GB2313234A

GB2313234A - Laser diode array

Info

Publication number: GB2313234A
Application number: GB9711573A
Authority: GB
Inventors: Takushi Itagaki
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1993-07-20
Filing date: 1994-07-19
Publication date: 1997-11-19
Also published as: GB9711573D0

Abstract

The semiconductor laser diodes 20, each including a primary diffraction grating 31, are radially arranged on an InP substrate 10 such that the primary diffraction grating faces a through hole 10a formed in the substrate. An optical fibre including a 45{ conic reflecting mirror 32a redirects light along the optical fibre. Alternatively, surface emission of laser light may be obtained using a prismatic element having an inclined conic recess which redirects light away from the surface of the substrate (figure 10).

Description

SURFACE-EMITTING LASER DIODE ARRAY AND DRIVING METHOD THEREOF, PHOTODETECTOR, PHOTODETECTOR ARRAY, OPTICAL INTERCONNECTION SYSTEM, AND MULTIWAVELENGTH OPTICAL COMMUNICATION SYSTEM FIELD OF TF INVENTION The present invention relates to a surface-emitting laser diode array and a method for driving the laser diode array. The invention also relates to a photodetector, a photodetector array, an optical interconnection system, and a multiwavelength optical communication system.

BACKGROUND OP E TMVPNTTOM Surface emitting laser diodes (hereinafter referred to as surface-emitting LDs) are classified into three types by shapes of cavity resonators: vertical cavity surface emitting LD, horizontal cavity surface-emitting LD, and bending cavity surface-emitting LD, as disclosed in Journal of Electronic Information Communication Institute, C-I, Vol.

J75-C-I, No. 5, pp. 245 - 256 (May 1992). These surfaceemitting LDs emit laser light in a direction perpendicular to a main surface of the substrate whereas conventional LDs emit laser light from a facet of the substrate.

Among those three types of surface-emitting laser diodes, the horizontal cavity surface-emitting LD is produced in a relatively simple process using a technique of fabricating a diffraction grating in an optical waveguide, which technique is usually employed in production of a DFB (Distributed Feedback) laser diode or a DBR (Distributed Bragg Reflector) laser diode. That is, the horizontal cavity LD is produced so that the diffraction grating is formed at second order, i.e., at a period satisfying k0(wavelength of laser light)/n(effective refractive index of waveguide). In the following description, this secondorder diffraction grating is called a secondary diffraction grating.

Figure 15 is a perspective view illustrating a prior art surface-emitting DBR laser disclosed in Applied Physics Letters, 50(24), 15 June 1987, pp.1705 - 1707. In the figure, a surface-emitting DBR laser 150 includes an n type GaAs substrate 52. There are successively disposed on the n type GaAs substrate 52 an n type GaAs buffer layer 53, an n type AlGaAs cladding layer 54, and a multiquantum well (hereinafter referred to as MOW) lightguide layer 55. The MQW lightguide layer 55 comprises alternating GaAs well layers and AlGaAs barrier layers. A p type AlGaAs cladding layer 56 having a stripe-shaped secondary diffraction grating 58 is disposed on the lightguide layer 55. A p type GaAs contact layer 59 is disposed on a part of the secondary diffraction grating 58 in a laser oscillation region 150A.

Insulating films 57 are disposed on the cladding layer 56 at opposite sides of the stripe-shaped secondary diffraction grating 58, on the top surface of the diffraction grating 58 except for that in the laser oscillation region 150A, and on the opposite side surfaces of the diffraction grating 58 and the contact layer 59 in the'laser oscillation region 150A.

A p side electrode 61 is disposed on the insulating film 57 and the contact layer 59 in the laser oscillation region 150A. A facet of the laser oscillation region 150A is coated with a high reflectivity film 60.

A description is given of the operation.

When current is injected into the laser oscillation region 150A through the p side electrode 61, fundamental laser oscillation occurs. When the laser light reaches into the p type AlGaAs cladding layer 56 including the secondary diffraction grating 58, i.e., the optical waveguide, a maximum reflectivity is obtained at a wavelength determined by the period of the secondary diffraction grating 58, and a resonator is produced between the lightguide layer 55 and the high reflectivity coating film 60, whereby the laser oscillates in single and longitudinal mode. The secondary diffraction grating 58 converts laser light parallel to the surface of the substrate 52 into laser light perpendicular to the surface of the substrate and outputs the laser light.

When a plurality of horizontal cavity LDs, such as the above-described surface-emitting DBR LDs 150, are twodimensionally integrated on the same substrate, a surfaceemitting DBR LD array is realized.

Figure 16 is a perspective view illustrating a prior art surface-emitting DBR LD array disclosed in Electronics Letters, Vol.24, No.5, 1988; p.283. In the figure, the same reference numerals as in figure 15 designate the same or corresponding parts. A two-dimensional surface-emitting DBR LD array 160 includes three surface-emitting DBR LDs respectively including secondary diffracting gratings 58a, 58b, and 58c and arranged parallel to each other on the same substrate 52, whereby output power of laser light is increased.

In this prior art surface-emitting DBR LD array 160, an output laser light from the LD array 160 comprises laser lights emitted from the respective laser resonators, so that higher output power is obtained compared to the DBR surfaceemitting LD 150 including the single laser resonator (secondary diffraction grating) shown in figure 15.

However, since a plurality of laser oscillating regions are disposed close to each other, when these lasers oscillate continuously, the temperature of the device significantly increases, so that the refractive index in the optical waveguide, i.e., the lightguide layer and the cladding layer, unfavorably varies. This variation in the refractive index causes a difference between the Bragg wavelength of the secondary diffraction grating fabricated in the optical waveguide and the laser oscillation wavelength, resulting in unstable beam output angle.

