JP2000183449A - Semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a semiconductor laser having a wide ridge structure shifted further to the high-frequency side of noises generated by horizontal mode conflict by providing an active layer and further an optical absorption layer having a wave loss absorbing an oscillating light of under fixed value. SOLUTION: This semiconductor laser having a wide-ridge structure at least 30 μm is produced by laminating on an n-type GaAs substrate 11 successively an n-type clad layer 12, an optical layer 13, an active layer 14, an optical guide layer 15, a p-type clad layer 16, a p-type optical absorption layer 17, a p-type clad layer 18, and a p-type contact layer 19. At that time, the wave loss of the absorption layer 17 is at most 3 cm-1. After depositing an insulating film 20 on the epitaxial growth substrate, a stripe-shaped window is opened on the insulating film 20, on which a p-side electrode 1 is formed. On one hand, an n-type electrode 2 is formed on the back side of the substrate 11. As a result, noises are shifted into the range of high frequency exceeding several MHz, and noises generated in the frequency in the range of several 10 kHz to several MHz, that is, changes of optical output can be reduced.

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a semiconductor laser, and more particularly, to a semiconductor laser provided with a layer for absorbing oscillating light in addition to an active layer in order to prevent noise generation. Semiconductor laser. 2. Description of the Related Art Conventionally, a so-called wide (generally 30 μm)
In the semiconductor laser having the ridge structure described above, the use of the refractive index waveguide structure suppresses the oscillation in the higher-order transverse modes among the originally existing transverse modes, thereby reducing noise and kink. Attempts have been made to do so. However, even in such a case, when the driving current is changed, mode fluctuation occurs in the active region, and the light output fluctuates under a constant driving current. Some elements generate noise (specified by a value divided by the intensity). In the case of a semiconductor laser having a wide structure, unlike the noise of a single transverse mode laser generally used for communication and optical disks, since a plurality of transverse modes originally exist, the transverse mode can be easily changed only by changing the drive current. Fluctuations have occurred, which has caused noise in the form of fluctuations in light intensity. FIG. 5 schematically shows how this noise is generated. When the light intensity distribution in the light emitting region of the semiconductor laser is as shown in FIG. 1A, when the light intensity distribution changes as indicated by the bold line in FIG. Noise is generated. Further, when the horizontal mode changes, the mode may change to the vertical mode, and noise and kink may increase.
In particular, noise greatly changes in the environment in which the semiconductor laser is used, particularly in temperature, and changes with time due to energization, which has been a great obstacle in actual use. [0005] When a semiconductor laser is actually used, the frequency of noise becomes a problem. For example, when the semiconductor laser is used with a constant optical output, an auto power control (hereinafter, referred to as APC) function is provided in the drive power supply of the semiconductor laser.
The feedback speed of the C circuit is several kilohertz (kHz)
, The fluctuation of the optical output at a frequency lower than this, that is, noise does not pose a practical problem. On the other hand, since the APC circuit does not respond to noise having a higher frequency, such noise becomes a practical problem. However, considering in more detail, there is an upper limit of the frequency of noise which is a particular problem depending on the application. For example, when YAG of a solid laser medium is excited using a wide semiconductor laser, noise having a frequency exceeding several tens of kHz to several MHz does not cause a problem because the YAG does not respond. The same applies to other solid-state laser media because they do not respond to the high-frequency noise of the semiconductor laser that is the pumping light source. SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has been made in view of the above circumstances. It is an object of the present invention to provide a semiconductor laser having a wide ridge structure in which the frequency of noise generated due to transverse mode competition is shifted to a higher frequency side in order to reduce (fluctuation in optical output). According to the semiconductor laser of the present invention, as described above, a semiconductor laser having a wide ridge structure of 30 μm or more has a waveguide loss, which absorbs oscillating light, in addition to an active layer. A light absorbing layer of 3 cm ^-1 or less is provided. [0009] In the semiconductor laser of the present invention, more preferably, the active layer is composed of _{In 1-x Ga x As 1} -y P y, constituted the light-absorbing layer is from Al _z Ga _1-z As . Preferably, the light absorbing layer has a structure that confine carriers in both the vertical and horizontal directions. In the semiconductor laser of the present invention having the above structure, the light absorption layer absorbs a part of the oscillation light of the semiconductor laser itself, and the electric field of the light in the direction perpendicular to the junction surface of the semiconductor. The intensity distribution is biased toward the side where the light absorbing layer exists,
Navigating to different transverse modes occurs easily. Here, FIG.
