JP2005086067A - Optical module, optical transmitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical module and an optical transmitter suitable for communication employing a multimode optical fiber. <P>SOLUTION: The optical module comprises a surface-emitting semiconductor laser 10 emitting a polarized light oriented preferentially to one given polarization axis and oscillating in multimode, and an optical member 30 exhibiting at least one of transmission function and reflecting function selectively with respect to a specified polarized light, and a light-receiving element 20 receiving at least a part of light P1 emitted from the surface-emitting semiconductor laser 10. The surface-emitting semiconductor laser 10 is arranged, such that the axis perpendicularly intersecting the polarization axis of the emitted light P1 does not overlap the polarization axis of the optical member 30. The optical member 30 is arranged, such that the major surface 30a intersects the advancing direction of the light P1 emitted from the surface-emitting semiconductor laser 10 and a reflected light P2 of the light P1, emitted from the surface emission semiconductor laser 10, is introduced to the light-receiving element 20 by reflecting at least a part of the emitted light P1 on the major surface 30a. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical module having a vertical cavity surface emitting laser (VCSEL) and an optical transmission device.

  In optical communication systems, for short-distance data transmission up to several hundred meters called LAN (local area network) or SAN (storage area network), the core diameter is larger than that of single-mode optical fiber and optical coupling with a light source is easy. Multimode optical fibers are often used.

When communication is performed using a multimode optical fiber, there is a problem of deterioration of the S / N ratio called mode selection loss. In order to improve mode selection loss, it is known that the use of a light source having low coherence of emitted light is effective. For example, in Japanese Patent Application Laid-Open No. 7-170231, a multimode oscillation VCSEL (vertical cavity surface emitting laser) is used as a light source to secure a transmission distance of several hundred meters in high-speed data transmission of 1.5 Gbps or more. Yes.
JP-A-7-170231

  However, in a multi-mode oscillation VCSEL, the polarization direction may be different for each oscillation mode, and if an optical element having polarization selectivity is interposed in the transmission path, the transmission of the optical element (or the transmission of the optical element due to mode competition of the emitted light) There is a possibility that the amount of reflected light will fluctuate and the S / N ratio will decrease. For this reason, development of a light source capable of obtaining signal light having good noise resistance in various communication modes using a multimode optical fiber is desired.

  An object of the present invention is to provide an optical module and an optical transmission apparatus suitable for a communication mode using a multimode optical fiber.

  (1) The present invention relates to a surface emitting semiconductor laser in which polarized light preferentially oriented to a given polarization axis is emitted by multimode oscillation, a transmission function selectively with respect to a predetermined polarization, and a reflection function An optical member that exhibits at least one of a refraction function and a diffraction function, and a light receiving element for receiving at least a part of light emitted from the surface-emitting semiconductor laser, the surface-emitting type The semiconductor laser is arranged so that the axis orthogonal to the polarization axis of the emitted light does not overlap the polarization axis of the optical member, and the optical surface of the optical member intersects with the traveling direction of the emitted light of the surface emitting semiconductor laser. The reflected light, refracted light, or diffracted light of the surface emitting semiconductor laser is reflected on the main surface by reflecting, refracting, or diffracting at least a part of the emitted light on the main surface. Related to the optical module lead.

  In the present invention, the preferential orientation of the polarization axis means that there is a polarization axis having a larger output light quantity than other polarization axes. Multimode oscillation refers to oscillation such that the transverse mode of the emitted light includes not only the fundamental (0th order) mode but also higher order modes higher than the primary mode.

  According to the present invention, since the surface emitting semiconductor laser oscillates in multimode, the emitted light has low coherence, and even when a multimode fiber is used, there is little deterioration in the S / N ratio due to mode selection loss.

  In addition, the polarization axis of the preferentially oriented light emitted from the surface emitting semiconductor laser is arranged so as not to overlap the axis orthogonal to the polarization axis of the optical member. There is little variation in the amount of light that has passed through (because of transmission, reflection, refraction, or diffraction). For this reason, according to the present invention, it is possible to realize a low noise light source suitable for a communication mode using a multimode fiber.