Figures 17(a) to 17(c) illustrate a surface-emitting DBR LD array including an annular diffraction grating disclosed in Japanese Published Patent Application No. Hei.

3-257888, wherein figure 17(a) is a perspective view of the DBR LD array, figure 17(b) is a sectional view taken along a line 17b-17b of figure 17(a), and figure 17(c) is a sectional view taken along a line 17c-17c of figure 17(a).

In these figures, a surface-emitting DBR LD array 170 includes an n type InP substrate 70 having opposite front and rear surfaces. An n type InGaAsP waveguide layer 83 having a band gap energy equivalent to a wavelength of about 1.3 um, an InGaAsP active layer 82 having an energy band gap equivalent to a wavelength of about 1.55 urn, a p type InP cladding layer 81, and a p type InGaAsP cap layer 80 are successively disposed on the n type InP substrate 70.

Portions of these layers 80, 81, and 82 in the center of the structure are selectively removed, and an annular secondary diffraction grating 79 is produced at the exposed surface of the n type InGaAsP waveguide layer 83. Further, these layers 80, 81, 82, and 83 are formed in a plurality of stripe-shaped mesas across the diffraction grating 79. More specifically, as shown in figure 17(a), four stripe-shaped laser resonators 71, 72, 73, and 74, each having the cross section shown in figure 17(b), are produced by selectively removing portions of those layers 80 to 83. As shown in figure 17(c), a semi-insulating InP layer 84 is disposed on the substrate 70, contacting opposite sides of each laser resonator. An n side electrode 78 is disposed on the rear surface of the substrate 70. A p side electrode 75 is disposed on the stripe-shaped resonators 71 to 74 and on the semi-insulating InP layer 84. Reference numeral 81a designates facets, numeral 76 designates an insulating film, and numeral 77 designates a metal film.

In this prior art surface-emitting DBR LD array, laser lights produced by laser oscillations of the respective resonators 71 to 74 are output upward from the annular diffraction grating 79, so that a high output power is obtained from the single aperture, in proportion to the number of the laser resonators. In addition, the space between adjacent resonators is larger than that of the DBR LD array shown in figure 16. Therefore, the unwanted increase in the temperature of the device during the continuous laser oscillation is suppressed,. whereby the instability of the beam output angle is reduced to some extent.

Since the prior art surface-emitting DBR-LD array outputs a high power laser light in the direction perpendicular to the main surface of the substrate, it is employed as a semiconductor light emitting element for optical interconnection of signals between a plurality of computers or for optical interconnection of signals in a computer, i.e., signals between a plurality of boards or on each board or signals between a plurality of chips. This optical interconnection system requires means for converting electric signal into light, means for transmitting optical signal, and means for restoring optical signal to electric signal. A semiconductor light emitting element (for example, a semiconductor laser), an optical waveguide, and a semiconductor light responsive element are respectively employed for those means. Figure 18(a) is a schematic diagram illustrating an optical interconnection system using an optical waveguide (hereinafter referred to as optical waveguide interconnection), and figure 18(b) is a schematic diagram illustrating an optical interconnection system using no waveguide (hereinafter referred to as optical spatial interconnection).

As shown in figures 18(a) and 18(b), in the optical waveguide interconnection and the optical spatial interconnection, the direction in which laser light emitted from the semiconductor laser is guided is changed by an optical waveguide or a reflecting mirror. Therefore, as shown in figure 19, if directions of laser lights 92a to 92c output from surface-emitting LD arrays 90a to 90c disposed on a transmitter chip 190a are controlled separately for each surface-emitting LD array, the physical means for changing the optical path, such as the waveguide or the reflecting mirror, can be dispensed with. In addition, informations from those three surface-emitting LD arrays 90a to 90c are received by six photodiodes 91a to 91f disposed on a receiver chip 190b, so that the device size is significantly reduced. In the prior art surface-emitting LD array shown in figure 16, however, a plurality of oscillation parts of the laser resonators are arranged parallel and close to each other, and each laser resonator is adversely affected by leakage light from the adjacent laser resonator. Therefore, even when the respective laser resonators are oscillated with different driving currents and a phase composite wave is produced, the respective laser resonators do not operate stably, so that the output direction of the obtained phase composite wave cannot be arbitrarily controlled.

On the other hand, in the prior art surface-emitting LD array 170 shown in figure 17, since a plurality of laser lights resulting from oscillations of the respective laser resonators 71 to 74 interfere with each other at the annular diffraction grating 79, a phase composite wave is not obtained, so that the output direction of the composite output light comprising the laser lights from the respective laser resonators cannot be arbitrarily controlled.

Meanwhile, multiwavelength optical communication in which a multiwavelength light comprising laser lights of difference wavelengths is guided through an optical fiber has been carried out recently. In the surface-emitting LD array shown in figure 16, if the pitches of the respective secondary diffraction gratings 58a to 58c are different from each other, laser lights with different wavelengths are output at the same time. However, in the prior art surfaceemitting LD array shown in figure 16, the secondary diffraction gratings 58a to 58c are arranged parallel and close to each other, laser lights emitted from the respective diffraction gratings interfere with each other, so that multiwavelength laser light having a prescribed wavelength cannot be output with stability. In the prior art surface-emitting LD array shown in figure 17, laser lights with difference wavelengths interfere with each other at the annular diffraction grating 79, so that multiwavelength laser light having a prescribed wavelength cannot be output with stability.

