JP5669364B2 - Surface emitting laser element, surface emitting laser array, optical scanning device, and image forming apparatus - Google Patents

Description

  The present invention relates to a surface emitting laser element, a surface emitting laser array, an optical scanning device, and an image forming apparatus, and more particularly, a surface emitting laser element that outputs light in a direction perpendicular to a substrate, and the surface emitting laser element. The present invention relates to a surface-emitting laser array in which the surface-emitting laser is integrated, an optical scanning device having the surface-emitting laser element or the surface-emitting laser array, and an image forming apparatus including the optical scanning device.

  The surface-emitting laser element has a feature that a low threshold current and low power consumption are easy due to its structure. In recent years, compared with ion-implanted surface-emitting laser elements that have been studied so far, an oxide constriction-type surface-emitting laser element capable of lowering the threshold current and excellent in high-speed response has been energetically studied. (For example, refer nonpatent literature 1).

  An oxide confinement type surface emitting laser element has the advantage that the transverse mode optical confinement by the oxide is good and the oscillation mode characteristic is stable. It is difficult to obtain basic transverse mode oscillation. Hereinafter, the oxidized constriction type surface emitting laser element is simply referred to as a “surface emitting laser element” for convenience.

  Conventionally, as a method of performing single basic transverse mode control, the area of the non-oxidized region, which is a current injection region (current passing region), is reduced so that the higher order transverse mode is confined and does not oscillate, in other words, higher order transverse mode A method of making the mode cut off has been widely adopted.

  In addition, as another method for performing the single fundamental transverse mode control, a method for reducing the intensity of light confinement in the transverse mode by the oxide has been proposed. When the light confinement strength in the transverse mode is small, higher-order mode oscillation is suppressed. In this case, since it is not necessary to reduce the area of the non-oxidized region, both the thermal characteristics and the electrical characteristics are improved. Therefore, the saturation output is improved and the modulation speed is also improved. In order to reduce the strength of light confinement by an oxide, conventionally, the position of the oxide has been moved away from the active layer, or the thickness of the oxide has been reduced.

  Since the surface emitting laser element can extract the laser output in the direction perpendicular to the substrate, it can be easily integrated two-dimensionally at a high density, and its application to a high-speed and high-definition electrophotographic system or the like has begun to be studied. . For example, Non-Patent Document 2 discloses a printer using a 780 nm band VCSEL array (surface emitting laser array). Patent Document 1 discloses a multi-spot image forming apparatus having a multi-spot light source. In general, higher-speed optical writing can be realized by using a surface emitting laser element capable of high output operation in a single basic transverse mode.

  In electrophotography and the like, the rising behavior in the response waveform of the light output of the light source when the drive current is supplied to the light source (time change of the light output, hereinafter also referred to as “light waveform”) is extremely large in image quality. Influence. For example, not only the rise time in the optical waveform, but also the light quality may deteriorate slightly even after the light output reaches a certain light quantity at the beginning of the rise.

  This is the contour of the image that is formed at the rise and fall of the optical waveform. This is because the image quality becomes unclear and the image quality lacks visual clarity.

  For example, if the time required to scan one line of A4 paper width (vertical) of about 300 mm is 300 μs, the width of about 1 mm is scanned in a time of 1 μs. It is said that the range in which the visual sensitivity of the human eye is highest with respect to fluctuations in image density is 1 to 2 mm. Therefore, if the image density fluctuates in a width of about 1 mm, the density change is sufficient to be detected by the human eye, giving an impression that the outline is unclear.

  FIG. 49 shows an optical waveform when the surface emitting laser element is driven under a pulse condition of a pulse width of 500 μs and a duty of 50% (pulse period 1 ms). As shown in FIG. 49, when viewed on a relatively long time scale, the light output once shows a peak immediately after rising, and then the light output decreases and becomes stable. This change in light output is due to self-heating of the surface emitting laser element, and is generally called “droop characteristic”.

  By the way, when the inventors conducted a detailed examination, as shown in FIG. 50 in which the vicinity of the rising edge of the optical waveform in FIG. 49 is enlarged, when viewed on a short time scale, the light different from the “droop characteristic”. I got a new finding that there was a change in output.

  In FIG. 50, the light output does not rise even after 10 ns elapses, and substantially rises after about 200 ns elapses, but then gradually increases during the time up to about 1 μs. Such a phenomenon (characteristic) has been newly found by the inventors. In this specification, such characteristics are referred to as “negative droop characteristics”. It should be noted that such a “negative droop characteristic” is not observed in the conventional edge-emitting laser element.

  In order to obtain a high-quality image with a surface-emitting laser element, it is necessary to appropriately control the optical response waveform at the time of start-up. With a surface-emitting laser element having the above “negative droop characteristics”, a high-quality image can be obtained. It became clear that it was difficult.

  The present invention has been made on the basis of the novel findings obtained by the inventors described above, and has the following configuration.

According to a first aspect of the present invention, there is provided a surface emitting laser element made of an AlGaAs-based material that outputs light in a direction perpendicular to a substrate, the resonator structure including an active layer; An oxide including at least an oxide formed by oxidizing a part of the selective oxidation layer containing aluminum and arsenic surrounds the current passing region, and the injection current and the transverse mode of the oscillation light are simultaneously displayed. A semiconductor multilayer reflector including therein a confinement structure that can be confined; and the semiconductor multilayer reflector reduces light confinement in a lateral direction on the substrate side with respect to the resonator structure. The semiconductor multi-layer film reflecting mirror has a plurality of pairs in which a low refractive index layer and a high refractive index layer are paired, and the light confinement reducing unit has at least one pair of the pair. The The optical thickness of at least one of the low refractive index layer and the high refractive index layer in the at least one pair is (2n + 1) λ / 4 using an oscillation wavelength λ, an integer n of 1 or more, Negative droop characteristics are suppressed by setting the thickness of the selective oxidation layer to at least 28 nm , the diameter of the current passing region to 4.0 μm or more, and the temperature at which the oscillation threshold current is minimized to 25 ° C. or less. When a square wave current pulse having a pulse period of 1 ms and a pulse width of 500 μs is supplied, the optical output P1 at 10 ns after supply and the optical output P2 at 1 μs after supply are used, and 0 ≧ (P1−P2) / The surface emitting laser element satisfies the relationship of P2 ≧ −0.1.

  This suppresses the “negative droop characteristic” and enables high output operation in single fundamental transverse mode oscillation.

  From a second viewpoint, the present invention is a surface emitting laser array in which the surface emitting laser elements of the present invention are integrated.

  According to this, since the surface emitting laser elements of the present invention are integrated, it is possible to suppress the “negative droop characteristic” and to perform high output operation in single fundamental transverse mode oscillation.

  From a third aspect, the present invention is an optical scanning device that scans a surface to be scanned with light, the light source having the surface-emitting laser element of the present invention; a deflector that deflects the light from the light source; A scanning optical system that condenses the light deflected by the deflector onto the surface to be scanned.

  According to a fourth aspect of the present invention, there is provided an optical scanning device that scans a surface to be scanned with light, a light source having the surface emitting laser array of the present invention; a deflector that deflects light from the light source; A scanning optical system that condenses the light deflected by the deflector onto the surface to be scanned.

  Since each of the above optical scanning devices has the surface emitting laser element of the present invention or the surface emitting laser array of the present invention, as a result, it becomes possible to perform highly accurate optical scanning.

  According to a fifth aspect of the present invention, there is provided at least one image carrier; and at least one optical scanning device according to the invention that scans light including image information on the at least one image carrier. An image forming apparatus.

  According to this, since at least one optical scanning device of the present invention is provided, a high-quality image can be formed.

It is a figure for demonstrating schematic structure of the laser printer which concerns on one Embodiment of this invention. It is the schematic which shows the optical scanning device in FIG. It is a figure for demonstrating schematic structure of the surface emitting laser element contained in the light source in FIG. 4A and 4B are diagrams for explaining the substrate in FIG. 3, respectively. It is the figure which expanded a part of lower semiconductor DBR in FIG. It is the figure which expanded the active layer vicinity in FIG. It is a figure for demonstrating the optical waveform when the conventional surface emitting laser element is driven by the square wave current pulse of 1 ms of pulse periods and 50% of a duty. It is a figure for demonstrating the optical waveform when the conventional surface emitting laser element is driven by the square wave current pulse of pulse period 100ns and duty 50%. FIG. 6 is a diagram (part 1) for explaining a built-in effective refractive index difference Δneff. FIGS. 10A and 10B are diagrams (No. 2) for explaining the built-in effective refractive index difference Δneff, respectively. FIG. 11A and FIG. 11B are diagrams for explaining the effective refractive index difference Δneff when the internal temperature rises. It is a figure for demonstrating the shift of the IL curve by the raise of internal temperature in the surface emitting laser element with insufficient light confinement of the horizontal direction at room temperature. It is a figure for demonstrating the optical waveform at the time of FIG. It is a figure for demonstrating each refractive index used for calculation. FIG. 6 is a diagram (part 1) for explaining the relationship between the optical confinement coefficient, the thickness of the selective oxidation layer, and the oxidized constriction diameter. FIG. 6 is a diagram (part 2) for explaining the relationship between the optical confinement factor, the thickness of the selective oxidation layer, and the oxidized constriction diameter. FIG. 6 is a diagram (No. 3) for explaining the relationship between the optical confinement factor, the thickness of the selective oxidation layer, and the oxidized constriction diameter; It is a figure for demonstrating the optical waveform of the surface emitting laser element in which the optical confinement coefficient of a fundamental transverse mode in 25 degreeC is about 0.983. It is a figure for demonstrating the optical waveform of the surface emitting laser element in which the optical confinement coefficient of a fundamental transverse mode in 25 degreeC is about 0.846. It is a figure for demonstrating the relationship between the thickness of a to-be-selected oxidation layer in a surface emitting laser element at 25 degreeC, and a droop rate. It is a figure for demonstrating (DELTA) (lambda) ₀ > 0. It is a figure for demonstrating (DELTA) (lambda) ₀ <0. It is a figure for demonstrating the relationship between an oscillation threshold current and measured temperature. It is a figure for demonstrating the relationship between the amount of detuning and the temperature where threshold current becomes the minimum. FIG. 6 is a diagram (No. 1) for describing a relationship between a droop rate and a temperature at which a threshold current is minimized. FIG. 6 is a diagram (No. 2) for explaining the relationship between the droop rate and the temperature at which the threshold current is minimized. It is a figure for demonstrating the relationship between the number of pairs of the optical confinement reduction area | region A, and the optical confinement coefficient of a fundamental transverse mode. It is a figure for demonstrating the structure of the conventional surface emitting laser element used for calculation. It is a figure for demonstrating the structure of the surface emitting laser element which has the optical confinement reduction area | region used for calculation. It is a figure for demonstrating the structure of lower semiconductor DBR in the conventional surface emitting laser element used for calculation. It is a figure for demonstrating the optical confinement reduction area | region A. FIG. It is a figure for demonstrating the optical confinement reduction area | region B. FIG. It is a figure for demonstrating the relationship between the number of pairs of the optical confinement reduction area | region B, and the optical confinement coefficient of a fundamental transverse mode. It is a figure for demonstrating the optical confinement reduction area | region C. FIG. It is a figure for demonstrating the relationship between the number of pairs of the optical confinement reduction area | regions C, and the optical confinement coefficient of a fundamental transverse mode. It is a figure for demonstrating the effect of a light confinement reduction area | region. It is FIG. (1) for demonstrating an absorption loss reduction layer. It is FIG. (2) for demonstrating an absorption loss reduction layer. It is a figure for demonstrating the influence which acts on the optical confinement coefficient of an absorption loss reduction layer. It is FIG. (1) for demonstrating the effect of an optical confinement reduction area | region and an absorption loss reduction layer. It is FIG. (2) for demonstrating the effect of an optical confinement reduction area | region and an absorption loss reduction layer. It is a figure for demonstrating the modification 1 of an optical confinement reduction area | region. It is a figure for demonstrating the modification 2 of an optical confinement reduction area | region. It is a figure for demonstrating the modification 3 of an optical confinement reduction area | region. It is a figure for demonstrating a surface emitting laser array. It is a figure for demonstrating the two-dimensional arrangement | sequence of the light emission part in FIG. It is AA sectional drawing of FIG. It is a figure for demonstrating schematic structure of a color printer. It is a figure for demonstrating the optical waveform of the conventional surface emitting laser element. It is the figure which expanded the standup vicinity in FIG.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a schematic configuration of a laser printer 1000 according to an embodiment of the present invention.

