decreasing loss with increasing wavelength. Semiconductor laser 9 typically comprises a plurality of epitaxial semiconductor layers grown on a monocrystalline semiconductor substrate (not shown). For example, layer 11 can be a layer of n-type semiconductor such as indium phosphide doped with silicon or tin to a concentration in the range 5 x 10 17 - 1 x 10 18 cm -3 . Layer 12, which is the optically active region in which photons are generated, can be a different semiconductor having a narrower bandgap than layer 11, such as InGaAsP, and layer 13 can be a semiconductor similar to layer 11 but having the opposite (here p-type) conductivity, such as indium phosphide doped with zinc to a concentration in the range 5 x 10 17 to 1 x 10 18 cm -3 . Transverse end surfaces 14 and 15 of the laser are polished facets to provide optical feedback, and electrical contact means 16 and 17 are provided for flowing electrical current transversely through the successive layers for producing lasing in layer 12. The layers constituting laser 9 can be grown using chemical beam epitaxy or metalorganic chemical vapor deposition techniques well known in the art. These layers, the facts at their ends and any coatings thereon define the laser cavity. The portion of the laser which interacts with the light generated in active region 12 is referred to as the laser cavity. The laser cavity includes layer 12, the reflecting facets at end surfaces 14 and 15, any coatings on the facets, and portions of layers 11 and 13 adjacent to layer 12. The laser cavity presents a characteristic loss to light traveling within the laser which is important in determining the threshold current for lasing. In conventional semiconductor lasers this characteristic loss is essentially independent of temperature. In this embodiment, one or both facets of the laser are provided with a stack 10 of reflection coatings for producing decreasing loss with increasing wavelength. Preferably the coatings are chosen to produce peak reflectivity at the lasing wavelength at the highest anticipated operating temperature. The general method of designing such multilayer stacks is described by O. S. Heavens, Optical Properties of Thin Solid Films , (Dover, 1965). In the preferred example of a 1.5 micrometer InGaAsP/InP laser, stack 10 can comprise alternate layers of silicon dioxide and silicon. Specifically layers 18, 20, 22 and 24 are SiO 2 having respective thicknesses of 2627 Å (1nm = 10Å) 2636 Å, 2653 Å and 2653 Å. Layers 19, 21, 23, and 25 are silicon having respective thicknesses of 1287 angstrom, 1292 Å, 1300 Å and 1300 Å. These layers can be conveniently deposited on laser facets using high vacuum evaporation techniques well known in the art. The thickness of the layers can be precisely monitored during deposition by a quartz transducer to produce the specified thicknesses. The result of the modified laser cavity is reduced temperature dependence. FIG. 2 is a plot of continuous wave threshold current as a function of temperature for two semiconductor lasers. Curve 1 is for a conventional semiconductor laser without coatings. Curve 2 is for a similar laser with coatings. As can be seen, at all temperatures between 20°C and 80°C, the coated laser exhibits a lower threshold current and a lower rate of increase of threshold current with increasing temperature. FIG. 3 is a schematic cross section of a semiconductor laser in accordance with the invention wherein a grating, rather than coatings, is used to achieve a laser cavity loss which decreases with increasing wavelength. In this embodiment the laser differs from laser 9 of FIG. 1 in that the coating stack 10 has been omitted and in that a longitudinal periodic grating 30 has been formed in n-layer 11 and filled in with a second epitaxially compatible n-type semiconductor 11 A. For an n-doped indium phosphide layer 11, layer 11 A can be n-doped InGaAsP. The period Λ of grating 30 is chosen to favor a lasing wavelength corresponding to the highest anticipated operating temperature, e.g. to favor 1.56 micrometer lasing at 85°C for a 1.5 micrometer laser. Specifically, if λ is the desired wavelength and N is the effective refractive index in the laser, then Λ = λ/2N. The depth of the grating is preferably chosen so that the optical feedback from the grating is weak compared to the feedback from the facets. As a specific example, for the above-described laser, Λ can be 2400 Å and the grating depth can be less than about 100 Å. Such gratings can be made using wet etching or reactive ion etching in conjunction