SUMMARY OF THE INVENTION An object of the present invention is to provide a surface-emitting LD array that provides stable output angles of laser lights from respective laser resonators (laser diodes) in the continuous operation at room temperature, that controls the respective laser resonators separately, and that directs a composite laser light comprising laser lights emitted from the respective laser resonators to an arbitrary direction with high controllability.

Another object of the present invention is to provide a method for driving the surface-emitting LD array.

A further object of the present invention is to provide a spatial optical interconnection system utilizing the surface-emitting LD array.

Another object of the present invention is to provide a surface-emitting LD array that controls a plurality of laser resonators (laser diodes) separately from each other and that outputs a composite laser light comprising laser lights of different wavelengths emitted from the respective laser resonators in a prescribed direction, with high stability, without mutual interference of those laser lights of different wavelengths.

Still another object of the present invention is to provide a photodetector that detects only light having a prescribed wavelength from the above-described multiwavelength laser light.

Yet another object of the present invention is to provide a multiwavelength optical communication system including a transmitter of the above-described surfaceemitting LD array and a receiver that detects the multiwavelength light transmitted from the transmitter at each wavelength.

Other objects and advantages of the invention will become apparent from the detailed description that follows.

The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the spirit and scope of the invention will be apparent to those of skill in the art from the detailed description.

According to a first aspect of the present invention, a surface-emitting LD array includes a plurality of surface emitting LDs respectively including secondary diffraction gratings produced at the same pitch. These LDs are radially arranged on the substrate with a prescribed point on the substrate as the center of the radial arrangement so that the secondary diffraction gratings face to the center point.

In this structure, the laser oscillation regions of the adjacent surface-emitting LDs are spaced apart from each other, so that the respective surface-emitting LDs are operated individually with high stability. As the result, when these surface-emitting LDs are oscillated with the same driving current, a high power laser light comprising a plurality of laser lights that are emitted from the respective LDs and have the same phase and the same wavelength is output stably in a prescribed direction. On the other hand, when these surface-emitting LDs are oscillated with different driving currents, a phase composite wave in which a plurality of laser lights having the same phase and different wavelengths are compounded is output stably in a prescribed direction.

According to a second aspect of the present invention, in the above-described surface-emitting LD array, the surface-emitting LDs are arranged so that projections of the secondary diffraction gratings of the adjacent LDs are not concyclic with the center point on the substrate as the center. Therefore, when these surface-emitting LDs are oscillated with different driving currents to produce a phase composite wave, the mutual interference of leakage lights having different phases between the adjacent surfaceemitting LDs is canceled, whereby the phase composite wave is output stably in a prescribed direction.

According to a third aspect of the present invention, in the above-described surface-emitting LD array, a plurality of grooves for heat radiation are disposed between adjacent LDs. Therefore, thermal interference between the adjacent surface-emitting LDs is prevented, whereby the respective LDs are operated with high reliability.

According to a fourth aspect of the present invention, in a method for driving the surface-emitting LD array, the respective surface-emitting LDs are operated with different operating currents to control these LDs individually, whereby output direction of a composite laser light of laser lights emitted from the respective surface-emitting is changed to an arbitrary direction without using a reflecting mirror or an optical waveguide.

According to a fifth aspect of the present invention, in the above-described surface-emitting LD array, the secondary diffraction gratings of the respective LDs are produced at different pitches. Therefore, the respective surface-emitting LDs are oscillated at different oscillation wavelengths, and a multiwavelength laser light comprising a plurality of laser lights having different wavelengths is obtained.

According to a sixth aspect of the present invention, a surface-emitting LD array includes a substrate having a through-hole, a plurality of surface-emitting LDs radially arranged on the substrate so that the laser emitting facets of the respective LDs face to the through-hole, and means for collecting laser lights emitted from the respective LDs and outputting the collected laser light in the direction perpendicular to the surface of the substrate. The laser light collecting means is disposed in the through-hole of the substrate. Therefore, the same operation as the abovedescribed surface-emitting LD array is achieved without using surface-emitting LDs.

According to a seventh aspect of the present invention, a photodetector comprises an optical waveguide layer, a light responsive part including a secondary diffraction grating that guides light having a prescribed wavelength to the optical waveguide layer, and a light detecting part for converting the light having a prescribed wavelength and traveling through the optical waveguide layer into photoelectric current and outputting the photoelectric current. Therefore, light having a prescribed wavelength is detected from multiwavelength light with high precision.

According to an eighth aspect of the present invention, a photodetector array includes a plurality of the abovedescribed photodetectors. These photodetectors are radially arranged on the substrate with a prescribed point on the substrate as the center of the radial arrangement so that the second diffraction gratings of the respective photodetectors face to the center point. Therefore, when the secondary diffraction gratings of the respective photodetectors are produced at different pitches, multiwavelength light is detected at each wavelength with high precision without using means for dividing the multiwavelength light into signal lights of the respective wavelengths.

According to a ninth aspect of the present invention, a spatial optical interconnection system includes, as a semiconductor light emitting device for transmission, a surface-emitting LD array in which secondary diffraction gratings of the respective surface-emitting LDs are produced at the same pitch. Therefore, when driving currents applied to the respective surface-emitting LDs are controlled, a composite laser light comprising laser lights emitted from the respective LDs is output in an arbitrary direction, whereby signal lights are transmitted to a plurality of light responsive elements.