  The laser printer 1000 includes an optical scanning device 1010, a photosensitive drum 1030, a charging charger 1031, a developing roller 1032, a transfer charger 1033, a charge eliminating unit 1034, a cleaning unit 1035, a toner cartridge 1036, a paper feeding roller 1037, a paper feeding tray 1038, A registration roller pair 1039, a fixing roller 1041, a paper discharge roller 1042, a paper discharge tray 1043, a communication control device 1050, a printer control device 1060 that comprehensively controls the above-described units, and the like are provided. These are housed in predetermined positions in the printer housing 1044.

  The communication control device 1050 controls bidirectional communication with a host device (for example, a personal computer) via a network or the like.

  The photosensitive drum 1030 is a cylindrical member, and a photosensitive layer is formed on the surface thereof. That is, the surface of the photoconductor drum 1030 is a scanned surface. The photosensitive drum 1030 rotates in the direction of the arrow in FIG.

  The charging charger 1031, the developing roller 1032, the transfer charger 1033, the charge removal unit 1034, and the cleaning unit 1035 are each disposed in the vicinity of the surface of the photosensitive drum 1030. Then, along the rotation direction of the photosensitive drum 1030, the charging charger 1031 → the developing roller 1032 → the transfer charger 1033 → the discharging unit 1034 → the cleaning unit 1035 are arranged in this order.

  The charging charger 1031 uniformly charges the surface of the photosensitive drum 1030.

  The optical scanning device 1010 irradiates the surface of the photosensitive drum 1030 charged by the charging charger 1031 with a light beam modulated based on image information from the host device. As a result, a latent image corresponding to the image information is formed on the surface of the photosensitive drum 1030. The latent image formed here moves in the direction of the developing roller 1032 as the photosensitive drum 1030 rotates. The configuration of the optical scanning device 1010 will be described later.

  The toner cartridge 1036 stores toner, and the toner is supplied to the developing roller 1032.

  The developing roller 1032 causes the toner supplied from the toner cartridge 1036 to adhere to the latent image formed on the surface of the photosensitive drum 1030 to visualize the image information. Here, the latent image to which the toner is attached (hereinafter also referred to as “toner image” for the sake of convenience) moves in the direction of the transfer charger 1033 as the photosensitive drum 1030 rotates.

  Recording paper 1040 is stored in the paper feed tray 1038. A paper feed roller 1037 is disposed in the vicinity of the paper feed tray 1038, and the paper feed roller 1037 takes out the recording paper 1040 one by one from the paper feed tray 1038 and conveys it to the registration roller pair 1039. The registration roller pair 1039 temporarily holds the recording paper 1040 taken out by the paper supply roller 1037, and in the gap between the photosensitive drum 1030 and the transfer charger 1033 according to the rotation of the photosensitive drum 1030. Send it out.

  A voltage having a polarity opposite to that of the toner is applied to the transfer charger 1033 in order to electrically attract the toner on the surface of the photosensitive drum 1030 to the recording paper 1040. With this voltage, the toner image on the surface of the photosensitive drum 1030 is transferred to the recording paper 1040. The recording sheet 1040 transferred here is sent to the fixing roller 1041.

  In the fixing roller 1041, heat and pressure are applied to the recording paper 1040, whereby the toner is fixed on the recording paper 1040. The recording paper 1040 fixed here is sent to the paper discharge tray 1043 via the paper discharge roller 1042 and is sequentially stacked on the paper discharge tray 1043.

  The neutralization unit 1034 neutralizes the surface of the photosensitive drum 1030.

  The cleaning unit 1035 removes the toner remaining on the surface of the photosensitive drum 1030 (residual toner). The surface of the photosensitive drum 1030 from which the residual toner has been removed returns to the position facing the charging charger 1031 again.

  Next, the configuration of the optical scanning device 1010 will be described.

  As shown in FIG. 2 as an example, the optical scanning device 1010 includes a deflector-side scanning lens 11a, an image plane-side scanning lens 11b, a polygon mirror 13, a light source 14, a coupling lens 15, an aperture plate 16, an anamorphic. A lens 17, a reflection mirror 18, a scanning control device (not shown), and the like are provided. These are assembled at predetermined positions in the housing 30.

  In the following, for convenience, the direction corresponding to the main scanning direction is abbreviated as “main scanning corresponding direction”, and the direction corresponding to the sub scanning direction is abbreviated as “sub scanning corresponding direction”.

  The coupling lens 15 converts the light beam output from the light source 14 into substantially parallel light.

  The aperture plate 16 has an aperture and defines the beam diameter of the light beam through the coupling lens 15.

  The anamorphic lens 17 forms an image of the light beam that has passed through the opening of the aperture plate 16 in the sub-scanning corresponding direction in the vicinity of the deflection reflection surface of the polygon mirror 13 via the reflection mirror 18.

  The optical system arranged on the optical path between the light source 14 and the polygon mirror 13 is also called a pre-deflector optical system. In the present embodiment, the pre-deflector optical system includes a coupling lens 15, an aperture plate 16, an anamorphic lens 17, and a reflection mirror 18.

  As an example, the polygon mirror 13 has a hexahedral mirror having an inscribed circle radius of 18 mm, and each mirror serves as a deflecting reflection surface. The polygon mirror 13 deflects the light flux from the reflection mirror 18 while rotating at a constant speed around an axis parallel to the sub-scanning corresponding direction.

  The deflector-side scanning lens 11 a is disposed on the optical path of the light beam deflected by the polygon mirror 13.

  The image plane side scanning lens 11b is disposed on the optical path of the light beam via the deflector side scanning lens 11a. Then, the light beam that has passed through the image plane side scanning lens 11b is irradiated onto the surface of the photosensitive drum 1030, and a light spot is formed. This light spot moves in the longitudinal direction of the photosensitive drum 1030 as the polygon mirror 13 rotates. That is, the photoconductor drum 1030 is scanned. The moving direction of the light spot at this time is the “main scanning direction”. The rotation direction of the photosensitive drum 1030 is the “sub-scanning direction”.

  The optical system arranged on the optical path between the polygon mirror 13 and the photosensitive drum 1030 is also called a scanning optical system. In the present embodiment, the scanning optical system includes a deflector side scanning lens 11a and an image plane side scanning lens 11b. Note that at least one folding mirror is disposed on at least one of the optical path between the deflector side scanning lens 11a and the image plane side scanning lens 11b and the optical path between the image plane side scanning lens 11b and the photosensitive drum 1030. May be.

  As an example, the light source 14 includes a surface emitting laser element 100 as illustrated in FIG. 3. In this specification, the laser oscillation direction is defined as the Z-axis direction, and two directions orthogonal to each other in a plane perpendicular to the Z-axis direction are described as the X-axis direction and the Y-axis direction.

  The surface emitting laser element 100 is a surface emitting laser element having a design oscillation wavelength of 780 nm band, and includes a substrate 101, a buffer layer 102, a lower semiconductor DBR 103, a lower spacer layer 104, an active layer 105, an upper spacer layer 106, and an upper semiconductor. A DBR 107, a contact layer 109, and the like are included.

  The surface of the substrate 101 is a mirror-polished surface, and as shown in FIG. 4A, the normal direction of the mirror-polished surface is crystal orientation [1 1 1] with respect to the crystal orientation [1 0 0] direction. ] An n-GaAs single crystal substrate inclined by 15 degrees (θ = 15 degrees) in the A direction. That is, the substrate 101 is a so-called inclined substrate. Here, as shown in FIG. 4B, the crystal orientation [0 1 -1] direction is the + X direction and the crystal orientation [0 -1 1] direction is the -X direction.

  The buffer layer 102 is laminated on the + Z side surface of the substrate 101 and is a layer made of n-GaAs.

  As shown in FIG. 5 as an example, the lower semiconductor DBR 103 includes a first lower semiconductor DBR 1031, a second lower semiconductor DBR 1032, and a third lower semiconductor DBR 1033.

The first lower semiconductor DBR 1031 is stacked on the + Z side of the buffer layer 102, and is a pair of a low refractive index layer 103a made of n-AlAs and a high refractive index layer 103b made of n-Al _0.3 Ga _0.7 As. 36.5 pairs. Between each refractive index layer, in order to reduce electrical resistance, a composition gradient layer (not shown) in which the composition is gradually changed from one composition to the other composition is provided. Each refractive index layer includes 1/2 of the adjacent composition gradient layer, and is set to have an optical thickness of λ / 4 when the oscillation wavelength is λ. By the way, when the optical thickness is λ / 4, the actual thickness d of the layer is d = λ / 4N (where N is the refractive index of the medium of the layer).

  The second lower semiconductor DBR 1032 is stacked on the + Z side of the first lower semiconductor DBR 1031 and has three pairs of the low refractive index layer 103a and the high refractive index layer 103b. Between each refractive index layer, in order to reduce electrical resistance, a composition gradient layer (not shown) in which the composition is gradually changed from one composition to the other composition is provided. The low refractive index layer 103a is set to have an optical thickness of 3λ / 4 including 1/2 of the adjacent composition gradient layer, and the high refractive index layer 103b is 1 / of the adjacent composition gradient layer. 2 so that the optical thickness is λ / 4. The second lower semiconductor DBR 1032 becomes an “optical confinement reduction region”.