  Further, according to the present invention, since the noise can be reduced even if the polarization axis of the surface emitting semiconductor laser and the polarization axis of the optical member are not aligned, the alignment margin during mounting (assembly) is large, and the optical module Can be produced at low cost. In addition, although the polarization axis of a surface emitting semiconductor laser may fluctuate depending on usage conditions such as drive current and ambient temperature, the present invention provides stable operation that is highly resistant to fluctuations in the polarization axis due to usage conditions. Can be realized.

  (2) In the optical module of the present invention, the surface-emitting type semiconductor laser may be arranged so that the angle of the polarization axis of the emitted light with respect to the polarization axis of the optical member is within ± 45 degrees. In this way, it is possible to secure a large alignment margin during mounting (assembly) and to realize a light source that is highly resistant to fluctuations in the polarization axis of the surface emitting semiconductor laser.

  (3) In the optical module of the present invention, the surface emitting semiconductor laser may include a resonator structure in which at least a part of the planar shape has anisotropy. In this way, since the superiority of the polarization axis appears in the major axis direction of the resonator, it becomes possible to obtain outgoing light with a uniform polarization plane.

  (4) The optical module can be applied to, for example, an optical transmission device used in a LAN, a SAN, or the like.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the drawings.

1. Optical Module FIG. 1 is a diagram schematically showing a first optical module according to an embodiment of the present invention.

  The optical module of this embodiment is made of ceramic on a metal stem 5 and a metal film is formed on the upper surface (mounting surface) (to ensure insulation from the stem 5). 1 includes a vertical cavity surface emitting semiconductor laser (light emitting element in a broad sense, hereinafter referred to as a VCSEL for short) 10 that is electrically connected to the substrate 10a via a submount 10a. . A monitor PD (light receiving element in a broad sense) 20 for monitoring the amount of light emitted from the VCSEL 10 is disposed on the stem 5.

  Further, the optical module of the present embodiment has lead pins 1 to 3 for mutually transmitting and receiving electrical signals to / from the outside, and using these, the VCSEL 10, the VCSEL submount 10a, and the monitor PD 20 are wire bonded. Are electrically connected.

  The VCSEL 10 may be one that can obtain linearly polarized light at a given one polarization axis (first polarization axis in a broad sense). Specifically, as shown in FIG. 2, the VCSEL 10 includes a lower mirror (multilayer reflective film) layer 102, an active layer 104, and an upper layer formed of a GaAs / AlGaAs compound semiconductor material on an n-type GaAs substrate 100. It has a resonator structure in which a mirror (multilayer reflective film) layer 106 is laminated. Note that reference numeral 108 in FIG. 2 denotes an insulating layer made of an oxide film. Further, reference numerals 112 and 114 in FIG. 2 denote a p-type electrode and an n-type electrode made of a metal multilayer film, respectively. Further, the VCSEL 10 has a current confinement layer 106 a formed by oxidizing a part of the upper mirror layer 106. In the VCSEL 10, the injection current density into the active layer 104 is controlled by the diameter φ of the current confinement layer 106a, so that the diameter of the active region (light emitting region) 104a is suitable for multimode oscillation (for example, 8 μm or more). . Instead of providing the current confinement layer 106a, the diameter of the active layer 104 may be processed to a diameter that causes multimode oscillation. In the VCSEL 10, the active layer 104 and the upper mirror layer 106 are shaped into an ellipse by anisotropic etching and have an anisotropic resonator structure. In the resonator having such an anisotropic shape, the polarization characteristic of the emitted light depends on the anisotropic long axis direction of the resonator, and specifically, the resonator is strong in the long axis direction of the resonator shape (light quantity) A polarization dominant axis (in a broad sense, the polarization axis) from which a linearly polarized wave can be obtained. Furthermore, if the p-type electrode 112 formed on the resonator is also formed so that the exit port has anisotropy, the current density distribution injected into the resonator becomes anisotropic, and the polarization of the VCSEL 10 The polarization dependence on the dominant axis becomes stronger. As a VCSEL resonator structure having one polarization dominant axis, the one exemplified in JP-A-8-116139 can be adopted in addition to the case where the planar shape is an elliptical shape. . Further, as exemplified in Japanese Patent Laid-Open No. 11-54838, by applying strain stress to the active layer, the resonator gain has anisotropy and a specific polarization mode is preferentially oscillated. A simple structure may be adopted.