According to a tenth aspect of the present invention, a multiwavelength optical communication system comprises a surface-emitting LD array in which secondary diffraction gratings of the respective surface-emitting LDs are produced at different pitches, a photodetector array in which secondary diffraction gratings of the respective photodetectors are produced at different pitches, and an optical fiber connecting the LD array and the photodetector array. Therefore, multiwavelength light is transmitted to the optical fiber with high stability and signal light having a prescribed wavelength is received from the transmitted multiwavelength light with high precision.

BREF DESCRIPTfON OF THE DRAWINGS Figure l(a) is a plan view illustrating a surfaceemitting LD array in accordance with a first embodiment of the present invention and figure l(b) is a perspective view of an LD included in the LD array, that is enclosed with alternate long and short dash line in figure l(a).

Figures 2 and 3 are sectional views for explaining the operation of the surface-emitting LD array shown in figure 1.

Figure 4 is a perspective view illustrating a surfaceemitting LD as shown in figure l(b) having a phase control function.

Figure 5 is a plan view illustrating parts of secondary diffraction gratings included in a surface-emitting LD array in accordance with a second embodiment of the present invention.

Figure 6 is a plan view illustrating a surface-emitting LD array in accordance with a third embodiment of the present invention.

Figure 7 is a perspective view illustrating a surfaceemitting LD included in the surface-emitting LD array shown in figure 6.

Figure 8 is a plan view illustrating a surface-emitting LD array in accordance with a fourth embodiment of the present invention.

Figures 9 and 10 are sectional views of the surfaceemitting LD array shown in figure 8.

Figure 11 is a plan view illustrating a surfaceemitting LD array in accordance with a fifth embodiment of the present invention.

Figures 12(a) and 12(b) are a perspective view and a sectional view illustrating a photodetector in accordance with a sixth embodiment of the present invention.

Figure 13 is a plan view illustrating a photodetector array in accordance with a seventh embodiment of the present invention.

Figure 14 is a schematic diagram illustrating a multiwavelength optical communication system in accordance with an eighth embodiment of the present invention.

Figure 15 is a perspective view illustrating a surfaceemitting LD according to the prior art.

Figure 16 is a perspective view illustrating a surfaceemitting LD array according to the prior art.

Figures 17(a)-17(c) are a perspective view and sectional views illustrating a surface-emitting LD array according to the prior art.

Figures 18(a) and 18(b) are schematic diagrams illustrating conventional optical interconnection systems.

Figure 19 is a schematic diagram illustrating a spatial optical interconnection system including a semiconductor light emitting element that can change the direction of output light.

nFTATrEn flSCRTPTTON Or THE PREFERRED wmPnDTMFNTS Figure l(a) is a plan view illustrating a surfaceemitting LD array in accordance with a first embodiment of the present invention, and figure l(b) is a perspective view of an LD included in the LD array, that is enclosed with alternate long and short dash line and designated by A in figure l(a). In these figures, a surface-emitting LD array 100 includes an n type InP substrate 10. A plurality of DBR (Distributed Bragg Reflector) surface-emitting LDs 101a to 1011, each having a secondary diffraction grating 16, are radially arranged on the n type InP substrate 10 with a prescribed point on the substrate 10 as the center of the radiation so that the secondary diffraction gratings 16 face to the center point. The secondary diffraction gratings 16 of the surface-emitting LDs 101a to 1011 are produced at the same pitch. Although it is not shown in figure l(a), the surface-emitting LDs 101a to 1011 are arranged so that the projections of the adjacent secondary diffraction gratings 16 are not concyclic with the center point of the radial arrangement as the center.

The surface-emitting LDs 101a to 1011 comprise the same semiconductor layers. More specifically, as shown in figure l(b), an n type InP cladding layer 11 having a thickness of 1 urn is disposed on the n type InP substrate 100. An InGaAsP lightguide layer 12 having a thickness of 300 A is disposed on the cladding layer 11. An active layer of a multiquantum well structure 13 (hereinafter referred to as MOW layer) is disposed on the lightguide layer 12. The MQW layer comprises, alternatingly arranged, five to ten 100 A thick InGaAsP barrier layers and five to ten 70 A thick InGaAsP well layers. An InGaAsP lightguide layer 14 having a thickness of 300 A is disposed on the MQW layer 13. A p type InP cladding layer 15 having a thickness of 0.5 um is disposed on the lightguide layer 14. An InGaAsP secondary diffraction grating 16 is disposed on the cladding layer 15.

These layers 11 - 16 are formed in a stripe-shaped mesa structure 30. An Fe-doped InP layer 17 is disposed on the n type InP cladding layer 11, contacting the opposite sides of the mesa structure 30 excluding the diffraction grating 16.

A p type InGaAsP contact layer 18 is disposed on a part of the diffraction grating 16 in the laser oscillation region 101A. An insulating film 19 is disposed on the opposite sides of the diffraction grating 16 and the contact layer 18 in the laser oscillation region 101A and on part of the Fedoped InP layer 17 in the vicinity of the diffraction grating 16. A p side electrode 20 comprising Cr/Au is disposed over the insulating film 19 and the top surface of the contact layer 18. Laser light is emitted from the secondary diffraction grating 16 in the laser light emitting region 101B in the direction perpendicular to the surface of the substrate 10. An n side electrode 22 comprising AuGe/Ni/Au is disposed on the rear surface of the substrate 10. A high reflectivity film 21 is disposed at an end of the secondary diffraction grating 16 and a portion of the p type InP cladding layer 15 protruding from the p side electrode 20. The high reflectivity film 21 may be disposed over the Fe-doped InP layer 17.

The production process of the surface-emitting LD array 100 shown in figures l(a)-l(b) will be described.