  The third lower semiconductor DBR 1033 is stacked on the + Z side of the second lower semiconductor DBR 1032 and has one pair of a low refractive index layer 103a and a high refractive index layer 103b. Between each refractive index layer, in order to reduce electrical resistance, a composition gradient layer (not shown) in which the composition is gradually changed from one composition to the other composition is provided. Each refractive index layer is set to have an optical thickness of λ / 4 including 1/2 of the adjacent composition gradient layer.

  Thus, in the present embodiment, the lower semiconductor DBR 103 has 40.5 pairs in total of the low refractive index layer 103a and the high refractive index layer 103b.

The lower spacer layer 104 is laminated on the + Z side of the third lower semiconductor DBR 1033 and is a layer made of non-doped (Al _0.1 Ga _0.9 ) _0.5 In _0.5 P.

  The active layer 105 is laminated on the + Z side of the lower spacer layer 104, and as an example, as shown in FIG. 6, is a triplet well active layer having a quantum well layer 105a made of GaInAsP and a barrier layer 105b made of GaInP. is there. The quantum well layer 105a is obtained by introducing As into a GaInP mixed crystal in order to obtain an oscillation wavelength in the 780 nm band, and has a compressive strain. In addition, the barrier layer 105b increases the band gap by introducing tensile strain, realizes high carrier confinement, and forms the strain compensation structure of the quantum well layer 105a.

  Here, since an inclined substrate is used as the substrate 101, anisotropy is introduced into the gain in the active layer, and the polarization direction can be aligned in a specific direction (polarization control).

The upper spacer layer 106 is laminated on the active layer 105 on the + Z side, and is a layer made of non-doped (Al _0.1 Ga _0.9 ) _0.5 In _0.5 P.

  A portion composed of the lower spacer layer 104, the active layer 105, and the upper spacer layer 106 is also called a resonator structure, and the thickness thereof is set to be an optical thickness of one wavelength. The PL wavelength of the active layer 105 is set to 772 nm, which is 8 nm shorter than the resonance wavelength of 780 nm of the resonator structure, and the threshold current is minimized at 17 ° C. The active layer 105 is provided at the center of the resonator structure at a position corresponding to the antinode in the standing wave distribution of the electric field so that a high stimulated emission probability can be obtained. This resonator structure is sandwiched between the lower semiconductor DBR 103 and the upper semiconductor DBR 107.

  The upper semiconductor DBR 107 includes a first upper semiconductor DBR 1071 and a second upper semiconductor DBR 1072.

The first upper semiconductor DBR 1071 is stacked on the + Z side of the upper spacer layer 106, and includes a low refractive index layer made of p- (Al _0.7 Ga _0.3 ) _0.5 In _0.5 P and p- (Al One pair of high refractive index layers made of _0.1 Ga _0.9 ) _0.5 In _0.5 P is provided. Between each refractive index layer, in order to reduce electrical resistance, a composition gradient layer is provided in which the composition is gradually changed from one composition to the other composition. Each refractive index layer is set to have an optical thickness of λ / 4 including 1/2 of the adjacent composition gradient layer.

  The first upper semiconductor DBR 1071 has a larger band gap energy than the AlGaAs layer and functions as a block layer for electrons injected into the active region.

  In addition, since an inclined substrate is used for the substrate 101, generation of hill-like defects (hillocks) of the AlGaInP material can be suppressed, crystallinity can be improved, and generation of natural superlattices can be suppressed, and a band gap can be suppressed. Energy reduction can be prevented. Therefore, the first upper semiconductor DBR 1071 can keep a large band gap energy and functions well as an electron blocking layer.

The second upper semiconductor DBR 1072 is stacked on the + Z side of the first upper semiconductor DBR 1071, and has a low refractive index layer made of p-Al _0.9 Ga _0.1 As and p-Al _0.3 Ga _0.7 As. 23 pairs of high refractive index layers made of Between each refractive index layer, in order to reduce electrical resistance, a composition gradient layer is provided in which the composition is gradually changed from one composition to the other composition. Each refractive index layer is set to have an optical thickness of λ / 4 including 1/2 of the adjacent composition gradient layer.

  In one of the low refractive index layers in the second upper semiconductor DBR 1072, a selective oxidation layer made of p-AlAs is inserted with a thickness of 30 nm. The insertion position of the selective oxidation layer is in the third pair of low refractive index layers from the upper spacer layer 106 and corresponds to the node in the standing wave distribution of the electric field.

  The contact layer 109 is stacked on the + Z side of the second upper semiconductor DBR 1072 and is a layer made of p-GaAs.

  Note that a structure in which a plurality of semiconductor layers are stacked on the substrate 101 is also referred to as a “stacked body” for convenience in the following.

  Moreover, the optical thickness of the refractive index layer shown below shall include 1/2 of an adjacent composition gradient layer.

  Next, a method for manufacturing the surface emitting laser element 100 will be briefly described.

(1) The laminate is formed by crystal growth by metal organic vapor phase epitaxy (MOCVD) or molecular beam epitaxy (MBE).

Here, trimethylaluminum (TMA), trimethylgallium (TMG), and trimethylindium (TMI) are used as Group III materials, and arsine (AsH ₃ ) gas is used as Group V materials. Further, carbon tetrabromide (CBr ₄ ) is used as a p-type dopant material, and hydrogen selenide (H ₂ Se) is used as an n-type dopant material. A phosphine (PH ₃ ) gas is used as the Group V P raw material for the AlGaInAsP-based material, and dimethyl zinc (DMZn) is used as the p-type dopant raw material for AlGaInP.

(2) A square resist pattern having a side of 25 μm is formed on the surface of the laminate.

(3) A quadrangular columnar mesa is formed by ECR etching using Cl2 gas, using the resist pattern as a photomask. Here, the bottom surface of the etching is positioned in the lower semiconductor DBR 103.

(4) The photomask is removed.

(5) The laminated body is heat-treated in water vapor. Here, Al in the selective oxidation layer is selectively oxidized from the outer periphery of the mesa. Then, an unoxidized region 108b surrounded by the Al oxide layer 108a is left in the center of the mesa (see FIG. 3). As a result, a so-called oxidized constriction structure is formed in which the drive current path of the light emitting part is limited to only the central part of the mesa. The non-oxidized region 108b is a current passage region (current injection region). Here, from the results of various preliminary experiments, the heat treatment conditions (holding temperature, holding time, etc.) are appropriately selected so that one side of the current passage region is approximately 4 μm. Specifically, the holding temperature was 360 ° C. and the holding time was 30 minutes.

(6) The protective layer 111 made of SiN or SiO ₂ is formed using a vapor phase chemical deposition method (CVD method).

(7) Flatten with polyimide 112.

(8) Open the window of the P-side electrode contact on the top of the mesa. Here, after masking with a photoresist, the opening at the top of the mesa is exposed to remove the photoresist at that portion, and then the polyimide 112 and the protective layer 111 are etched and opened with BHF.

(9) A square resist pattern having a side of 10 μm is formed in a region to be a light emitting portion on the top of the mesa, and a p-side electrode material is deposited. As the electrode material on the p side, a multilayer film made of Cr / AuZn / Au or a multilayer film made of Ti / Pt / Au is used.

(10) Lift off the electrode material of the light emitting portion to form the p-side electrode 113.

(11) After polishing the back side of the substrate 101 to a predetermined thickness (for example, about 100 μm), the n-side electrode 114 is formed. Here, the n-side electrode 114 is a multilayer film made of AuGe / Ni / Au.

(12) Ohmic conduction is established between the p-side electrode 113 and the n-side electrode 114 by annealing. Thereby, the mesa becomes a light emitting part.

(13) Cut for each chip.

  When a square wave current pulse having a target light output of 1.4 mW, a pulse period of 1 ms, and a pulse width of 500 μs (duty 50%) is supplied to the surface emitting laser element 100 manufactured in this manner, Assuming that the optical output at 10 ns is P1, and the optical output at 1 μs after supply is P2, (P1−P2) /P2=−0.06. Hereinafter, the value (unit:%) of (P1−P2) / P2 × 100 is also referred to as “droop rate”. Therefore, in the surface emitting laser element 100 according to the present embodiment, the droop rate is −6%. By the way, when a surface emitting laser element having a droop rate smaller than −10% is used, an image output from a laser printer is frequently blurred at least partially when observed with the naked eye.

  Further, the surface emitting laser element 100 was able to obtain a single fundamental transverse mode output of 2 mW or more.

  Furthermore, the surface emitting laser element 100 has almost the same threshold current characteristics and external differential quantum efficiency (slope efficiency) as the conventional surface emitting laser element.

  By the way, the inventors examined in detail the optical waveforms when a conventional surface emitting laser element having an oxidized constriction structure is driven by various square wave current pulses. FIG. 7 shows an optical waveform when the pulse period is 1 ms and the duty is 50%, and FIG. 8 shows an optical waveform when the pulse period is 100 ns and the duty is 50%.

  Looking at the optical waveform of FIG. 7, after rising, the optical output gradually increases, and a “negative droop characteristic” appears. Further, even after 60 ns, the light output does not reach the target value (1.5 mW). On the other hand, in the optical waveform of FIG. 8, the output after the rise is stable, and the “negative droop characteristic” does not appear.

  In this way, when a square wave current pulse is supplied to a conventional surface emitting laser element, even if it has the same duty, that is, the same calorific value, a “negative droop characteristic” when the pulse period is long. It was found that the “negative droop characteristic” was not observed in the short case.

  If the pulse period is different, the state of the internal temperature of the surface emitting laser element is considered to be different. That is, when the pulse period is long, the time during which heat is generated and the time during which the pulse is cooled are both long, so that the internal temperature of the surface emitting laser element varies greatly. On the other hand, when the pulse period is short, the continuous cooling time cannot be taken sufficiently, so that the fluctuation of the internal temperature of the surface emitting laser element is small and stable on average at a higher temperature. In other words, under the driving conditions where “negative droop characteristics” can be seen, the internal temperature of the surface emitting laser element varies greatly, and “negative droop characteristics” is a phenomenon caused by the internal temperature of the surface emitting laser element. Can think.

  When the internal temperature of the surface emitting laser element changes, the electric field strength distribution in the lateral direction of the oscillation mode (hereinafter also referred to as “lateral mode distribution” for convenience) changes.

  Since the refractive index of the oxide layer in the oxide confinement structure is about 1.6, which is smaller than the refractive index of the peripheral semiconductor layer (about 3), so-called built-in effective laterally inside the surface emitting laser element. There is a refractive index difference Δneff (see FIG. 9).

  Oscillation modes such as the fundamental transverse mode are confined in the lateral direction by this effective refractive index difference Δneff. At this time, the lateral spread of the oscillation mode is determined by the magnitude of Δneff. The larger the Δneff is, the smaller the lateral spread is (see FIGS. 10A and 10B).

