GB2082380A - Injection laser - Google Patents

Abstract

A semiconductor injection laser with a stripe active region (13a) is provided with a quarter wavelength anti-reflection layer (22) of alumina on one reflecting facet to broaden the spectral emission, increase the range of single fundamental transverse mode operation, and reduce the spread on the far-field pattern. The other facet may have a protective half-wavelength coating. <IMAGE>

Description

SPECIFICATION Injection laser This invention relates to injection lasers of the Fabry Perot type, and in particular to the provision of an anti-reflection coating on one reflecting facet of such a laser.

In the case of non-lasing light emissive diodes the use of ani-reflection coatings has been described in order to improve the light output efficiency by reducing the reflection coefficient of the interface between the material of the semiconductor, which has a relatively high refractive index, and the surrounding air, which has a low index.

In contrast to this, in the conventional Fabry Perot type injection laser whose optical resonator cavity is defined by two plane parallel reflectors, it is this reflection at such interfaces that is used to form the Fabry Perot reflectors of the laser resonant cavity. In consequence the position of an anti-reflection coating on the reflecting facet of an injection laser appears in general undesirable because it would lower the Q of the cavity and thus put up the lasing threshold.

However, in the case of pulsed power lasers, it is known, for instance from U.K. Patent Specifications Nos. 1275886 and 1407351 (P.R. Selway, A.R. Goodwin, G.H. Backhouse 11-2-1 and G.H.B. Thompson 15), that antireflection coatings can be used with advantage in special applications where they raise the threshold of peak power dissipation at which occurs the onset of catastrophic damage to the facets.

The present invention relates to the provision of anti-reflection coatings on the altogether different type of injection laser in which low order transverse mode operation is required and which are normally designed to be capable of continuous wave operations. In this instance a primary object of providing the anti-reflection coating is to broaden the spectral bandwidth of the fundamental transverse mode emission lines and to increase the dynamic range of single transverse mode operation by raising the first order transverse mode lasing threshold with respect ot that of the fundamental transverse mode. Neither of these effects is observed with the anti-reflection coated high power lasers of the prior art because such devices are essentially physically broad emission devices operating in high order transverse modes.

According to the present invention there is provided an injection laser having a pair of plane parallel reflecting facets forming a Fabry Perot type optical resonator cavity, with an active region in which laser action is capable of being stimulated, and with means to restrict the current flow in the neighbourhood of said active region to a stripe which extends normally from one reflecting facet to the other and whose width is substantially less than the full width of the semiconductive material of the laser, and wherein at least one of the reflecting facets is provided with a reflection reducing coating.

There follows a description of a laser embodying the invention in a preferred form. This description refers to the accompanying drawings in which: Figure 1 depicts a perspective view of the laser; Figure 2 depicts comparative graphs of the light output of uncoated and coated lasers; Figure 3 depicts comparative graphs of the spectral output of the lasers of Fig. 2 at various power levels, and Figure 4 depicts comparative graphs of the far-field patterns of the lasers of Figs. 2 and 3.

Many different ways of achieving current confinement in an injection laser are known, of which perhaps the simplest to comprehend is the stripe contact laser in which the active region is so close to one surface of the semiconductor that current is confined to a narrow stripe in the active region solely by means of using an electrode contact in the form of a stripe on the adjacent surface. This stripe may be defined for instance by a stripe window in an oxide layer deposited direct upon the epitaxial layers of the laser. However, for the purpose of exemplifying the present invention, the preferred method of current confinement is that achieved by the design of laser known as the channel substrate narrow stripe laser hereinafter referred to as the CNS laser.

For an introduction to the principles of operation of channel substrate lasers reference may be made to the article entitled "Channelled substrate buried heterostructure GaAs (GaAl)AS injection lasers" by P.A. Kirkby and G.H.B. Thompson appearing in the Journal of Applied Physics Volume 47 No. 10 pages 4578 to 4589 (October 1976), while the CNS laser is described in the article entitled "Channelled-Substrate Narrow-Stripe GaAs/ GaAIAs Injection Lasers with Extremely Low Threshold Currents" by P.A. Kirkby appearing in Electronics Letters 6th December 1979 Volume 15 No. 25 pages 824 to 826. Channel substrate lasers are also described in our Patent Specification No. 1 531 908 (P.A.

Kirkby 2).

Referring to Fig. 1 a CNS laser has an ntype GaAs substrate 10 which has a Veeshaped groove 11 extending in a surface that is tilted by about 14 from a 100 plane. This tilt is about an axis lying in or near one of the 100 directions lying in the 100 plane, and the groove itself extends in a direction at right angles to this 100 direction. The substrate is placed in a multi-well liquid phase epitaxy graphite boat (not shown) provided with melts for growing three layers, 12, 1 3 and 14, on the substrate. The boat is placed in an epitaxy furnace and the three layers are grown without interruption. Layers 12 and 14 are layers of GaAIAs, whose conductivity types are respectively n-type and p-type so as to provide the structure with a single p-n junction which is located between these layers.Layer 1 3 is grown in lower band-gap higher refractive index material than the other two layers. This may contain either a reduced AlAs content or substantially none. Layer 1 3 which is the active layer of the device may be of either conductivity type, though generally it is preferred to make it p-type. Typically it is about 0.2 microns thick in the regions where it is grown on flat portions of the surface of the underlying layer 12, and the thickness and growth conditions of the active layer and of the underlying layer are chosen so that the active layer grows in three separated regions comprising a central region overlying the groove 11 and the two flanking regions overlying the flat portions of layer 12.The profile of the central region depends in part upon the thickness of the underlying layer 1 2 and also in part upon the rate of growth of layer 1 3. In a typical example the grpwth performed at 750"C with a cooling rate of 0.4"C per minute to provide a crescent shaped cross-section for the central region 1 3a of layer 1 3 about 8 microns wide and 0.21 microns maximum thickness. In this instance layer 1 3 was of GaAs and layers 1 2 and 14 were of GaO,6AI0,4As.