The process of fabricating the LDs lOla to 1011 of the surface-emitting LD array 100 is basically identical to the fabrication process of the prior art surface-emitting LD.

However, in the fabrication process of the secondary diffraction grating including depositing and prebaking a photoresist film, patterning the photoresist film in a diffraction grating at prescribed pitch using two-beam interference exposure method with a laser source, and forming the secondary diffraction grating in a semiconductor layer using the photoresist pattern as a mask, one exposure process produces a diffraction grating photoresist pattern only in one direction. Therefore, in the present invention, utilizing that the surface-emitting LD array 100 has a rotation symmetric structure with the center point of the device as the center of the symmetry, the substrate 10 is partially masked so that surface-emitting LDs on which secondary diffraction gratings extending in the same direction are to be formed are exposed, and the substrate is subjected to multiple beam interference exposure. The substrate 10 or the optical source for the interference exposure (not shown) is turned on the center of the device, and secondary diffraction gratings extending in the same direction are successively produced by multiple beam interference exposure. Alternatively, the photoresist film may be patterned in a radial diffraction grating using EB (Electron Beam) exposure, or the diffraction grating pattern may be formed directly on the semiconductor layer by FIBE (Focused Ion Beam Etching) without using the photoresist film.

In the surface-emitting LD array 100 according to the first embodiment of the invention, the respective LDs lOla to 1011 are separated from each other by mesa etching. In the mesa etching process for conventional InGaAsP-InP LDs, wet etching using Br base solution is employed. However, this wet etching much depends on the surface orientation of the substrate, so that different planes and shapes are produced according to the directions in which the patterns are produced. Therefore, favorable separation of the LDs is not achieved by the wet etching. In this embodiment of the invention, dry etching, such as RIE (Reactive Ion Etching) or FIBE (Focused Ion Beam Etching), is employed to achieve element separation with no dependency on surface orientation.

A description is given of the operation.

When the respective LDs 101a to 1011 included in the surface-emitting LD array 100 are oscillated with injection currents of the same magnitude, as shown in figure 2, laser lights 23a and 23b emitted from the secondary diffraction gratings 16 of the surface-emitting LDs 101a to 1011 have the same wavelength and the same phase, and these laser lights interfere each other at the center of the device, resulting in a high power laser light 23. Figure 2 is a sectional view taken along the opposite LDs lOla and 101g, and only two laser lights 23a and 23b are shown in the figure. However, laser lights of the same wavelength and the same phase are emitted from all of the LDs lOla to lO11.

On the other hand, when the respective LDs lOla to 1011 included in the surface-emitting LD array 100 are oscillated with injection currents of different magnitudes, since laser lights 24a and 24b emitted from the secondary diffraction gratings 16 of the respective surface-emitting LDs lOla to 1011 have the same wavelength and different phases as shown in figure 3, a far-field laser light 24 that is output from the center of the LD array becomes a phase composite wave of the respective LDs. Therefore, it is possible to vary the intensity peak angle e determined by the difference in phases between the respective LDs 101a to 1011 by controlling the driving current applied to each of the surface-emitting LDs lOla to 1011 to change the phase of laser light emitted from the LD. The peak angle e variable direction is set conically with respect to the light emitting direction, i.e., the direction perpendicular to the substrate, since the surface-emitting LDs 101a to 1011 are radially arranged with the prescribed point on the substrate as the center. Figure 3 is a sectional view taken along the opposite LDs 101a and 101g, and only two laser lights 24a and 24b are shown in the figure. However, laser lights are emitted from all of the LDs 101a to 1011.

In the above-described operation, as a method for controlling the oscillation phase of each LD, a phase control means included in a wavelength variable threeelectrode DBR laser disclosed in Electronics Letters, Vol.23, p.404, 1987 may be employed. More specifically, a phase control region is formed in a prescribed portion of an optical waveguide, and current is injected into the phase control region to change the carrier concentration in that region. The change in the carrier concentration causes the refractive index in that region to change. The variation in phase caused by variation in the laser oscillation wavelength is compensated and controlled by the change in the refractive index. Figure 4 is a perspective view illustrating a part of a surface-emitting LD including such phase control means. In figure 4, the same reference numerals as in figure l(b) designate the same or corresponding parts. The structure of figure 4 is identical to the structure of figure l(b) except that a phase control region 101c comprising an additional insulating film 19a and an additional p side electrode 20a is disposed between the laser oscillation region lOlA and the laser light emitting region 101B. The insulating film 19a and the p side electrode 20a are produced simultaneously with the insulating film 19 and the p side electrode 20 in the laser oscillation region 101A.

In the surface-emitting LD array 100 according to the first embodiment of the invention, a plurality of separated surface-emitting LDs lOla to 1011, each including a secondary diffraction grating 16 produced at the same pitch, are radially disposed on the semiconductor substrate 10 so that a prescribed point on the substrate 10 is the center of the radial arrangement and the secondary diffraction gratings 16 of the respective LDs face to the center point.

Since the surface-emitting LDs lOla to 1011 are separated from each other and the adjacent laser oscillation regions are adequately spaced apart from each other, when the respective surface-emitting LDs 101a to 1011 are continuously operated at room temperature with injection currents of the same magnitude, the respective LDs are stably oscillated without adversely affected by leakage light from the adjacent LD. As the result, high power laser light is stably output at a prescribed output angle.