  When a current (driving current) is injected into this surface emitting laser element, the current concentrates in the center of the mesa. It rises locally with respect to the surrounding area. The semiconductor material has the property that when the temperature rises, the band gap energy decreases and the refractive index increases. For this reason, when the temperature at the center of the mesa rises locally, the refractive index of the center increases with respect to the peripheral region, and lateral light confinement becomes stronger.

  As shown in FIG. 10A, when the built-in effective refractive index difference Δneff is small and the temperature at the center of the mesa rises locally, the effective refractive index difference as shown in FIG. The change in Δneff increases and the transverse mode distribution changes greatly. In this case, the overlap between the gain region in which current injection is performed and the transverse mode increases, and the optical confinement in the lateral direction becomes stronger. As a result, the light intensity in the gain region increases, the stimulated emission rate increases, and the threshold current decreases.

  As described above, in the surface emitting laser element in which the built-in effective refractive index difference Δneff is small and the lateral light confinement at room temperature is insufficient, when the internal temperature rises, the IL curve is Shifting to the low current side improves the light emission efficiency (see FIG. 12). In this case, the optical output at the same drive current value increases with time, and a “negative droop characteristic” is observed (see FIG. 13). FIG. 12 shows an IL characteristic that is predicted at time t = t0 seconds before the internal temperature rises, and is predicted at time t = t1 seconds when the drive current is supplied in pulses and the internal temperature sufficiently rises. IL characteristics are shown. As the temperature rises, the light emission efficiency is improved and the threshold current is reduced. Therefore, the IL characteristic at t1 seconds is shifted to the low current side with respect to t0 seconds. Since the value Iop of the driving current is constant, the light output becomes larger in the case of t1 seconds. The optical waveform in this case is shown in FIG.

  On the other hand, as shown in FIG. 10B, when the built-in effective refractive index difference Δneff is large, even if the temperature at the center of the mesa rises locally, as shown in FIG. The change in the effective refractive index difference Δneff is small and the transverse mode distribution does not change much.

  As described above, in a surface emitting laser element having a large built-in effective refractive index difference Δneff and sufficiently large lateral light confinement at room temperature, the lateral mode distribution is stable even when the internal temperature rises, and the luminous efficiency is increased. Almost no change occurs. In this case, the optical output at the same drive current value is almost constant over time, and “negative droop characteristics” are not observed.

  As an index representing the strength of light confinement in the lateral direction, there is a light confinement factor in the lateral direction (hereinafter simply referred to as “light confinement factor” for convenience). Here, the optical confinement factor can be obtained from the ratio of “the integrated strength of the electric field in the same radius region as the current passage region” to “the integrated strength of the electric field in the XY section passing through the center of the surface emitting laser element”. The larger the value of the optical confinement factor, the sharper the electric field intensity distribution is concentrated in the gain region. In other words, the larger the value of the optical confinement coefficient at room temperature, the more confined by the oxide confinement structure, which means that the transverse mode distribution is more stable against local temperature changes in the gain region. ing.

  The transverse mode distribution of the surface emitting laser element can be estimated by calculating the electric field intensity distribution from the following Helmholtz equations (Equations (1) and (2)).

  However, since the equations (1) and (2) are difficult to solve analytically, numerical analysis by a finite element method using a computer is usually performed. There are various types of finite element solvers that can be used, and commercially available VCSEL simulators (for example, LASER MOD) can be used.

  As an example, a fundamental transverse mode distribution in a surface emitting laser element in the 780 nm band is calculated.

In the surface emitting laser element used for the calculation, the active layer has an Al _0.12 Ga _0.88 As / Al _0.3 Ga _0.7 As triple quantum well structure with a thickness of 8 nm / 8 nm, and each spacer layer has Al _0.6 Ga _0.4 As. The lower semiconductor DBR is composed of 40.5 pairs of Al _0.3 Ga _0.7 As (high refractive index layer) / AlAs (low refractive index layer), and the upper semiconductor DBR is Al _0.3 Ga _0.7 As. It consists of 24 pairs of (high refractive index layer) / Al _0.9 Ga _0.1 As (low refractive index layer).

  This surface emitting laser element has a cylindrical mesa shape with a diameter of 25 μm, and mesa etching is performed up to the interface between the lower semiconductor DBR and the lower spacer layer, and the etched region is occupied by the atmosphere. It was. That is, a simple etched mesa structure was used. The diameter of the portion of the lower semiconductor DBR that is not mesa-etched is 35 μm, which is the maximum lateral width considered in the calculation. The material of the selectively oxidized layer is AlAs, and the position of the selectively oxidized layer is in the low refractive index layer having an optical thickness of 3λ / 4 in the upper semiconductor DBR, and the standing wave distribution is counted from the active layer. The position corresponds to the third section.

  Note that the calculation does not consider the gain of the active layer and the absorption by the semiconductor material, and only the eigenmode distribution determined by the structure is obtained. The surface emitting laser element has a uniform temperature of 300K. Moreover, the value shown in FIG. 14 was used for the refractive index of each material. Hereinafter, for convenience, the oxide layer of the oxidized constriction structure is also simply referred to as an “oxide layer”, and the diameter of the current passage region is also referred to as an “oxidized constriction diameter”.

  Based on the fundamental transverse mode distribution calculated as described above, the optical confinement coefficient Γl was calculated using the following equation (3). Here, a corresponds to the radius of the current passage region.

  FIG. 15 shows the result of calculating the optical confinement factor of the fundamental transverse mode at room temperature in the above-described surface emitting laser element of the 780 nm band with respect to the thicknesses of various selective oxidation layers and oxidation confinement diameters. According to this, the optical confinement coefficient depends on the thickness of the selective oxidation layer and the oxidized constriction diameter, and takes a higher value as the thickness of the selective oxidation layer increases and the oxidized constriction diameter increases.

  FIG. 16 shows the calculation result of FIG. 15 with the optical confinement factor as the vertical axis and the thickness of the selectively oxidized layer as the horizontal axis. Looking at the change in the optical confinement factor with respect to the increase in the thickness of the selective oxidation layer, even if the oxidation confinement diameter is different, the change is abrupt in the region where the thickness of the selective oxidation layer is 25 nm or less, and 25 nm or more. Then, it turns out that it shows a saturation tendency.

  FIG. 17 shows the results of actually producing a plurality of surface emitting laser elements having different thicknesses of selective oxidation layers and oxidation constriction diameters and evaluating their droop characteristics. In FIG. 17, a droop rate of −10% or more is represented as “◯”, and a droop rate smaller than −10% is represented as “x”. 15 and 17, it can be seen that a droop rate of −10% or more is obtained in an element structure in which the optical confinement coefficient of the fundamental transverse mode at room temperature is 0.9 or more.

  FIG. 18 shows an optical waveform of the surface emitting laser element in which the fundamental transverse mode optical confinement coefficient at room temperature is about 0.983. The droop rate at this time was about −4.3%.

  FIG. 19 shows an optical waveform of the surface emitting laser element in which the fundamental transverse mode optical confinement coefficient at room temperature is about 0.846. The droop rate at this time was about -62.8%.

  As described above, various surface emitting laser elements having different optical confinement factors were manufactured and examined in detail. When the optical confinement factor was about 0.9, the droop rate was about -5%. . Increasing the optical confinement factor further increased the droop rate with the increase. On the other hand, in the surface emitting laser element having an optical confinement factor smaller than 0.9, the droop rate tends to decrease as the optical confinement factor decreases, and a surface emitting laser element having a droop rate of −70% or less was also observed. .

  Thus, by setting the optical confinement coefficient of the fundamental transverse mode at room temperature to 0.9 or more, the “negative droop characteristic” can be suppressed.

  In general, the effective refractive index difference Δneff at room temperature increases as the thickness of the selective oxidation layer is thicker and the position of the selective oxidation layer is closer to the active layer. However, when these two influences are compared, the influence degree of the thickness of the selective oxidation layer is much larger. Therefore, the intensity of lateral light confinement at room temperature is mainly determined by the thickness of the selectively oxidized layer.

  In addition, the oxidation confinement diameter that is generally used is 4.0 μm or more. As shown in FIG. 15, when the thickness of the selective oxidation layer is 25 nm or more, the optical confinement coefficient is 0.9 or more. Can be secured.

  FIG. 20 shows the relationship between the thickness of the selective oxidation layer 108 and the droop rate in a surface emitting laser element having a square columnar mesa and an oxidation confinement diameter of 4 μm or more. The droop rate in FIG. 20 was obtained from an optical waveform when driven by a square wave current pulse with a pulse period of 1 ms and a duty of 50%. According to this, when the thickness of the selective oxidation layer decreases, the droop rate decreases exponentially, and the “negative droop characteristic” appears remarkably. In addition, variation in the droop rate for each element becomes remarkable. And in order to make droop rate -10% or more, it is necessary to make the thickness of a selective oxidation layer into 25 nm or more.

  In addition, since the optical confinement factor of the fundamental transverse mode is determined mainly depending on the oxidation confinement diameter and the thickness of the selective oxidation layer, how can the combination of the oxidation confinement diameter and the thickness of the selective oxidation layer be determined? It is important to choose.

  When the inventors tried various fitting methods, the calculation result of FIG. 15 shows that the oxidized constriction diameter (d [μm]) and the thickness of the selective oxidation layer (t [nm]) are variables. As a result, it was possible to perform fitting in these secondary forms. The following equation (4) is the result of fitting the optical confinement factor (Γ) of the fundamental transverse mode in a quadratic form of the oxidized constriction diameter d and the thickness t of the selective oxidation layer. By substituting the specific values of FIG. 15, the optical confinement factor of the fundamental transverse mode of FIG. 15 can be obtained with an error of approximately 1%.

Γ (d, t) = − 2.54d ² −0.14t ² −0.998d · t + 53.4d + 12.9t−216 (4)

  In order to effectively suppress the “negative droop characteristic”, it is necessary to set the optical confinement factor to 0.9 or more. Here, the oxidized confinement diameter (d) at which the optical confinement factor is 0.9 or more. And the combination (range) of the thickness (t) of the selectively oxidized layer can be obtained from the above equation (4). That is, this range is a combination of d and t that satisfies the inequality of Γ (d, t) ≧ 0.9, and more specifically is expressed as the following equation (5).

−2.54d ² −0.14t ² −0.998d · t + 53.4d + 12.9t−216 ≧ 0.9 (5)

  Therefore, by selecting the oxidized constriction diameter (d) and the thickness (t) of the selective oxidation layer so that the above equation (5) is satisfied, the optical confinement coefficient in the fundamental transverse mode becomes 0.9 or more. An element in which the “negative droop characteristic” is suppressed can be obtained.

  It has not been known so far that Δneff affects the droop characteristics, and as described above, this has been revealed for the first time by the inventors of the present application.