2 shows the electric field intensity distribution of light in a direction perpendicular to the junction surface of the semiconductor, with and without a light absorbing layer. In the figure, a thin line indicates a case without a light absorbing layer, and a thick line indicates a case with a light absorbing layer. Note that the position of the light absorbing layer and the position of the active layer in FIG. In the case of a wide semiconductor laser, the ridge width is generally about 30 to 300 μm. However, since the back and forth occur randomly between innumerable transverse modes in the active region, the light absorbing layer is made thin to some extent. If formed, the light absorption is saturated, so that transverse mode competition and longitudinal mode competition in a relatively high frequency band exceeding several MHz occur in a complex manner. As a result, as described above, the frequency of noise, which has been a problem in the conventional device, shifts to a high frequency range exceeding several MHz, and noise generation is suppressed. The light absorbing layer has a waveguide loss of 3 cm ^-1.
If the light absorption effect is so high that the light absorption efficiency exceeds the threshold value, the luminous efficiency of the semiconductor laser is reduced to such an extent that it becomes a problem in practical use.
cm ⁻¹ or less. According to the present invention, it is difficult to completely suppress the above-mentioned noise generation.
While semiconductor lasers that generate noise in the frequency range of several MHz are generated from 20% to 50%, according to the present invention, they are reduced from 10% to 20%. Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a semiconductor laser having an oscillation wavelength of 800 nm band according to a first embodiment of the present invention. Hereinafter, the configuration of this semiconductor laser will be described together with its manufacturing process. First, on an n-type GaAs substrate 11, an n-type Al _x Ga _{1 -x} As clad layer (x = 0.5, film thickness 2 μm)
m) 12, an Al _y Ga _1-y As optical guide layer (y = 0.3, film thickness 70 nm) 13, an Al _z Ga _1-z As active layer (z = 0.0
8, thickness 10 nm) 14, Al _y Ga _1-y As optical guide layer (thickness 70 nm) 15, p-type Al _x Ga _1-x As cladding layer (thickness 100 nm) 16, p-type Al _z Ga _{1- z} As light absorbing layer (z = 0.08, film thickness 8 nm) 17, p-type Al _x Ga _{1 -x} A
s cladding layer 18, p-type GaAs contact layer (film thickness 20
0 nm) 19 are sequentially stacked by MOCVD or the like. The light absorbing layer 17 has a characteristic that the waveguide loss is 3 cm ^-1 or less. Next, an insulating film 20 made of SiO ₂ or SiNx is deposited on the epitaxial growth substrate by a plasma CVD method or the like. Open it. A p-side electrode 1 made of Ti / Pt / Au is formed thereon. On the other hand, on the back side of the n-type GaAs substrate 11, an n-side electrode 2 made of, for example, AuGe / Ni / Au is formed. These electrodes 1 and 2 can be formed by, for example, an electron beam evaporation method. Through the above steps, a semiconductor laser having the structure shown in FIG. 1 is obtained. In this semiconductor laser, since the light absorption layer 17 is formed as described above, noise shifts to a high frequency range exceeding several MHz, and noise generated at a frequency in the range of several tens kHz to several MHz or less. That is, the fluctuation of the light output is reduced.
The reason is as described in detail above. Next, a second embodiment of the present invention will be described with reference to FIG. In the semiconductor laser according to the second embodiment, the active layer is formed from InGaAsP. Hereinafter, the configuration of this semiconductor laser will be described together with its manufacturing process. In FIG. 2, elements that are the same as the elements in FIG. 1 are given the same reference numerals, and overlapping descriptions thereof will be omitted (hereinafter the same). First, on an n-type GaAs substrate 21, an n-type Al _x Ga _{1 -x} As clad layer (x = 0.6, film thickness 2 μm) is formed.
m) 22, InGaP light guide layer (100 nm thick) 23,
InGaAsP active layer (10 nm thick) 24, InGaP
Light guide layer (film thickness 100 nm) 25, p-type Al _x Ga _{1 -x} A
s cladding layer (film thickness 100 nm) 26, p-type Al _z Ga _1-z
As light absorbing layer (z = 0.08, film thickness 8 nm) 27, p-type A
An l _x Ga _{1 -x} As clad layer 28 and a p-type GaAs contact layer (thickness: 200 nm) 29 are sequentially laminated by MOCVD or the like. The light absorbing layer 27 has a waveguide loss of 3 cm ^-1.