When the respective surface-emitting LDs 101a to 1011 are operated with currents of different magnitudes, since the respective LDs are separated from each other and the adjacent laser oscillation regions and the adjacent secondary diffraction gratings are adequately spaced apart from each other, thermal interference and mutual interference of leakage light between adjacent LDs are prevented. As the result, the respective LDs 101a to 1011 stably emit laser lights of prescribed phases, and a phase composite wave having a prescribed phase is output in a prescribed direction with high reliability. Further, the intensity peak angle 6 of the phase composite wave can be changed by controlling the current applied to each of the surface-emitting LDs 101a to 1011 to change the oscillation phase of the LD. For example, when the surface-emitting LD array 100 is mounted on a transmitter chip, it is possible to transmit informations from the LD array 100 on the transmitter chip to any photodiode among a plurality of photodiodes arranged in matrix on a receiver chip without using any physical optical path changing means, such as the optical waveguide or the reflecting mirror shown in figure 19.

Furthermore, the surface-emitting LDs 101a to 1011 are radially arranged so that the projections of the secondary diffraction gratings 16 of the adjacent LDs are not concyclic with the prescribed point on the n type InP substrate 10 as the center, so that mutual interferences of phases of leakage lights from adjacent LDs are canceled each other. As the result, the respective surface-emitting LDs 101a to 1011 are operated at desired oscillation wavelengths and phases with high reliability.

Figure 5 is a schematic diagram illustrating a part of a surface-emitting LD array in the vicinity of the secondary diffraction gratings, in accordance with a second embodiment of the present invention. In figure 5, the same reference numerals as in figure l(a) designate the same or corresponding parts. Reference numerals 16a to 16c designate secondary diffraction gratings produced at different pitches. The surface-emitting LD array according to this second embodiment is basically identical to the surface-emitting LD array shown in figures l(a) and I(b) except that the respective surface-emitting LDs includes secondary diffraction gratings produced at different pitches.

In the DBR surface-emitting LD, output laser light has a single wavelength determined by the pitch of the secondary diffraction grating in view of the operating principle.

Therefore, laser light output from the surface-emitting LD array of this second embodiment is a multiwavelength light comprising a plurality of laser lights having different wavelengths. The number of the wavelengths is equal to the number of the surface-emitting LDs disposed on the substrate at the maximum.

According to the second embodiment of the invention, since the secondary diffraction gratings of the respective surface-emitting LDs are produced at different pitches, a multiwavelength output light comprising laser lights of different wavelengths is attained, whereby a plurality of informations are transmitted at the same time.

Figure 6 is a plan view illustrating a surface-emitting LD array in accordance with a third embodiment of the present invention, and figure 7 is a perspective view illustrating each surface-emitting LD included in the array of figure 7. In these figures, the same reference numerals as in figures l(a) and l(b) designate the same or corresponding parts. A surface-emitting LD array 200 comprises a plurality of surface-emitting LD chips 201a to 2011 radially arranged on an insulating substrate 40 with a prescribed point on the substrate 40 as the center of the radial arrangement so that the secondary diffraction gratings 16 of the respective LDs face to the center point.

Although only the surface-emitting LD 201a is shown in figure 7, other LDs 201b to 2011 have the same structure as shown in figure 7. The secondary diffraction gratings 16 are produced at the same pitch.

In the surface-emitting LD array 200 according to this third embodiment of the invention, the surface-emitting LD chips 201a to 2011 which are fabricated individually are mounted on the insulating substrate 40. Each of those LD chips 201a to 2011 or the insulating substrate 40 has an electrode pad for leading the n side electrode.

Also in this surface-emitting LD array 200, the same effects as described in the first embodiment are achieved.

Further, since the respective LDs 201a to 2011 are spaced apart from each other, optical and thermal interferences between adjacent LDs are prevented, whereby the respective LDs are operated stably with desired oscillation wavelengths and phases.

Figure 8 is a plan view illustrating a surface-emitting LD array in accordance with a fourth embodiment of the present invention, and figure 9 is a sectional view taken along a line 9-9 of figure 8. In the figure, the same reference numerals as in figure l(a) designate the same or corresponding parts. A surface-emitting LD array 300 includes an n type InP substrate 10 having a through-hole 10a. A plurality of DBR-LDs 301a to 3011, each including a primary diffraction grating 31, are radially arranged on the n type InP substrate 10 with the primary diffraction gratings 31 of the respective LDs facing to the through-hole 10a of the substrate 10. As shown in figure 9, an optical fiber 32 including a 45 e inclined conic reflecting mirror 32a is inserted in the through-hole 10a of the substrate 10 so that laser lights emitted from the respective LDs 301a to 3011 strike the reflecting mirror 32a. The reflecting mirror 32a is produced by mechanical processing. In this structure, a composite light of the laser lights emitted from the respective LDs 301a to 3011 is output upward in the direction perpendicular to the surface of the substrate.

In this fourth embodiment of the invention, since the optical fiber 32 for collecting the laser lights emitted from the respective primary diffraction grating DBR-LDs is disposed in the through-hole 10a of the substrate 10, an LD array having the same function as the surface-emitting LD array 100 according to the first embodiment is obtained. In place of the optical fiber 32, an optical coupling element comprising quartz glass and having a 450 inclined conic recess 33a as shown in figure 10 may be employed. The conic recess 33a is produced by mechanical processing.

In place of the conic recess 33a, the optical coupling element 33 may include a region where the refractive index is conically varied. If an Er-doped optical fiber or optical coupling element is used, the surface-emitting LD array of this embodiment can serve as a fiber amplifier.

Also in this surface-emitting LD array, when the LDs 301a to 3011 are arranged so that the projections of the primary diffraction gratings of the adjacent LDs are not concyclic with the through-hole 10a of the substrate 10 as the center, the respective LDs 301a to 3011 are operated stably with prescribed oscillation wavelengths and phases.