  In the step of selectively oxidizing Al (the above step (5)), the oxidation is performed not only in the direction parallel to the substrate surface (here, in the XY plane direction) but also in the vertical direction (here, the Z-axis direction). However, it progresses slightly. Therefore, when the cross section of the mesa after selective oxidation is observed with an electron microscope, the thickness of the oxide layer is not uniform, the thickness at the outer periphery of the mesa (oxidation start portion) is thick, and the oxidation end portion is thin. . However, in the region from the oxidation end portion to the outer peripheral direction of the mesa with a thickness of 2 to 3 μm, the thickness of the oxide layer substantially matches the thickness of the selective oxidation layer. Since the oscillation light is mainly affected by the effective refractive index difference at the oxidation end portion, the thickness of the selective oxidation layer is controlled to a desired value (25 nm or more) in the step (1), thereby oxidizing the oxidation layer. The thickness of the end portion can be set to a desired value.

  Incidentally, in addition to the optical confinement factor, the detuning amount also changes when the internal temperature of the surface emitting laser element changes. Therefore, the relationship between the detuning amount and the “negative droop characteristic” will be described next.

  In the edge-emitting laser element, since the resonance longitudinal mode is densely present, laser oscillation occurs at the gain peak wavelength λg. On the other hand, in the surface emitting laser element, the resonance wavelength is usually one wavelength, and only a single longitudinal mode can exist in the reflection band of the semiconductor DBR. Further, since laser oscillation occurs at the resonance wavelength λr, the emission characteristics of the surface emitting laser element depend on the relationship between the resonance wavelength λr and the gain peak wavelength λg of the active layer.

Here, the detuning amount Δλ ₀ is defined by the following equation (6). λr ₀ is a resonance wavelength, and λg ₀ is a gain peak wavelength. The subscript 0 means a value when CW (Continuous Wave Oscillation) driving is performed with a threshold current at room temperature. Hereinafter, when there is no subscript 0, in other cases, for example, a value when operating at a threshold current or more is meant.

Δλ ₀ = λr ₀ −λg ₀ (6)

FIG. 21 shows the case of Δλ ₀ > 0, and FIG. 22 shows the case of Δλ ₀ <0.

Since the oscillation wavelength is determined not by the gain peak wavelength but by the resonance wavelength, the laser characteristics of the surface emitting laser element greatly depend on the positive / negative of Δλ ₀ and its value. For example, the threshold current at room temperature tends to increase as the absolute value of Δλ ₀ increases.

  Both the resonance wavelength and the gain peak wavelength change to the longer wavelength side as the temperature rises. At this time, the change in the resonance wavelength is caused by the change in the refractive index of the material constituting the resonator structure, and the change in the gain peak wavelength is caused by the change in the band gap energy of the active layer material. However, the rate of change in the band gap energy is about an order of magnitude greater than the rate of change in the refractive index. Therefore, the light emission characteristics at the time of temperature change are determined mainly depending on the amount of change in the gain peak wavelength. Note that the temperature change rate of the resonance wavelength is about 0.05 nm / K, and the change with respect to the temperature can be substantially ignored.

In the surface emitting laser element, when the internal temperature (temperature of the active layer) rises due to a change in injection current or the like, the gain peak wavelength shifts to the long wavelength side. Therefore, when Δλ ₀ > 0 (see FIG. 21), the absolute value of Δλ (degree of detuning) decreases once and then increases.

  In general, in a surface emitting laser element, the oscillation efficiency (light emission efficiency) is highest when the gain peak wavelength and the resonance wavelength coincide with each other.

When Δλ ₀ > 0 and the element temperature (environment temperature) is increased from room temperature and the threshold current is measured, the threshold current starts to decrease as the element temperature increases. The threshold current becomes the minimum value when the gain peak wavelength matches the resonance wavelength, and starts to increase when the temperature is further increased. That is, a temperature at which the threshold current is minimum exists on the higher temperature side than room temperature.

In the case of Δλ ₀ <0 (see FIG. 22), as the internal temperature (the temperature of the active layer) increases, the absolute value of Δλ only increases, so the threshold temperature is increased by increasing the element temperature from room temperature. When measured, the threshold current only increases with increasing device temperature.

In this case, when the element temperature is lowered from room temperature, the gain peak wavelength Δλg is shifted to the short wavelength side. Therefore, when the threshold temperature is measured by lowering the element temperature from room temperature, the threshold current starts to decrease and becomes minimum when the gain peak wavelength and the resonance wavelength coincide. When the temperature is further lowered, the threshold current starts to increase. That is, when Δλ ₀ <0, the temperature at which the threshold current is minimum exists on the lower temperature side than room temperature.

Δλ ₀ is different from each other (Δλ ₀ <0, Δλ ₀ ≈0, Δλ ₀ > 0). The result of measuring the oscillation threshold current of three elements while changing the element temperature (environment temperature) is shown as an example. 23. The vertical axis in FIG. 23 is a value obtained by normalizing the oscillation threshold current (Ith) at each temperature with the oscillation threshold current (Ith (25 ° C.)) at 25 ° C. (room temperature). FIG. 23 shows that the threshold current is lower than room temperature when Δλ ₀ <0, near the room temperature when Δλ ₀ ≈0, and higher than room temperature when Δλ ₀ > 0. It can actually be confirmed that it is minimum.

In conventional surface-emitting laser elements, Δλ ₀ > 0 is usually set so that the threshold current at high temperatures is low in order to prevent degradation of light emission characteristics at high temperature and high power operation.

However, when a conventional surface emitting laser element set to Δλ ₀ > 0 is driven by a square wave current pulse, the IL characteristic shifts to the low current side as the internal temperature increases, and the threshold current decreases. Therefore, the light output at the same drive current value increases with time. That is, a “negative droop characteristic” occurs. On the other hand, when Δλ ₀ <0, the IL characteristic shifts to the high current side as the internal temperature rises, so that the light output does not rise. That is, the “negative droop characteristic” does not occur. Thus, in order to suppress the “negative droop characteristic”, it is necessary to set Δλ ₀ <0 in addition to the thickness of the oxide layer so that the threshold current does not become minimum at a temperature of room temperature or higher. .

In order to set λ ₀ to a desired value, it is necessary to know the gain peak wavelength λg ₀ . In the edge-emitting laser element, the oscillation wavelength coincides with the gain peak wavelength, so that the gain peak wavelength can be known from the oscillation wavelength. However, in the surface emitting laser element, the resonance wavelength is determined by the structure, so that it is difficult to estimate the gain peak wavelength as in the edge emitting laser element.

  Therefore, (1) an edge-emitting laser device having the same active layer is manufactured and a gain peak wavelength is estimated from an oscillation wavelength at room temperature, or (2) a double heterostructure having the same active layer is manufactured, Either a method of estimating the gain peak wavelength from the photoluminescence wavelength (PL wavelength) is taken.

When the method (1) is adopted, as an example, an oxide film stripe type edge emitting laser element having the same active layer structure and a stripe width of 40 μm and a resonator length of 500 μm is manufactured, and the room temperature of the edge emitting laser element is obtained. The wavelength at the threshold current of CW oscillation at ₁ is used as the gain peak wavelength λg ₀ .

  Further, when the method (2) is adopted, the wavelength at the time of laser oscillation is shifted to the long wavelength side (wavelength shift) with respect to the PL wavelength, so adjustment for this is necessary. The wavelength shift is due to differences in excitation processes such as photoexcitation and current excitation, and the influence of heat generated by current in the case of current excitation. In general, the oscillation wavelength of the edge emitting laser element is about 10 nm longer than the PL wavelength λPL. Therefore, the wavelength shift amount in this case is 10 nm.

  Therefore, considering the PL wavelength as a reference, the above equation (6) becomes the following equation (7).

Δλ ₀ = λr ₀ −λg ₀ = λr ₀ − (λPL + 10) = λr ₀ −λPL− 10 (7)

  The wavelength shift amount of 10 nm is a general value, but may be changed according to the material system used.

A plurality of surface-emitting laser elements having different Δλ ₀ were manufactured, and the temperature at which the threshold current in each surface-emitting laser element was minimum was determined. The result is shown in FIG. From FIG. 24, it can be seen that when Δλ ₀ is actually 0, the threshold current is minimum at room temperature.

  Next, a plurality of surface-emitting laser elements having different thicknesses of the selective oxidation layers (30, 31, 34 nm) are manufactured, and each surface-emitting laser element is driven by changing the output of the light pulse, so that the threshold current is minimized. The temperature and droop rate were calculated. 25 and 26 show the relationship between the droop rate and the temperature at which the threshold current is minimized for each thickness of the selective oxidation layer.

  Here, FIG. 25 shows the droop rate when the surface emitting laser element is driven by a current pulse with an optical output of 1.4 mW. FIG. 26 shows the droop rate when the same surface emitting laser element as shown in FIG. 25 is driven by a current pulse with an optical output of 0.3 mW.

  First, comparing FIG. 25 with FIG. 26, it can be seen that the droop rate differs depending on the light output. The smaller the optical output (0.3 mW), the smaller the droop rate, and the “negative droop characteristic” appears remarkably.

  When the optical output is large, the amount of injected current is large and the amount of heat generated by the element is also increased, and it is considered that the influence of output saturation due to heat appears remarkably from the beginning of energization. That is, it is considered that normal droop characteristics appear on a relatively fast time scale. The “negative droop characteristic” is a phenomenon in which the output of the light pulse gradually increases on the time scale from the initial stage of energization to 1 μS. It is thought that “negative droop characteristics” have been improved.

  As described above, even in the same element, the droop rate takes different values by changing the light output of the element, and the “negative droop characteristic” appears more noticeably as the light output is lower.

  By the way, a method of modulating the intensity of an optical pulse is taken in order to express the density of an image in a printer system. Therefore, in order to realize a high-definition image, it is very important that the “negative droop characteristic” is suppressed in a wide output range from low output to high output. As described above, the “negative droop characteristic” becomes more noticeable as the output becomes lower. Therefore, it is very important to suppress the “negative droop characteristic” at the time of low output. This is a problem that has been newly found by the inventors having studied the droop characteristics in detail by changing the driving conditions of the element.

  Next, in FIG. 25 and FIG. 26, the relationship between the droop rate and the thickness of the selective oxidation layer is considered for an element having a temperature at which the threshold current is minimized at 25 ° C. or less. The results of the thickness of the selective oxidation layer of 30 nm and 31 nm overlap in the range of variation, but the droop rate is larger (close to 0) as the thickness of the selective oxidation layer is thicker than the result of 34 nm. It can be seen that the “negative droop characteristic” is suppressed. The broken line A in FIGS. 25 and 26 indicates the average droop rate when the temperature at which the threshold current is minimum is 25 ° C. or less and the thickness of the selective oxidation layer is 34 nm, and the broken line B indicates the threshold current. The average droop rate when the minimum temperature is 25 ° C. or less and the thickness of the selectively oxidized layer is 30 nm and 31 nm is shown. This is because, as described above, the thicker the selective oxidation layer is, the larger the optical confinement coefficient by the oxide layer is, and the fundamental transverse mode is more stable with respect to temperature changes.