with conventional holographic patterning techniques. FIG. 4 is a graphical illustration showing the continuous wave threshold current of the device of FIG. 3 as a function of temperature. The applicants offer the following explanation of the reduced temperature dependence of semiconductor lasers. In a semiconductor laser, the total cavity loss α total is given by where Γ is the confinement factor of the laser waveguide, α i is the effective internal loss, L is the cavity length, is the effective distributed mirror loss, and R 1 and R 2 are the facet reflectivities. The laser reaches the lasing threshold when the gain G of the active medium overcomes the total cavity losses, i.e. when G th = α total where G th is the gain at losing threshold. At a given temperature, the G of the medium increases with increasing injection current and gradually becomes saturated. As shown in FIG. 5, as the temperature is increased from T 1 to T 4 , the gain decreases and the gain peak shifts in wavelength from λ 1 to λ 4 . For conventional semiconductor lasers, the total cavity loss α total is essentially independent of temperature and wavelength. This is illustrated by the dashed line in FIG. 5. The threshold currents for conventional lasers are the currents at the intersections between the α total dashed line and the respective (T,λ) curves. For the lasers described herein, however, the total cavity losses decrease with increasing wavelength. Thus the α total is represented by the dotted line of FIG. 5. As illustrated, the threshold currents are the currents at the intersections (shown by triangles) between the α total dotted line and the respective (T,λ) curves. For each (T,λ), the threshold currents are lower than for the conventional structure. This effect is clearly illustrated in FIG. 6 which plots the threshold currents in FIG. 5 versus temperature.

Claims: A semiconductor laser for operating over a range of temperatures and comprising a layer (11) of n-type semiconductor, a layer (13) of p-type semiconductor, an active layer (12) of semiconductor disposed between said n-type and p-type layers, and a laser cavity (14, 10) for interacting with light generated in said optically active semiconductor, CHARACTERISED IN THAT said laser comprises reflecting means such that said laser cavity has a total cavity loss which decreases with increasing optical wavelength up to the peak lasing wavelength at the preselected highest temperature of said range whereby said laser exhibits reduced temperature dependence of threshold current over said temperature range. A semiconductor laser according to claim 1 wherein said total cavity loss decreases with increasing wavelength up to a peak wavelength corresponding to the lasing wavelength at about 85°C. A semiconductor laser according to claim 1 or claim 2 wherein said laser further comprises transverse end facets (14,15), and at least one of said end facets (15) is provided with reflecting means (18-25) providing increasing reflectivity for increasing wavelength. A semiconductor laser according to claim 1 or claim 2 wherein said laser further comprises longitudinal grating means (30) for providing decreasing loss for increasing wavelength. A semiconductor laser as claimed in claim 3 wherein said n-type layer (11) is of n-type indium phosphide, said p-type layer (13) is of p-type indium phosphide, said active layer (12) is InGaAsP, and said reflecting means comprises a plurality of reflective coatings (18-25) on at least one of said end facets (15) for producing a total cavity loss which decreases with increasing wavelength. A laser according to claim 5 wherein said reflective coatings comprise alternate layers of silicon and silicon dioxide. A semiconductor laser as claimed in claim 4 wherein said n-type layer (11) is of n-type indium phosphide, said p-type layer (13) is of p-type indium phosphide, and said active layer (12) is of of InGaAsP said layers having transverse end facets (14,15) defining said laser cavity therebetween, and said grating means being a longitudinally extending grating (30) in one of said layers for producing a total cavity loss which decreases with increasing wavelength. A semiconductor laser according to claim 7 wherein said laser exhibits an effective refractive index N, an optical wavelength λ at a preselected highest operating temperature, and said longitudinally extending grating has Λ=λ/2N.

Inventors: TSANG WON-TIEN, TSANG, WON-TIEN
Applicants: AT & T CORP, AT&T CORP.
Application year: 1992
Main classification: H01S3
IPC codes: H01S5