Layer 14 is covered with a p-type layer 1 5 of GaAs provided to facilitate the provision of a top contact. This layer 1 5 is then covered with a thin layer 16 of plasma deposited silica typically about 90 nm thick and then standard techniques of photolithography are used to open up a stripe 17, 2 to 3 microns wide registering with the line of the Vee groove 11.

Next conventional metal contact layer 18 and 19 are deposited on the top and bottom surfaces, the end surfaces 20 and 21 are prepared by clearing to provide a cavity length of 200 microns, and finally transparent coatings 22 and 23 are deposited upon the cleared surfaces. For the sake of clarity the outline of these coatings has been represented in Fig. 1 by broken lines.

In this particular instance the two coatings 22 and 23 are coatings of alumina deposited by electron beam evaporation from a source of alumina in high vacuum. During the coating process the laser is heated by radiation from a quartz-iodine lamp to a temperature lying in the range 100 to 200"C. The thicknesses of the two coatings are monitored with a standard quartz crystal thickness monitor.

Coating 22 is an anti-reflection coating and so needs to be an odd number of quarter-wavelengths thick, and the deposition rate is typically such as to achieve the required thickness in between 2 and 5 minutes of deposition time. Coating 23 is optional, and is not applied to provide any optical effect but merely to serve as a protective layer. In order to produce no optical effect it is made one halfwavelength thick. Alternatively it could have been made an integral number of half-wavelengths thick.

In a comparison between similar CNS lasers, some with the quarter-wavelength coating, and some without, it was found that those without the coating operated in the single fundamental transverse made up to powers in the range 2 to 5 mW whereas those with the coating typically operated up to.

about 10 mW in single fundamental transverse mode.

The quarter-wavelength coating causes a higher fraction of the circulating optical power in the laser cavity to be emitted, and so the cavity becomes more lossy. The increased optical output from the cavity has to be made up by increases in the spontaneous optical input and optical gain and so the quarterwavelength coated lasers threshold currents about 30 percent higher than similar uncoated ones (typically 44 mA at 25eC instead of 32 mA). This is not a severe disadvantage because the CNS laser has a relatively low threshold current and current density compared with for instance an oxide insulated stripe contact laser. Above the lasing threshold the slope efficiency of the output from the quarter-wavelength coated end of the CNS laser is typically 60 percent greater than that of an equivalent uncoated CNS laser.Comparative characteristics are depicted in Fig.

2,-characteristics (A) and (B) pertaining to an uncoated and to a quarter-wavelength coated CNS laser respectively.

It is believed that the increased cavity loss of the quarter-wavelength coated lasers causes the spectral width to increase, particularly at moderate powers. This is evident from Fig. 3 which shows at (A) the spectrum of an uncoated laser at various power levels, and at (B) the equivalent spectrum of a coated laser at the same power levels. In the case of the uncoated laser it is seen that operation in the range 4 to 8 mW is confined to a single longitudinal mode, whereas, in the case of the coated laser over the same range of power levels, several longitudinal modes are stimulated.

The displacement in the centre of gravity of the emission lines in going from one laser to another is attributable not only to the Burstein effect, in which a shorter wavelength is expected for the coated laser because of the band filling effects produced by the stronger optical pumping, but also to small differences in residual aluminium content in the active layer.

Fig. 4 depicts the far-field patterns (parallel to the junction) of typical uncoated (A) and quarter-wavelengths coated (B) CNS lasers at various power levels, and the lack of appreciable broadening of the far-field pattern (B), even at 10 mW, shows that the onset of the first order transverse mode occurs at a much higher power level than is the case with the uncoated equivalent laser.

Claims (6)

1. An injection laser having a pair of plane parallel reflecting facets forming a Fabry Perot type optical resonator cavity, with an active region in which laser action is capable of being stimulated, and with means to restrict the current flow in the neighbourhood of said active region to a stripe which extends normally from one reflecting facet to the other and whose width is substantially less than the full width of the semiconductive material of the laser, and wherein at least one of the reflecting facets is provided with a reflection reducing coating.

2. An injection laser as claimed in claim 1 wherein the reflection reducing coating has an optical thickness substantially equal to a quarter-wavelength of the laser emission.

3. An injection laser as claimed in claim 1 or 2 wherein only one of the reflecting facets is provided with a reflection reducing coating, and wherein the other reflecting facet is provided with a protective coating whose optical thickness is substantially equal to a half-wavelength of the laser emission.

4. An injection layer as claimed in any preceding claim wherein said coating or coatings are of alumina.

5. An injection laser as claimed in any preceding claim wherein said means to restrict the current flow are provided at least in part by the use of a grooved semiconductor substrate upon which the epitaxial layers of the injection laser are grown.

6. An injection laser substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.