  As described above, the standard of the droop rate at which the “negative droop characteristic” starts to affect the image quality is −10%. If the droop rate is smaller than this, there is a problem that a part of the image becomes unclear at a high frequency. As shown in FIG. 25, when the optical output is 1.4 mW, although there is some variation, the average droop rate in the element having the thickness of the selective oxidation layer of 34 nm is about −3%. Since the average droop rate in the elements having the selective oxide layer thickness of 30 nm and 31 nm is about −5%, from these change rates, if the thickness of the selective oxidation layer is 25 nm or more, −10% or more The droop rate can be obtained.

  In addition, as shown in FIG. 26, when the optical output is 0.3 mW, although there is some variation, the average droop rate in the element having the thickness of the selective oxidation layer of 34 nm is about −5%. Since the average droop rate in the elements having the thickness of the selective oxidation layer of 30 nm and 31 nm is about −7%, from these change rates, if the thickness of the selective oxidation layer is 25 nm or more, the − A droop rate of 10% or more can be obtained.

  As described above, in an element in which the thickness of the selective oxidation layer is 25 nm or more and the temperature at which the oscillation threshold current is minimum is 25 ° C. or less, in the wide output range from low output to high output, it is approximately −10%. It is possible to obtain an element having the above droop rate.

  A surface-emitting laser element in which the temperature at which the oscillation threshold current is minimum is higher than room temperature (25 ° C.) is an element whose oscillation efficiency is improved when the temperature of the active layer is increased by current injection. As shown, the “negative droop characteristic” appears. As shown in FIG. 26, the tendency is remarkable in the case of driving with a pulse current with an optical output of 0.3 mW.

  In common with the temperature (detuning amount) at which the optical confinement factor and the threshold current are minimized, in order to suppress “negative droop characteristics”, when the temperature of the active layer rises, That is, it is important to set the surface emitting laser element so that the efficiency (light emitting efficiency) is not improved. Even in an element in which the thickness of the selective oxidation layer is set to be somewhat thick, it is fundamental that “negative droop characteristics” are more likely to occur as the temperature at which the threshold current is minimized is set higher. Inevitable.

  As shown in FIG. 26, when an element having a minimum threshold current temperature of 25 ° C. or more is driven with a current pulse with an optical output of 0.3 mW, “negative droop characteristics” begin to appear prominently. However, if the temperature at which the threshold current is minimum is 35 ° C. or less and the thickness of the selective oxidation layer is 30 nm or more, a droop rate of −10% or more is obtained in terms of the average value.

  Further, as shown in FIG. 25, when driven by a current pulse with an optical output of 1.4 mW, the thickness of any selectively oxidized layer within the temperature range where the threshold current shown in FIG. 25 is minimized. Even at (30 nm, 31 nm, 34 nm), a droop rate of −10% or more is obtained.

  In other words, when these are considered together, if the thickness of the selective oxidation layer is 30 nm or more, an element having a minimum threshold current of 35 ° C. or less has a droop rate of −10% or more in a wide output range. It is possible to obtain. By using these surface emitting laser elements as a writing light source of a printer, a high-definition image without density unevenness can be obtained. Referring to FIG. 24, the detuning amount at room temperature of the element whose temperature at which the threshold current is minimized is 35 ° C. is about 4 nm.

  By the way, in order to use the surface emitting laser element as a writing light source, it is advantageous that the single fundamental transverse mode output is large. In order to increase the single fundamental transverse mode output, it is effective to weaken the optical confinement. This is contrary to the suppression of the “negative droop characteristic”.

  Therefore, the inventors have investigated the relationship between the structure of the resonator structure and the optical confinement strength in the surface emitting laser element in order to improve the single fundamental transverse mode output while maintaining the effect of suppressing the “negative droop characteristic”. Detailed examination was conducted. As a result, it has been found that it is effective to provide a light confinement reduction region described below in the lower semiconductor DBR (substrate-side n-type multilayer mirror) in order to improve the above two characteristics simultaneously.

  The effect of the optical confinement reduction region will be described.

The fundamental transverse mode optical confinement coefficient at room temperature (300 K) of a conventional surface emitting laser element without an optical confinement reduction region and a surface emitting laser device having an optical confinement reduction region was calculated. The result is shown in FIG. Each surface emitting laser element used for the calculation here has an oscillation wavelength of 780 nm band, and its basic structure is n-AlAs (low refractive index layer) / n-Al _0.3 Ga _0.7 As ( 40.5 pairs of lower semiconductor DBRs (substrate-side n-type multilayer mirrors) paired with a high refractive index layer) and p-Al _0.9 Ga _0.1 As / p-Al _0.3 Ga _{0. It has} 24 pairs of upper semiconductor DBRs (outgoing side p-type multilayer mirrors) in which ₇ As is paired and a spacer layer of Al _0.6 Ga _0.4 As. The active layer has a triplet well structure of Al _0.12 Ga _0.88 As / Al _0.3 Ga _0.7 As and is provided at the center of the spacer layer. The selective oxidation layer is provided in the upper semiconductor DBR at a position corresponding to the third standing wave node from the active layer. The oxide layer has a thickness of 28 nm and an oxidized constriction diameter of 4 μm.

  As shown in FIG. 28, the conventional surface-emitting laser element has a mesa post structure in which the lower semiconductor DBR is processed into a circle having a diameter of 25 μm. Further, in the surface emitting laser element having the light confinement reduction region, as shown in FIG. 29, the light confinement reduction region is provided in a region in contact with the lower semiconductor DBR.

  The structure of the lower semiconductor DBR in the conventional surface emitting laser element is shown in FIG. Each refractive index layer has an optical thickness of λ / 4. The structure of the lower semiconductor DBR in the surface emitting laser element having the optical confinement reduction region is shown in FIG. The optical confinement reduction region has three pairs of a high refractive index layer having an optical thickness of 3λ / 4 and a low refractive index layer having a wavelength of λ / 4. In addition, the light confinement reduction region composed of a pair of a high refractive index layer having an optical thickness of 3λ / 4 and a low refractive index layer having a wavelength of λ / 4 is hereinafter also referred to as “light confinement reduction region A” for convenience. .

  The number of pairs of the lower semiconductor DBRs in the conventional surface emitting laser element and the surface emitting laser element having the light confinement reduction region A is the same. Further, the high refractive index layer included in the light confinement reduction region A has an optical thickness that is an odd multiple of λ / 4 and satisfies the multiple reflection phase condition, so that absorption of free carriers and the like of the semiconductor material is considered. Otherwise, the vertical direction (Z-axis direction) has a reflectance equivalent to that of the lower semiconductor DBR in the conventional surface emitting laser element.

  FIG. 27 shows the results for the case where the optical confinement reduction region A is 1 to 3 pairs. Here, the number of pairs of 0 indicates a conventional surface emitting laser element.

  According to FIG. 27, by providing the optical confinement reduction region A, the optical confinement factor of the fundamental transverse mode is reduced as compared with the conventional surface emitting laser element. Further, as the number of pairs in the light confinement reduction region A increases, the light confinement factor decreases.

FIG. 32 shows another light confinement reduction region. This optical confinement reduction region has a pair of a high refractive index layer (Al _0.3 Ga _0.7 As) having an optical thickness of λ / 4 and a low refractive index layer (AlAs) having a wavelength of 3λ / 4. The rate layer is formed thicker than the conventional surface emitting laser element. In addition, the light confinement reduction region composed of a pair of a high refractive index layer having an optical thickness of λ / 4 and a low refractive index layer having a wavelength of 3λ / 4 is also referred to as “light confinement reduction region B” for convenience hereinafter. .

  FIG. 33 shows the optical confinement factor of the fundamental transverse mode at room temperature (300 K) of the surface emitting laser element having the light confinement reduction region B.

  According to FIG. 33, by providing the light confinement reduction region B, the light confinement factor in the fundamental transverse mode is reduced as compared with the conventional surface emitting laser element, similarly to the light confinement reduction region A. Further, as the number of pairs in the optical confinement reduction region B increases, the optical confinement factor decreases. Note that, when compared with the same number of pairs, it can be seen that the light confinement reduction region B has a greater reduction effect than the light confinement reduction region A. By the way, AlAs has the highest thermal conductivity among AlGaAs mixed crystals which are materials for semiconductor multilayer mirrors. When the layer made of AlAs is provided thick, thermal diffusion in the lateral direction is improved, and there is an effect of reducing the temperature rise in the active layer. As a result, the temperature rise in the central portion of the element is alleviated and the change in the effective refractive index difference is reduced, so that the effect of suppressing negative droop characteristics can also be obtained.

FIG. 34 shows still another light confinement reduction region. This optical confinement reduction region C is a pair of a high refractive index layer (Al _0.3 Ga _0.7 As) with an optical thickness of 3λ / 4 and a low refractive index layer (AlAs) with a 3λ / 4. Both the refractive index layer and the high refractive index layer are formed thicker than the conventional surface emitting laser element. As described above, the light confinement reduction region composed of a pair of a high refractive index layer having an optical thickness of 3λ / 4 and a low refractive index layer having a 3λ / 4 is also referred to as “light confinement reduction region C” for convenience. .

  FIG. 35 shows the optical confinement factor of the fundamental transverse mode at room temperature (300 K) of the surface emitting laser element having the optical confinement reduction region C.

  According to FIG. 35, by providing the optical confinement reduction region C, the fundamental lateral mode optical confinement factor is reduced as compared with the conventional surface emitting laser element, as in the optical confinement reduction region A. Further, as the number of pairs in the light confinement reduction region C increases, the light confinement factor decreases. Note that, when compared with the same number of pairs, it can be seen that the light confinement reduction region C has a greater reduction effect than the light confinement reduction region A and the light confinement reduction region B.

  As described above, the inventors more effectively reduce the optical confinement coefficient of the fundamental transverse mode as the thickness of the high refractive index layer and the low refractive index layer in the light confinement reduction region is increased and the number of pairs is increased. It was found that an effect can be obtained.

  Further, it should be noted that this light confinement reduction effect has the effect of improving the single fundamental transverse mode output without degrading the “negative droop characteristic” as will be described later. Generally speaking, the reduction of the optical confinement factor is advantageous in improving the output of the single fundamental transverse mode, but it is expected that the stability of the transverse mode will deteriorate and the “negative droop characteristic” will likely occur. The

FIG. 36 shows the results of an experimental investigation of the relationship between the single fundamental transverse mode output and the droop rate of a surface emitting laser element having an oscillation wavelength band of 780 nm. The black circles in FIG. 36 are the results of the conventional surface emitting laser element, and the white circles are the results of the surface emitting laser element having the light confinement reduction region B. All areas of the current passing region are 16 μm ² . In FIG. 36, “28” indicates the case where the thickness of the selective oxidation layer is 28 nm, and “30” indicates the case where the thickness of the selective oxidation layer is 30 nm.