It has the following characteristics. Subsequent element conversion steps conform to the above-described first embodiment, and a description thereof will be omitted. Also in this embodiment, the same effect as in the first embodiment can be obtained by providing the light absorbing layer 27. Since the refractive index of a material having a higher gain changes depending on the carrier concentration, when the active layer is formed of an InGaAsP material having a low gain as in the present embodiment, the light absorbing layer is formed of the same material. Rather than using Al
When GaAs is used, the effect as a light absorbing layer becomes more remarkable. Next, a third embodiment of the present invention will be described with reference to FIG. Hereinafter, the configuration of the semiconductor laser according to the third embodiment will be described together with its manufacturing process. It is considered that the effect of the light absorbing layer in the semiconductor laser of the present invention becomes more remarkable as the carrier density in this layer increases. In view of this point, the semiconductor laser according to the present embodiment employs a wide ridge structure to confine carriers not only in the layer thickness direction but also in the horizontal direction perpendicular thereto. First, on an n-type GaAs substrate 31, an n-type Al _x Ga _{1 -x} As cladding layer (x = 0.5, film thickness 2 μm)
m) 32, Al _y Ga _1-y As optical guide layer (y = 0.3, film thickness 70 nm) 33, Al _z Ga _1-z As active layer (z = 0.0
8, thickness 10 nm) 34, Al _y Ga _1-y As optical guide layer (thickness 70 nm) 35, p-type Al _x Ga _1-x As cladding layer (thickness 100 nm) 36, p-type Al _z Ga _{1- z} As light absorption layer (z = 0.08, film thickness 8 nm) 37, p-type Al _x Ga _{1 -x} A
s cladding layer 38, p-type GaAs contact layer (film thickness 20
0 nm) 39 are sequentially stacked by MOCVD or the like. The light absorbing layer 37 has a characteristic that the waveguide loss is 3 cm ^-1 or less. Next, after a resist is formed in a layer on the epitaxial growth substrate, the resist is opened in two stripes by photolithography. Next, the light absorption layer 37 is etched to a depth at which the light absorption layer 37 is etched with a citric acid-based etchant or the like to form a ridge groove. After that, using a plasma CVD method or the like,
After depositing an insulating film 40 made of O ₂ , SiNx or the like, the insulating film 40 on the top of the mesa sandwiched between the ridge grooves is opened in a stripe shape for electrode contact by photolithography. A p-side electrode 1 made of Ti / Pt / Au is formed thereon. On the other hand, on the back side of the n-type GaAs substrate 11, an n-side electrode 2 made of, for example, AuGe / Ni / Au is formed. The electrodes 1 and 2 can be formed by, for example, an electron beam evaporation method. Also in this embodiment, the same effect as in the first or second embodiment can be obtained by providing the light absorbing layer 37.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view showing a semiconductor laser according to a first embodiment of the present invention. FIG. 2 is a schematic sectional view showing a semiconductor laser according to a second embodiment of the present invention. FIG. 4 is a schematic cross-sectional view showing a semiconductor laser according to a third embodiment of the invention. FIG. 4 is a schematic view showing how the electric field intensity distribution of light emitted by the semiconductor laser changes depending on the presence or absence of a light absorbing layer. schematic diagram illustrating the cause of noise generation in [eXPLANATION oF sYMBOLS] 1 p-side electrode 2 n-side electrode 11 n-type GaAs substrate 12 n-type Al _x Ga _1-x As cladding layer 13 Al _y Ga _1-y As optical guide layer 14 Al _z Ga _1-z As active layer 15 Al _y Ga _1-y As optical guide layer 16 p-type Al _x Ga _1-x As cladding layer 17 p-type Al _z Ga _1-z As light-absorbing layer 18 p-type al _x Ga _1-x As cladding layer 19 p-type GaAs contact layer 20 an insulating 21 n-type GaAs substrate 22 n-type Al _x Ga _1-x As cladding layer 23 InGaP optical guide layer 24 InGaAsP active layer 25 InGaP optical waveguide layer 26 p-type Al _x Ga _1-x As cladding layer 27 p-type Al _z Ga _{1 -z} As light-absorbing layer 28 p-type Al _x Ga _1-x As cladding layer 29 p-type GaAs contact layer 31 n-type GaAs substrate 32 n-type Al _x Ga _1-x As cladding layer 33 Al _y Ga _1-y As optical guide layer 34 Al _z Ga _1-z As active layer 35 Al _y Ga _1-y As optical guide layer 36 p-type Al _x Ga _1-x As cladding layer 37 p-type Al _z Ga _1-z As light-absorbing layer 38 p Al _x Ga _{1 -x} As clad layer 39 p-type GaAs contact layer 40 insulating film

Claims (1)

Claims: 1. A semiconductor laser having a ridge structure having a width of 30 μm or more, wherein, in addition to an active layer, a waveguide loss that absorbs oscillation light and has a waveguide loss of 3 cm ^−1.
A semiconductor laser having the following light absorption layer. 2. The semiconductor laser according to claim 1, wherein the active layer is made of In _1-x Ga _x As _{1-y Py} , and the light absorbing layer is made of Al _z Ga _1-z As. 4. The light absorbing layer according to claim 1, wherein the light absorbing layer has a structure for confining carriers in both directions of a layer thickness direction and a horizontal direction perpendicular to the layer thickness direction.
9. The semiconductor laser according to item 1.