Figure 11 is a plan view illustrating a surfaceemitting LD array in accordance with a fifth embodiment of the present invention. In the figure, the same reference numerals as in figure l(a) designate the same or corresponding parts. A surface-emitting LD array 400 of this fifth embodiment is fundamentally identical to the surface-emitting LD array 100 of the first embodiment except that a plurality of separation grooves 41 for heat radiation are formed in the Fe-doped InP layer 17 between the adjacent LDs. More specifically, three grooves, i.e., a center groove as long as the total length of the LD and two grooves as long as the laser oscillation region of the LD and sandwiching the center groove, are produced between adjacent two LDs. The grooves 41 reach the n type InP substrate 10 or the vicinity of the substrate 10.

In the surface-emitting LD array 400 of this fifth embodiment, since the separation grooves 41 for heat radiation are formed in the Fe-doped InP layer 17 between adjacent LDs, the surface area between the adjacent LDs is increased. Therefore, radiation of heat into space above the device is improved, so that the respective LDs are operated' with no thermal interference between the adjacent LDs. As the result, laser light having prescribed wavelength and phase is output with high reliability.

Figure 12(a) is a perspective view illustrating a photodetector in accordance with a sixth embodiment of the present invention, and figure 12(b) is a sectional view taken along line 12b-12b of figure 12(a). In the figure, the same reference numerals as in figure l(b) designate the same or corresponding parts. A photodetector 120 includes an n type InP substrate 10. An n type InP buffer layer 42 having a thickness of 1000 A is disposed on the substrate 10. An optical waveguide layer 43 is disposed on the buffer layer 42. The optical waveguide layer 43 has a multiquantum well (hereinafter referred to as MQW) structure comprising, alternatingly arranged, five to ten 100 A thick InGaAsP barrier layers and five to ten 70 A thick InGaAsP well layers. A p type InP buffer layer 44 having a thickness of 1000 A is disposed on the optical waveguide layer 43. These layers 42, 43, and 44 grown on the substrate 10 are formed in a stripe-shaped mesa. A part of the p type InP buffer layer 44 in the light responsive region of the photodetector is patterned in a secondary diffraction grating 16. An Fedoped InP layer 17 is disposed on the n type InP buffer layer 42, contacting the opposite sides of the mesa structure excluding the p type InP buffer layer 44. A p type InGaAsP contact layer 18 is disposed on a part of the p type InP buffer layer 44 in the light detecting region of the photodetector. An insulating film 19 is disposed on the opposite side surfaces of the p type InGaAsP contact layer 18 and on the upper surface of the Fe-doped InP layer 17 in the light detecting region. A p side electrode 20 comprising Cr/Au is disposed on the upper surface of the p type InGaAsP contact layer 18 and on the insulating film 19.

The energy band gap of the MOW optical waveguide layer 43 in the light responsive region beneath the secondary diffraction grating 16 is wider than the wavelength of light to be detected in the light detecting region, and the energy band gap of the MOW optical waveguide layer 43 beneath the electrode 20 is narrower than the wavelength of light to be detected in the light detecting region.

In production of the MQW optical waveguide layer 43, a region of the buffer layer 42 to be the light detecting region, i.e., to be under the p side electrode 20, is sandwiched with SiO2 or SiN films, and the MQW layer 43 is epitaxially grown on the buffer layer 42, whereby the thickness of the well layers of the MQW structure is increased in the light detecting region. The abovedescribed difference in the energy band gaps is produced in this way.

A description is given of the operation.

For example, when 1.55 um band multiwavelength communication laser light comprising a plurality of signal lights of different wavelengths (interval between the wavelengths: several angstroms to zero point several angstrom) is input to the light responsive region of the photodetector 120 in the direction perpendicular to the 'surface of the substrate 10, only light of single wavelength that is selected by the secondary diffraction grating 16 having the pitch of about 4400 A is turned to the longitudinal direction of the diffraction grating, i.e., the horizontal direction with respect to the surface of the substrate 10, and the light is transmitted through the optical waveguide layer 43 to the light detecting region.

The optical waveguide layer 43 in the light detecting region has an energy band gap narrower than the wavelength of the light to be detected, so that the light is absorbed by the optical waveguide layer 43 and converted into photoelectric current. On the other hand, lights other than the selected single wavelength light are reflected by or transmitted through the secondary diffraction grating 16, or turned to directions other than the above-described horizontal direction. Anyway, those lights are not transmitted to the light detecting region.

In the photodetector 120 according to the sixth embodiment of the invention, light of prescribed wavelength can be detected from multiwavelength signal light without adversely affected by lights of other wavelengths, whereby signal detection with high S/N ratio is achieved.

In the structure shown in figures 12(a)-12(b), an antireflection film that makes the reflectivity at the detected wavelength zero may be disposed on the surface of the secondary diffraction grating 16. In this case, the detected photoelectric current is increased, whereby the S/N ratio is further improved.

The single wavelength light turned to the horizontal direction by the secondary diffraction grating 16 and traveling through the optical waveguide 43 is guided in both directions toward the light detecting region and toward the end facet of the device. Therefore, when the end facet of the device is vertically shaped and an anti-reflection film that makes the reflectivity at the detected wavelength 100 % is disposed at the end facet, the single wavelength light guided to the light detecting region is further increased, whereby the S/N ratio is further improved.