  The “negative droop characteristic” and the single fundamental transverse mode output are both related to the optical confinement factor, and these characteristics are mutually contradictory as shown in FIG. For example, as the optical confinement factor is larger, the stability of the transverse mode is improved, so the “negative droop characteristic” is suppressed. In addition, when the optical confinement factor is increased, confinement in the higher-order transverse mode is improved and oscillation is facilitated, so that the single fundamental transverse mode output is lowered. If the droop rate and the single fundamental transverse mode output are determined merely by the optical confinement factor, the correlation should be expressed by one straight line regardless of the presence or absence of the optical confinement reduction region.

  However, as shown in FIG. 36, when the light confinement reduction region is provided and when the light confinement reduction region is not provided, the correlation represented by mutually different straight lines is shown. Furthermore, even with element structures having the same droop rate, when the light confinement reduction region is provided, a higher single fundamental transverse mode output is obtained than when the light confinement reduction region is not provided. That is, this indicates that the optical confinement reduction region has an effect of improving the single fundamental transverse mode output without affecting the droop rate.

  In this way, the inventors provide a light confinement reduction region, set the detuning amount so that the temperature at which the threshold current becomes minimum is 25 ° C. or less, and set the thickness of the selective oxidation layer to 25 nm or more. Thus, it has been newly found that it is possible to effectively suppress the “negative droop characteristic” and improve the single fundamental transverse mode output while maintaining the droop characteristic well.

  In the surface emitting laser element 100, a third lower semiconductor DBR 1033 is provided between the optical confinement reduction region and the resonator structure. This role will be described below.

  In general, light absorption by free carriers exists in a semiconductor material. This light absorption increases in proportion to the electric field strength of light and the free carrier concentration. The energy of the light absorbed by the free carriers becomes the kinetic energy of the carriers and is finally converted into the energy of lattice vibration. This increases the oscillation threshold current and decreases the external differential quantum efficiency (slope efficiency).

  In addition, the semiconductor DBR has a large reflection effect (high reflectivity) by being superimposed so that the reflected light from the interface of each refractive index layer has the same phase and the opposite phase to the incident light. ing. At this time, since reflection occurs only at the interface, the electric field strength (amplitude) in each refractive index layer is not attenuated and is constant. In the laser oscillation state, the electric field distribution in the semiconductor DBR generates a standing wave, and nodes and antinodes appear alternately for each optical thickness of λ / 4. In either case, in an ordinary semiconductor DBR composed of a low refractive index layer and a high refractive index layer having an optical thickness of λ / 4, the interface between the low refractive index layer and the high refractive index layer corresponds to the nodes and antinodes of the standing wave. Position. Further, in the region close to the resonator structure, the intensity of the standing wave of the oscillation light is large, and light absorption by free carriers in this region is remarkable.

  Here, when a pair of a low refractive index layer and a high refractive index layer having an optical thickness of λ / 4 is provided between the optical confinement reduction region and the resonator structure as shown in FIG. The reduction effect of absorption loss was calculated. Hereinafter, for convenience, a pair of a low refractive index layer and a high refractive index layer each having an optical thickness of λ / 4 provided between the light confinement reduction region and the resonator structure is referred to as an “absorption loss reduction layer”. Also called.

As a result, when the absorption loss in the conventional semiconductor DBR (see FIG. 30) is 100%, when a pair of absorption loss reduction layers is provided as shown in FIG. Compared to FIG. 32), the absorption loss could be reduced by 23%. Further, when three pairs of absorption loss reduction layers were provided, the absorption loss could be reduced by 54% compared to the case where no layer was provided. Here, in the calculation, the n-type carrier concentration was changed within the range of ³ to 5 × 10 18 cm ^{−3 and} the number of pairs included in the optical confinement reduction region was changed within the range of 1 to 5 pairs. It didn't change.

  The surface emitting laser element 100 reduces light absorption by free carriers by providing a pair of absorption loss reduction layers. In other words, in the light confinement reduction region, since the thickness of the low refractive index layer is increased, the thickness itself increases and the increase in the thickness includes extra antinodes of standing waves. Light absorption by carriers increases. In this case, by providing a pair of absorption loss reduction layers, the electric field strength in the optical confinement reduction region can be reduced and the absorption loss can be reduced.

In FIG. 39, the high refractive index layer is Al _0.3 Ga _0.7 As, the low refractive index layer is AlAs, the light confinement reduction region A (type A), the light confinement reduction region B (type B), and the light confinement reduction region. For C (typeC), the optical confinement factor of the fundamental transverse mode with and without a pair of absorption loss reduction layers is shown. Note that the number of pairs in each optical confinement reduction region is three.

  FIG. 39 shows the reduction rate of the optical confinement factor in each structure. The reduction rate of the optical confinement factor is the percentage of reduction of the optical confinement factor of each structure when the fundamental transverse mode optical confinement factor in the conventional semiconductor DBR (see FIG. 30) is 100%. It is a representation.

  From FIG. 39, it can be seen that the change in the optical confinement coefficient due to the provision of one pair of absorption loss reduction layers is slight.

Also, a surface emitting laser element VCSEL1 (see FIG. 30) in which no optical confinement reduction region is provided, and a surface emitting laser element VCSEL2 (in FIG. 30) in which an optical confinement reduction region B is provided but no absorption loss reduction layer is provided. 32), and a surface emitting laser element VCSEL3 (see FIG. 38) provided with the optical confinement reduction region B and a pair of absorption loss reduction layers was actually fabricated and subjected to comparative evaluation. Each surface emitting laser element has an oscillation wavelength in the 780 nm band, and the area of the current passing region is all 16 μm ² . The number of pairs of the high refractive index layer and the low refractive index layer in the lower semiconductor DBR of each surface emitting laser element is the same.

  As shown in FIG. 40, in the surface-emitting laser element VCSEL2 and the surface-emitting laser element VCSEL3, it was confirmed that the clear single basic transverse mode output was substantially equivalent to that of the surface-emitting laser element VCSEL1. In FIG. 40, “30” is the case where the thickness of the selective oxidation layer is 30 nm, and “28” is the case where the thickness of the selective oxidation layer is 28 nm.

  40, the surface emitting laser element VCSEL2 and the surface emitting laser element VCSEL3 have a single fundamental transverse mode output of about 0.3 to 0.5 mW as compared with the surface emitting laser element VCSEL1 at the same droop rate. The improvement can be confirmed.

  FIG. 41 shows the relationship between the single fundamental transverse mode output (calculated value) and the fundamental mode optical confinement factor (calculated value). The meanings of symbols in FIG. 41 are the same as those in FIG. Also, the meanings of “30” and “28” in FIG. 41 are the same as those in FIG.

  41, it can be confirmed that the surface emitting laser element VCSEL3 has a clear reduction in the optical confinement factor and a corresponding improvement in the single fundamental transverse mode output as compared with the surface emitting laser element VCSEL1.

  Further, the surface emitting laser element VCSEL3 was able to obtain a lower threshold current and a higher external differential quantum efficiency (slope efficiency) than the surface emitting laser element VCSEL2. This is because absorption loss has been reduced.

  As described above, the surface emitting laser element 100 according to the present embodiment includes the resonator structure including the active layer 105, the lower semiconductor DBR 103 and the upper semiconductor DBR 107 provided with the resonator structure interposed therebetween. Yes. In the upper semiconductor DBR 107, an oxide layer 108a containing at least an oxide generated by oxidizing a part of a 30 nm thick selective oxidation layer containing aluminum surrounds the current passing region 108b, and a lateral mode of injection current and oscillation light It contains a constriction structure that can be confined simultaneously.

  The lower semiconductor DBR 103 includes a second lower semiconductor DBR 1032 that is provided on the substrate 101 side with respect to the resonator structure and serves as an optical confinement reduction region that reduces optical confinement in the lateral direction. This suppresses the “negative droop characteristic” and enables high output operation in single fundamental transverse mode oscillation.

Further, the PL wavelength of the active layer is set to 772 nm with respect to the resonance wavelength of 780 nm of the resonator, the detuning amount Δλ ₀ at room temperature is set to −2 nm, and the threshold current is minimized at 17 ° C. Yes. Thereby, the “negative droop characteristic” can be further suppressed.

  Further, since the second lower semiconductor DBR 1032 is set so that the optical thickness of the low refractive index layer 103a is 3λ / 4, the interface of each refractive index layer corresponds to the antinodes or nodes of the standing wave. It can be a position. Note that the optical thickness of the low refractive index layer 103a in the second lower semiconductor DBR 1032 may be (2n + 1) λ / 4 using an integer n of 1 or more. Also in this case, the interface of each refractive index layer can be set at a position corresponding to the antinode or node of the standing wave.

  Further, when a square wave current pulse having a pulse period of 1 ms and a pulse width of 500 μs is supplied, since (P1−P2) /P2=−0.06, the “negative droop characteristic” can be further suppressed. .

  In addition, since the third lower semiconductor DBR 1033 is provided between the second lower semiconductor DBR 1032 and the resonator structure, absorption loss can be reduced.

  According to the optical scanning device 1010 according to the present embodiment, since the light source 14 includes the surface emitting laser element 100, high-precision optical scanning can be performed.

  Further, according to the laser printer 1000 according to the present embodiment, since the optical scanning device 1010 is provided, a high quality image can be formed.

  In the above embodiment, the case where the third lower semiconductor DBR 1033 has one pair of the low refractive index layer 103a and the high refractive index layer 103b has been described, but the present invention is not limited to this.

  In the above embodiment, when it is not necessary to consider the absorption loss, the second lower semiconductor DBR 1032 may be adjacent to the resonator structure as shown in FIG.

  In the above embodiment, the case where the second lower semiconductor DBR 1032 has three pairs of the low refractive index layer 103a and the high refractive index layer 103b has been described, but the present invention is not limited to this.

  In the above embodiment, the fourth lower semiconductor DBR 1034 shown in FIG. 43 may be used in place of the second lower semiconductor DBR 1032. The fourth lower semiconductor DBR 1034 has three pairs of a low refractive index layer 103a and a high refractive index layer 103b. Like the light confinement reduction region B, the low refractive index layer 103a has an optical thickness of λ / 4. The high refractive index layer 103b is set to have an optical thickness of 3λ / 4.

  In the above embodiment, the fifth lower semiconductor DBR 1035 shown in FIG. 44 may be used instead of the second lower semiconductor DBR 1032. The fifth lower semiconductor DBR 1035 has three pairs of a low refractive index layer 103a and a high refractive index layer 103b. Like the optical confinement reduction region C, the low refractive index layer 103a has an optical thickness of 3λ / 4. The high refractive index layer 103b is set to have an optical thickness of 3λ / 4.

  In the above embodiment, instead of the surface emitting laser element 100, a surface emitting laser array 500 may be used as shown in FIG. 45 as an example.

  In the surface emitting laser array 500, a plurality of (herein, 32) light emitting portions are two-dimensionally arranged and formed on the same substrate. Here, the M direction in FIG. 45 is the main scanning corresponding direction, and the S direction is the sub scanning corresponding direction. Note that the number of light emitting units is not limited to 32.