  Here, the inventors of the present application measured the dependence of the polarization angle on the output light of the VCSEL 10 using a polarizing filter. FIG. 4 shows the relative intensity noise (RIN) of the emitted light of the VCSEL 10 when measured at 937.5 MHz, which is the use band of Gigabit Ethernet (GbE), as noise characteristics. In the measurement, the short axis direction of the elliptical resonator (axis perpendicular to the polarization dominant axis) was used as a reference (0 degree). According to FIG. 4, it is confirmed that the noise is large in the minor axis direction, and the noise is greatly reduced at an intermediate angle between the minor axis direction and the major axis direction of the resonator. Further, the same noise reduction can be achieved if the polarization dominant axis that appears in the major axis direction of the resonator of the VCSEL 10 is within ± 45 degrees.

  The monitor PD 20 may employ a known light receiving element such as a PN photodiode or a PIN photodiode using a semiconductor such as Si or Ge, or a compound semiconductor (GaAs or InP). Specifically, one having a light receiving wavelength band suitable for the oscillation wavelength of the VCSEL 10 is used.

  Further, in the optical module of the present embodiment, the above-described VCSEL 10, the VCSEL submount 10a, and the monitor PD 20 are enclosed in a metal container 7 joined (or integrated) with the stem 5. The container 7 is disposed above the VCSEL 10 and the monitor PD 20 so that a reflection mirror (optical member in a broad sense) 30 having polarization selectivity as an exit window of the exit light of the VCSEL 10 intersects the traveling direction of the exit light P1 of the VCSEL 10. Has been. The reflection mirror 30 is reflected by a metal film (which may be a single layer or a multilayer) or a dielectric film (which may be a single layer or a multilayer) on one surface (main surface in a broad sense) of a glass plate. It has a reflective surface 30a on which a film is formed. In this reflection surface 30a, the p-polarized light has a higher transmittance than the s-polarized light, and has a predetermined polarization transmission axis (second polarization axis in a broad sense). The light transmittance at the reflecting surface 30a is determined by the thickness and material of the reflecting film, and when the reflecting film is multilayered, it also depends on the number of layers. The reflection mirror 30 transmits a part of the emission light P1 of the VCSEL 10 and outputs the output light P2 to the outside, and reflects the remaining light to guide (inject) the monitor light P3 to the monitor PD.

  Next, the arrangement relationship between the VCSEL 10 and the reflection mirror 30 will be described.

  In the present embodiment, the noise of the optical module is reduced by the positional relationship between the polarization dominant axis of the outgoing light P1 of the VCSEL 10 and the polarization transmission axis of the reflection mirror 30. Specifically, both are arranged so that the axis orthogonal to the polarization dominant axis of the VCSEL 10 does not overlap the polarization transmission axis of the reflection mirror 30. Thereby, the relatively low noise linearly polarized light shown in FIG. 4 can be reliably extracted from the reflecting mirror 30. In particular, if the VCSEL 10 is arranged so that the polarization transmission axis of the reflection mirror 30 is located within ± 45 degrees with the polarization dominant axis of the VCSEL 10 as the center, the low-noise output light P2 can be obtained. That is, even if the polarization dominant axis of the VCSEL 10 and the polarization transmission axis of the reflection mirror 30 are not necessarily aligned, a sufficiently low noise can be achieved. For this reason, the alignment margin at the time of mounting (assembly) is large, and low-cost production of the optical module is possible. Further, the polarization dominant axis of the VCSEL 10 may vary depending on usage conditions such as drive current and environmental temperature, but according to the present embodiment, stable operation that is highly resistant to fluctuations in the polarization dominant axis depending on the usage conditions. Possible light sources can be realized.

  As described above, according to the optical module of the present embodiment, since the VCSEL 10 oscillates in multimode, the emitted light P1 has low coherence, and even if a multimode fiber is used, it is caused by mode selection loss. There is little deterioration of S / N ratio.

