Semiconductor Laser Including a Reflection Film

The present invention relates to a semiconductor laser for a laser printer, and more particularly, to a semiconductor laser capable of improving the printing uniformity of a laser printer.

FIG. 4 shows a prior art semiconductor laser device for a laser printer. In FIG. 4, reference numeral 1 designates a laser chip for a laser printer. Facet protection films 2 comprising, for example, Al.sub.2 O.sub.3 , are produced at the front and rear laser facets of the laser chip. A metal wire 4 is connected to the laser chip 1. The laser chip 1 is mounted on a sub-mount 5 and the sub-mount 5 is disposed on a heat sink 6. An output laser light beam 7 is emitted from the front facet of the laser chip 1 and a monitor laser light beam 8 is emitted from the rear facet of the laser chip 1.

FIG. 5 shows a semiconductor laser used for the device. In FIG. 5, reference numeral 11 designates a p-type GaAs substrate. A p-type AlGaAs cladding layer 12 is disposed on the p-type GaAs substrate 11. An n-type GaAs current blocking layer 13 is disposed on the p-type AlGaAs cladding layer 12. A p-type AlGaAs cladding layer 14 is disposed on the n-type GaAs current blocking layer 13. An undoped AlGaAs active layer 15 is disposed on the p-type AlGaAs cladding layer 14. An n-type AlGaAs cladding layer 16 is disposed on the undoped active layer 15. An n-type GaAs contact layer 17 is disposed on the n-type AlGaAs cladding layer 16. A p-side electrode 18 is disposed on the p-type GaAs substrate 11 and an n-side electrode 19 is disposed on the n-type GaAs contact layer 17.

When a current larger than a threshold current flows through the semiconductor laser 1, laser oscillation occurs and laser light beams are emitted from both of the front and rear facets of the laser chip. The laser beam 8 emitted from the rear facet is utilized as a monitor light. Facet protection films 2 comprising, for example, Al.sub.2 0.sub.3 , are provided at the laser light emission facets in order to prevent oxidation of the facets. When the facet is oxidized, light absorption at the facet surfaces increases, generating heat due to the laser light absorption at the surface. The crystal in the vicinity of the facet may melt thereby destroying the laser device. Herein, the reflectivity of the facet is about 30 % regardless of the existence of the protection film 2.

Generally, a semiconductor laser used in a laser printer is pulsed. In this case, the laser light output gradually decreases with the passage of time during each pulse, as shown in FIG. 6. In FIG. 6, reference characters (P.sub.A) and (P.sub.B) respectively represent laser light output direct after the start of laser oscillation and directly before the end of laser oscillation and reference character (t) represents the time period during which the laser is switched on.

When the laser is switched on and off at a constant operating current as described above, the laser light output transiently decreases. The degree of the transient decrease in the laser light output during pulsed operation is represented as a droop. The droop is defined in the following equation, referring to FIG. 6. ##EQU1##

Because the printing density of the laser printer is determined uniquely by the laser light output, when the droop .DELTA.P is large, printing intensity variations are generated. Accordingly, it is necessary to reduce the droop .DELTA.P in order to obtain a printing result having no variation. In the semiconductor laser for 9 laser printer having a construction as shown in FIG. 1, however, the droop .DELTA.P is larger than 10 %, and there is a possibility that printing variations will occur with that value of droop.

An object of the present invention is to provide an improved semiconductor laser for 9 laser printer which has a lower value of droop and no possibility of generating printing blurs.

Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the spirit and the scope of the invention will become apparent to those skilled in the art from this detailed description.

According to the present invention, a reflection film having reflectance of 45 to 65 % is provided at the laser facet from which laser light is output. Thus, the external differential quantum efficiency is lowered and the difference between the operation current and the threshold current is increased. Accordingly, the influence on light output caused by variations in the threshold current are reduced, and the droop .DELTA.P is lowered.

FIG. 1(a) is a sectional view of semiconductor laser for a laser printer according to an embodiment of the present invention and FIG. 1(b) is a detailed sectional view of a reflectivity film according to an embodiment of the invention;

FIG. 2 is a diagram of the relationship between the droop .DELTA.P and the front facet reflectivity Rf;

FIG. 3 is a diagram of the relationship between the external light output level at COD P.sub.0 and the front facet reflectivity Rf.

FIG. 4 is a diagram showing a semiconductor laser for laser printer according to the prior art;

FIG. 5 is a perspective view of a semiconductor laser which is used in both laser devices according to the prior art and according to an embodiment of the present invention;

FIG. 6 is a diagram of the reduction of laser light output during pulsed operation of a laser.

An embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1(a) shows a semiconductor laser for a laser printer according to an embodiment of the present invention and FIG. 1(b) shows an enlarged view of a portion of the front facet of the semiconductor laser chip of FIG. 1(a). In FIG. 1, the semiconductor chip 1 has a cross-section having the structure shown in FIG. 5. A pair of surface protection films 2 comprising, for example, Al.sub.2 O.sub.3 are produced on the front and rear facets of the laser chip 1 from which laser light is output. A metal wire 4 is connected to the laser chip 1. The laser chip 1 is mounted on a sub-mount 5 and the sub-mount 5 is disposed on a heat sink 6. An output laser light beam 7 is emitted from the front facet of the laser chip 1 and a monitor laser light beam 8 is emitted from the rear facet of the laser chip 1. A reflection film 3 is disposed on the front facet of the laser chip 1. In FIG. 1(b), reference numeral 3a designates an Al.sub.2 O.sub.3 film, reference numeral 3b designates an amorphous silicon film and reference numeral 3c designates an Al.sub.2 O.sub.3 film.