  Here, the plurality of light emitting units are arranged such that the intervals between adjacent light emitting units are equal to c when all the light emitting units are orthogonally projected onto a virtual line extending in the S direction. In this specification, the “light emitting portion interval” refers to the distance between the centers of two light emitting portions.

  Here, the interval c is 3 μm, the interval d is 24 μm, and the light emitting portion interval X (see FIG. 46) in the M direction is 30 μm.

  Each light emitting section has the same structure as the surface emitting laser element 100 described above, as shown in FIG. The surface emitting laser array 500 can be manufactured by a method similar to that of the surface emitting laser element 100 described above.

  Thus, since the surface emitting laser array 500 is a surface emitting laser array in which the surface emitting laser elements 100 are integrated, the same effect as the surface emitting laser element 100 can be obtained. In particular, as an effect in the case of an array, the variation in thickness profile of the oxide layer 108a and the oxidation distance between the light emitting portions is extremely small. In addition, the variation in the life of each light emitting unit is small and long.

  In this case, in the surface-emitting laser array 500, since all the light emitting portions are orthogonally projected on the virtual line extending in the sub-scanning corresponding direction, the light emitting portion intervals are equal intervals c. On the body drum 1030, it can be understood that the configuration is the same as the case where the light emitting units are arranged at equal intervals in the sub-scanning direction.

  Since the distance c is 3 μm, if the magnification of the optical system of the optical scanning device 1010 is about 1.8, high-density writing of 4800 dpi (dot / inch) can be performed. Of course, it is possible to increase the density by increasing the number of light emitting portions in the main scanning direction, making the array arrangement in which the distance d is narrowed and the distance c is further reduced, or by reducing the magnification of the optical system. Quality printing is possible. Note that the writing interval in the main scanning direction can be easily controlled by the lighting timing of the light emitting unit.

  In this case, the laser printer 1000 can perform printing without decreasing the printing speed even if the writing dot density increases. Further, when the writing dot density is the same, the printing speed can be further increased.

  By the way, it is preferable that the groove | channel between two light emission parts shall be 5 micrometers or more for the electrical and spatial separation of each light emission part. This is because if it is too narrow, it becomes difficult to control etching during production. Further, the mesa size (length of one side) is preferably 10 μm or more. This is because if it is too small, heat will be accumulated during operation and the characteristics may be deteriorated.

  In the above embodiment, the case where the mesa shape in the cross section orthogonal to the laser oscillation direction is square has been described, but the present invention is not limited to this, and may be any shape such as a circle, an ellipse, or a rectangle. Can do.

  Moreover, although the said embodiment demonstrated the case where the oscillation wavelength of a light emission part was a 780 nm band, it is not limited to this. The oscillation wavelength of the light emitting unit may be changed according to the characteristics of the photoreceptor.

  The surface emitting laser element 100 and the surface emitting laser array 500 can be used for applications other than the image forming apparatus. In that case, the oscillation wavelength may be a wavelength band such as a 650 nm band, an 850 nm band, a 980 nm band, a 1.3 μm band, or a 1.5 μm band depending on the application.

  In the above embodiment, a surface emitting laser array in which light emitting units similar to those of the surface emitting laser element 100 are arranged one-dimensionally may be used instead of the surface emitting laser element 100.

  In the above embodiment, the laser printer 1000 is described as the image forming apparatus. However, the present invention is not limited to this.

  For example, an image forming apparatus that directly irradiates laser light onto a medium (for example, paper) that develops color with laser light may be used.

  Further, an image forming apparatus using a silver salt film as the image carrier may be used. In this case, a latent image is formed on the silver salt film by optical scanning, and this latent image can be visualized by a process equivalent to a developing process in a normal silver salt photographic process. Then, it can be transferred to photographic paper by a process equivalent to a printing process in a normal silver salt photographic process. Such an image forming apparatus can be implemented as an optical plate making apparatus or an optical drawing apparatus that draws a CT scan image or the like.

  As an example, as shown in FIG. 48, a color printer 2000 including a plurality of photosensitive drums may be used.

  The color printer 2000 is a tandem multicolor printer that forms a full-color image by superimposing four colors (black, cyan, magenta, and yellow). The black “photosensitive drum K1, charging device K2, "Developing device K4, cleaning unit K5, and transfer device K6", cyan "photosensitive drum C1, charging device C2, developing device C4, cleaning unit C5, and transfer device C6", and magenta "photosensitive drum" M1, charging device M2, developing device M4, cleaning unit M5, and transfer device M6 ”,“ photosensitive drum Y1, charging device Y2, developing device Y4, cleaning unit Y5, and transfer device Y6 ”for yellow, and light A scanning device 2010, a transfer belt 2080, a fixing unit 2030, and the like are provided.

  Each photoconductor drum rotates in the direction of the arrow in FIG. 48, and a charging device, a developing device, a transfer device, and a cleaning unit are arranged around each photoconductor drum along the rotation direction. Each charging device uniformly charges the surface of the corresponding photosensitive drum. The surface of each photoconductive drum charged by the charging device is irradiated with light by the optical scanning device 2010, and a latent image is formed on each photoconductive drum. Then, a toner image is formed on the surface of each photosensitive drum by a corresponding developing device. Further, the toner image of each color is transferred onto the recording paper on the transfer belt 2080 by the corresponding transfer device, and finally the image is fixed on the recording paper by the fixing unit 2030.

  The optical scanning device 2010 has a light source similar to the light source 14 for each color. Therefore, the same effect as that of the optical scanning device 1010 can be obtained. In addition, since the color printer 2000 includes the optical scanning device 2010, the same effect as the laser printer 1000 can be obtained.

  By the way, in the color printer 2000, color misregistration may occur due to manufacturing error or position error of each component. Even in such a case, if each light source of the optical scanning device 2010 has a surface-emitting laser array similar to the surface-emitting laser array 500, color misregistration is reduced by changing the light-emitting unit to be lit. can do.

  As described above, according to the surface emitting laser element and the surface emitting laser array of the present invention, it is suitable for suppressing the “negative droop characteristic” and enabling high output operation in single fundamental transverse mode oscillation. ing. Moreover, the optical scanning device of the present invention is suitable for performing high-precision optical scanning. The image forming apparatus of the present invention is suitable for forming a high quality image.

DESCRIPTION OF SYMBOLS 11a ... Deflector side scanning lens (a part of scanning optical system), 11b ... Image surface side scanning lens (a part of scanning optical system), 13 ... Polygon mirror (deflector), 14 ... Light source, 100 ... Surface emitting laser Element 101 ... Substrate 103 ... Lower semiconductor DBR (part of semiconductor multilayer reflector), 103 ₂ ... Second lower semiconductor DBR (light confinement reduction region), 103 ₄ ... Fourth lower semiconductor DBR (light confinement) Reduction region), 103 ₅ ... fifth lower semiconductor DBR (light confinement reduction region), 104 ... lower spacer layer (part of the resonator structure), 105 ... active layer, 106 ... upper spacer layer (resonator structure) , 107... Upper semiconductor DBR (part of semiconductor multilayer reflector), 108 a... Oxidized layer (part of constricted structure), 108 b... Current passing region (part of constricted structure), 500. Surface emitting array Array, 1000 ... Laser printer (image forming apparatus), 1010 ... Optical scanning apparatus, 1030 ... Photosensitive drum (image carrier), 2000 ... Color printer (image forming apparatus), 2010 ... Optical scanning apparatus, K1, C1, M1 , Y1... Photosensitive drum (image carrier).

JP 11-48520 A

K. D. Choquette, R.A. P. Schneider, Jr. K. L. Lear, K.M. M.M. Geib, “Low threshold voltage vertical-cavity lasers fabricated by selective oxidation,” “Electronics Letters, No. 4”. 24, Vol. 30, p. 2043-2044, 1994 H. Nakayama, T .; Nakamura, M .; Funada, Y. et al. Ohashi, M .; Kato, “780 nm VCSELs for Home Networks and Printers”, Electronic Components and Technology Conference Processings, 54th, Vol. 2, June, 2004, p. 1371-1375

Claims (8)

A surface emitting laser element made of an AlGaAs-based material that outputs light in a direction perpendicular to a substrate,
A resonator structure including an active layer;
An oxide including at least an oxide formed by oxidizing a part of the selective oxidation layer containing aluminum and arsenic is provided between the resonator structure and surrounds the current passing region, and the injected current and the oscillation light A semiconductor multilayer reflector including therein a constriction structure capable of simultaneously confining a transverse mode;
The semiconductor multilayer mirror has a light confinement reduction unit that reduces light confinement in the lateral direction on the substrate side with respect to the resonator structure.
The semiconductor multilayer film reflector has a plurality of pairs in which a low refractive index layer and a high refractive index layer are paired,
The optical confinement reduction unit has at least one pair of the pair, and an optical thickness of at least one of the low refractive index layer and the high refractive index layer in the at least one pair is an oscillation wavelength λ, an integer n of 1 or more (2n + 1) λ / 4, and
By setting the thickness of the selective oxidation layer to at least 28 nm , the diameter of the current passage region to 4.0 μm or more, and the temperature at which the oscillation threshold current is minimized to 25 ° C. or less , negative droop characteristics can be obtained. Suppress,
When a square wave current pulse having a pulse period of 1 ms and a pulse width of 500 μs is supplied,
A surface-emitting laser element satisfying a relationship of 0 ≧ (P1−P2) /P2≧−0.1 using the light output P1 at 10 ns after supply and the light output P2 at 1 μs after supply.   The semiconductor multilayer mirror has at least one pair of the pair between the optical confinement reduction unit and the resonator structure, and the optical thicknesses of the low refractive index layer and the high refractive index layer in the at least one pair. The surface emitting laser element according to claim 1, wherein is λ / 4. Using the width d [μm] of the current passing region and the thickness t [nm] of the oxide surrounding the current passing region, −2.54d ² −0.14t ² −0.998d · t + 53.4d + 12.9t The surface emitting laser element according to claim 1, wherein a relationship of −216 ≧ 0.9 is satisfied.   A surface emitting laser array in which the surface emitting laser elements according to claim 1 are integrated. An optical scanning device that scans a surface to be scanned with light,
A light source comprising the surface-emitting laser element according to claim 1;
A deflector for deflecting light from the light source;
A scanning optical system for condensing the light deflected by the deflector onto the surface to be scanned. An optical scanning device that scans a surface to be scanned with light,
A light source comprising the surface emitting laser array according to claim 4;
A deflector for deflecting light from the light source;
A scanning optical system for condensing the light deflected by the deflector onto the surface to be scanned. At least one image carrier;
An image forming apparatus comprising: at least one optical scanning device according to claim 5 or 6 that scans the at least one image carrier with light including image information. The image forming apparatus according to claim 7, wherein the image information is multicolor color image information.

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