Figure 13 is a plan view illustrating a photodetector array in accordance with a seventh embodiment of the present invention. In the figure, the same reference numerals as in figures 128a)-12(b) designate the same or corresponding parts. A photodetector array 130 includes an n type InP substrate 10. A plurality of photodetectors 120a to 1201, each having the structure shown in figures 12(a)-12(b), are radially arranged on the n type InP substrate 10 with a prescribed point on the substrate 10 as the center of the radial arrangement so that secondary diffraction gratings 16 of the respective photodetectors face to the center point.

The secondary diffraction gratings 16 of the respective photodetectors 120a to 1201 are produced at different pitches.

When the photodetector array 130 is used as a receiver for a multiwavelength optical communication using an optical fiber, signal light having each wavelength is detected from the multiwavelength light without using means for diverging signal lights of the respective wavelengths from the optical fiber. In addition, since the size of the photodetector array 130 is reduced, the cost of the photodetector array is reduced. Further, since the optical waveguide layers of the adjacent two photodetectors are not arranged parallel to each other, overlapping of wave surfaces of signal lights traveling through the adjacent optical waveguide layers is insignificant, so that the mutual interference due to leakage of optical signals between the adjacent optical waveguide layers. As the result, the S/N ratio of the detected signal is increased, whereby high-purity signal light is detected.

Further, when the photodetectors 120a to 1201 are arranged so that the projections of the secondary diffraction gratings of the adjacent photodetectors are not concyclic with the prescribed point on the substrate as the center, the phases of lights leaked from the optical waveguide layers of the respective photodetectors are shifted and canceled each other, whereby the mutual interference is prevented.

Furthermore, when pitches of the secondary diffraction gratings of at least two of the photodetectors 120a to 1201 are the same, single wavelength light can be detected by a plurality of photodetectors having the same pitch, whereby the S/N ratio of the optical signal having a prescribed wavelength to be detected is further increased. Usually, as the principle of light turned by 90, i.e., turned to the horizontal direction, the intensity of the light is reduced compared to rectilinear propagating light although the monochromaticity of the wavelength of the light is increased. However, the above-described means compensates the reduction in the intensity.

Figure 14 is a schematic diagram illustrating a multiwavelength optical communication system in accordance with an eighth embodiment of the present invention, including a surface-emitting LD array as described in the second embodiment as a transmitter and a photodetector array as described in the seventh embodiment as a receiver. In the multiwavelength optical communication system 140 shown in figure 14, a surface-emitting LD array 140a that outputs multiwavelength laser light as described in the second embodiment is connected to an end of an optical fiber 47 using an ordinary optical coupling means 46, and a photodetector array 130 including a plurality of photodetectors, each detecting light having a prescribed wavelength, as described in the seventh embodiment is connected to the other end of the optical fiber 47 using the optical coupling means 46.

As the optical coupling means 46, confocal compound lens coupling means in which two lenses having relatively long focal length, such as a sphere lens and a GRIN (graded index) lens, are disposed at confocal positions is employed.

This means is usually employed as an optical coupling means between an LD and an optical fiber.

In the multiwavelength optical communication system shown in figure 14, laser lights having multiple wavelengths are simultaneously output from the surface-emitting LD array 140a to the optical fiber 47 without using laser light composite means, and the photodetector array 130 detects laser light of each wavelength from the multiwavelength laser light transmitted through the optical fiber 47 without using light diverging means. Therefore, the structure of the system is simplified compared to a conventional communication system of this kind, reducing the cost.

While in the above-described first, second third, fifth, and eighth embodiments a DBR-LD is used as a surfaceemitting LD, a DFB-LD may be employed with the same effects as described above.

Claims

CLAIMS:

1. A surface-emitting laser diode array including: a substrate having a main surface and a through-hole perpendicular to the main surface; a plurality of laser diodes each including a facet from which laser light is emitted in a direction parallel to the main surface of the substrate and radially arranged on the main surface of the substrate with the laser emitting facets of the respective laser diodes facing to the through-hole of the substrate; and means for collecting laser lights emitted from the respective laser diodes and outputting the collected laser light upward in a direction perpendicular to the main surface of the substrate.

2. A surface-emitting laser diode array as defined in claim 1 wherein said laser diodes are distributed feedback laser diodes or distributed Bragg reflector laser diodes including primary diffraction gratings.

3. A surface-emitting laser diode array as defined in claim 2 wherein said primary diffraction gratings of the respective laser diodes are produced at the same pitch and oscillate at the same oscillation wavelength.

4. A surface-emitting laser diode array as defined in claim 2 wherein said primary diffraction gratings of the respective laser diodes are produced at different pitches and oscillate at different oscillation wavelengths.

5. A surface-emitting laser diode array as defined in claim 2 or 4 wherein said laser diodes are arranged so that projections of the primary diffraction gratings of the adjacent laser diodes are not concyclic with the through-hole of the substrate as the center.

6. A surface-emitting laser diode array as defined in any of claims 1 to 5 wherein said substrate is a semiconductor substrate and said laser diodes are simultaneously produced on the semiconductor substrate.

7. A surface-emitting laser diode array as defined in any of claims 1 to 5 wherein said substrate is an insulating substrate and said laser diodes are fabricated individually and mounted on the insulating substrate.

8. A surface-emitting laser diode array substantially as herein described with reference to Figures 1 to 4 Figure 5, Figures 6 and 7, Figures 8 to 10, or Figure 11 of the accompanying drawings.

9. A photodetector array substantially as herein described with reference to Figure 12 or Figure 13 of the accompanying drawings.

10. A multiwavelength optical communication system substantially as herein described with reference to Figure 14 ofthe accompanying drawings