The method for analyzing the droop quantitatively will be described.

The droop phenomenon is caused by a reduction in laser light output due to an increase in of the threshold current which is also due to a temperature increase of the active region of the semiconductor laser. When the temperature increase of the active region is Tj, P.sub.A and P.sub.B are represented by ##EQU2## Herein, .eta..sub.df :external differential quantum efficiency of the front facet

.nu.:frequency of laser light

h:Planck's constant

q:electron charge

I.sub.th :threshold current

I.sub.op :operating current

T.sub.o :characteristic temperature

Herein, external differential quantum efficiency is the conversion efficiency from electrical current to light in percent (%) when a current is injected into the laser.

When the formulae (2) and (3) are introduced into the formula (1), the droop becomes: ##EQU3##

The characteristic temperature of the semiconductor laser T.sub.0 is 180.degree. K and this is quite large compared with .DELTA.T.sub.j. Accordingly,

and in this case, the formula (4) is approximated by: ##EQU4##

From the formula (5), it can be seen that droop P can be reduced by increasing the term (I.sub.op /I.sub.th)-1. Because P.sub.A and h/q take constant values in the above-described formula (2), in order to increase (I.sub.op /I.sub.th), it is required that the external differential quantum efficiency .eta..sub.df be decreased.

In order to calculate .DELTA.P from formula (5), the value of .DELTA.T.sub.j has to be obtained. The value of .DELTA.T.sub.j is obtained by solving differential equation (6). ##EQU5## Herein, C:thermal capacitance of the laser

V.sub.op :operating voltage between electrodes of the laser

When the differential equation (6) is solved, the following equation is obtained. ##EQU6##

Herein, R.sub.th :thermal resistance of the laser.

When the formula (4) or formulae (5) and (7) are calculated, the droop .DELTA.P is obtained.

From the above, as .eta..sub.df increases, .DELTA.p increases.

Accordingly, in order to reduce the .DELTA.P, it is required to reduce the external differential quantum efficiency.

As a concrete example of reducing the external differential quantum efficiency .eta..sub.df, the front facet reflectivity Rf of the laser chip 1 may be increased. The relationship between the .eta..sub.df, Rf and the rear facet reflectivity Rr is represented by the following formula. ##EQU7## Herein, .alpha.:internal loss

L:cavity length

From the formula (8), when the Rf is increased, the .eta..sub.df is decreased.

FIG. 2 shows the relationship between the droop .DELTA.P and the front facet reflectivity Rf of the laser chip. From this FIG. 2, it can be seen that as Rf increases, .DELTA.P is reduced. The upper limit of the droop .DELTA.P where no printing depth variations occur in a laser printer is 8%. From FIG. 2, it can be seen that for .DELTA.P<8%, Rf should be above 45%.

As discussed above, in order to lower the droop .DELTA.P, Rf must be increased. When the Rf is increased, however, the light density in the neighborhood of the facet is raised while the droop .DELTA.P is lowered. Accordingly, laser facet destruction due to COD (Catastrophic Optical Damage) is likely to occur, reducing reliability. The following relationship exists between external light output P.sub.0 in the COD and the internal light intensity at the neighborhood of the laser facet, using Rf. ##EQU8##

FIG. 3 shows the relationship between the external light output P.sub.0 at COD and the front facet reflectivity Rf which is obtained calculated from formula (9). Herein, the internal light intensity Pi in the neighborhood of the laser facet at COD is constant at 30 mW regardless of the value of Rf. Because the laser for a printer is generally used at a light output of about 3 mW, the output level at COD is desired to be above about 6 mW which is twice of the above-described practical level. Accordingly, Rf has to be below 65% from FIG. 3.

As described above, as a result of research into the relationship between the droop and the front facet reflectivity and the relationship between the external light output at COD and the front facet reflectivity, the front facet reflectivity Rf of the laser chip has to be within 45%<Rf<65%.

As a means for realizing the above-described Rf, the construction of the reflection film 3 as shown in FIG. 1b, that is, a triple layer structure comprising Al.sub.2 O.sub.3 /a-Si/Al.sub.2 O.sub.3 is preferred.

Then, if the layer thicknesses of d.sub.1, d.sub.2, and d.sub.3 are as in the following, ##EQU9## Rf becomes 50%. Herein, .lambda.0:wavelength of laser light

n.sub.1 :refractive index of Al.sub.2 O.sub.3 (at light of wavelength .lambda..sub.0)

n.sub.2 :refractive index of a-Si (at light of wavelength .lambda..sub.0)

While in the above-described enbodiment the construction of reflection film 3 comprises a triple layer structure comprising Al.sub.2 O.sub.3, a-Si, and Al.sub.2 O.sub.3, the present invention may not restricted thereto. Table 1 shows other construction of a reflection film 3 and the reflectivity Rf thereof together with the above-described construction. In the table, reference numerals 3a, 3b, 3c, and 3d represent the first, second, third, and fourth layer in the order from below, and all the layers have a quarter wavelength layer thickness.

As discussed above, according to the present invention, a reflection film is provided at the front facet of a semiconductor laser for a laser printer, and the facet reflectivity thereof is made 45% to 65%. Therefore, a semiconductor laser superior in droop characteristic, having no printing variations and superior reliability is obtained.