  Further, the polarization dominant axis of the emitted light P1 of the VCSEL 10 (long axis in the case of an elliptical resonator; broad axis in the broad sense) (short axis in the case of an elliptical resonator; broad sense) Since the VCSEL 10 is disposed so that the short axis of the anisotropic resonator does not overlap with the polarization axis of the reflection mirror 30, the reflected light of the reflection mirror 30 is reflected even when mode competition occurs. And there is little fluctuation in the amount of transmitted light. For this reason, according to the optical module of this Embodiment, the low noise light source suitable for the communication form using a multimode fiber is realizable.

  As a modification of the first optical module, as shown in FIG. 4, a diffraction grating (optical member in a broad sense) 32 may be used instead of the reflection mirror 30. Even when the diffraction grating 32 is used, one surface (main surface in a broad sense) 32a of the diffraction grating 32 is disposed above the VCSEL 10 so as to intersect the emitted light P1 of the VCSEL 10. In this case, the arrangement of the diffraction grating 32 and the monitor PD 20 is determined so that the first-order diffracted light (low-order diffracted light in a broad sense) is guided to the monitor PD 20. Since the diffraction grating also has polarization selectivity along the direction of the grating, the optical module according to this modification also has a low noise suitable for a communication mode using a multimode fiber, as in the optical module shown in FIG. Can be obtained as the output light P2. In the present embodiment, a configuration in which light is guided to the monitor PD 20 by using refraction in addition to the one using transmission, reflection, or diffraction may be adopted.

2. Optical Transmission Device FIG. 5 is a diagram showing an optical transmission device to which the optical module of the present embodiment is applied.

  The optical transmission device 90 connects electronic devices 92 such as a computer, a display, a storage device, and a printer to each other. The electronic device 92 may be an information communication device. The optical transmission device 90 may be one in which plugs 96 are provided at both ends of the cable 94. The cable 94 is specifically a multimode optical fiber. The plug 96 includes the optical module of the present embodiment as a light source for optical transmission. According to this optical transmission device, data transmission between electronic devices can be performed by an optical signal.

  Note that electronic devices interconnected by the optical transmission device of this embodiment include a liquid crystal display monitor, a digital CRT (may be used in the fields of finance, mail order, medical care, and education), and a liquid crystal projector. Plasma display panel (PDP). It may be a digital TV, a retail store cash register (for POS), a video, a tuner, a game device, or the like.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the gist of the invention. For example, the material constituting each layer of the VCSEL 10 is not limited to a GaAs type material, and may be an InP type or GaN type compound semiconductor.

The figure which shows the optical module of this embodiment. The figure which shows the surface emitting semiconductor laser suitable for the optical module of this embodiment. The figure which shows the polarization dependence of the emitted light of a surface emitting semiconductor laser. The figure which shows the modification of the optical module of this embodiment. The figure which shows the optical transmission apparatus of this embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Surface emitting semiconductor laser, 20 Monitor PD (light receiving element), 30 Reflection mirror (optical member), 30a Reflective surface (main surface), 32 Diffraction grating (Optical member)

Claims (4)

A surface emitting semiconductor laser in which polarized light preferentially oriented to a given polarization axis is emitted by multimode oscillation;
An optical member that exhibits a transmission function and at least one of a reflection function, a refraction function, and a diffraction function selectively with respect to a predetermined polarization;
A light receiving element for receiving at least a part of the emitted light of the surface emitting semiconductor laser;
Including
The surface-emitting type semiconductor laser is arranged so that an axis orthogonal to the polarization axis of the emitted light does not overlap the polarization axis of the optical member,
The optical member is disposed such that a main surface intersects a traveling direction of the emitted light of the surface emitting semiconductor laser, and reflects, refracts, or diffracts at least a part of the emitted light on the main surface, An optical module that guides reflected light, refracted light, or diffracted light emitted from a surface emitting semiconductor laser to the light receiving element. In claim 1,
The surface emitting semiconductor laser is an optical module arranged such that an angle of a polarization axis of outgoing light with respect to a polarization axis of the optical member is within ± 45 degrees. In claim 1 or 2,
The surface emitting semiconductor laser is an optical module including a resonator structure in which at least a part of a planar shape has anisotropy.   An optical transmission device comprising the optical module according to claim 1.

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