Optical transmission module

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2008-023443 filed Feb. 4, 2008.

1. Technical Field

This invention relates to an optical transmission module.

2. Related Art

In technical fields such as optical communication or optical storage, there has been a growing interest in Vertical-Cavity Surface-Emitting Lasers (hereinafter referred to as VCSELs) for use as a light source. VCSELs have excellent characteristics which edge-emitting semiconductor lasers do not have. For example, a VCSEL has a lower threshold current and consumes less power, and VCSELs can be arranged in two-dimensional arrays. Because of these characteristics, applications for an optical transmission module as a light source have been expected in the fields such as optical communication or optical data processing.

In order to achieve a lower cost of an optical transmission module, it is required that a VCSEL and a driving circuit for driving the VCSEL are mounted in a same package. The optical transmission module performs a burn-in test and a defect determination.

An aspect of the present invention provides an optical transmission module that includes: a semiconductor light emitting element for emitting laser light; a first driving unit for providing a first driving current to the semiconductor light emitting element; a switching unit connected between the semiconductor light emitting element and the first driving unit; and a package for accommodating the semiconductor light emitting element, the first driving unit, and the switching unit. The switching unit includes a first input for receiving a first driving current outputted from at least from the first driving unit, a second input for receiving a second driving current for testing the semiconductor light emitting element, and an output connected to the semiconductor light emitting element. The switching unit connects the first input or the second input to the output.

Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein:

FIG. 1 is a schematic cross sectional view of an optical transmission module according to an example of the present invention;

FIG. 2 is a schematic cross sectional view illustrating a configuration of a VCSEL shown in FIG. 1 ;

FIG. 3 is a block diagram illustrating a functional configuration of an optical transmission module according to a first example of the present invention;

FIG. 4 illustrates a detailed circuit configuration of the optical transmission module shown in FIG. 3 ;

FIG. 5 is a table illustrating the relation between internal control signals and operation modes;

FIG. 6 is a block diagram illustrating a functional configuration of an optical transmission module according to a second example of the present invention;

FIG. 7 is a block diagram illustrating a detailed circuit configuration of the optical transmission module shown in FIG. 6 ;

FIG. 8 is a block diagram illustrating a functional configuration of an optical transmission module according to a third example of the present invention;

FIG. 9 is a block diagram illustrating a detailed circuit configuration of the optical transmission module shown in FIG. 8 ;

FIG. 10 is a block diagram illustrating a functional configuration of an optical transmission module according to a fourth example of the present invention;

FIG. 11 is a block diagram illustrating a functional configuration of an optical transmission module according to a fifth example of the present invention; and

FIG. 12 is a block diagram illustrating a functional configuration of an optical transmission module according to a sixth example of the present invention.

Referring to the accompanying drawings, exemplary embodiments for implementing the present invention will be described. In the specification, an optical transmission module in which an electronic device including a VCSEL is implemented in a CAN package is described as an example. However, it is exemplary only, and a ceramic package or a resin package can be used for the optical transmission module. In addition, the exemplary embodiment of the electronic device including a VCSEL can be varied.

FIG. 1 is a cross sectional view illustrating a schematic configuration of an optical transmission module according to an example of the present invention. An optical transmission module 10 according to an example may include a disc-shaped metallic stem 12 , a submount 14 mounted on an upper surface of the stem 12 , an IC chip 16 mounted on the submount 14 , a VCSEL 18 mounted on the IC chip 16 , a cylindrical cap 20 fixed on the stem 12 , plural lead terminals 22 made of a conductive metal and mounted on a bottom surface of the stem 12 .

In a center portion of the cap 20 , a circular opening 20 a may be formed. For closing the opening 20 a , a flat glass plate 20 b is mounted inside the cap 20 . The cap 20 and the flat glass plate 20 b forms a preferably hermetically sealed internal space on the stem 12 , and protects the IC chip 16 and the VCSEL 18 from external environment. In this example, the flat glass plate is mounted to the CAN package; however, other than the flat plate glass, an optical material such as a spherical lens, a convex lens, a concave lens, or the like may be mounted instead.

FIG. 2 is a cross sectional view of the VCSEL shown in FIG. 1 . In this example, a single-spot type VCSEL in which one light emitting spot is formed is illustrated; however, it is exemplary only, and a multi-spot type VCSEL in which plural light emitting spots are formed may be used.

As shown in FIG. 2 , the VCSEL 18 may be manufactured as follows. An n-side electrode 102 is formed on a back surface of an n-type GaAs substrate 100 . Stacked on the substrate 100 are an n-type GaAs buffer layer 104 , an n-type lower Distributed Bragg Reflector (DBR) 106 made of alternately stacked AlGaAs layers each having a different Al-composition, an active region 108 , a current confining layer 110 made of p-type AlAs and including an oxidized region on the periphery thereof, a p-type upper DBR 112 made of alternately stacked AlGaAs layers each having a different Al-composition, and a p-type GaAs contact layer 114 . A cylindrical post (or mesa) P is formed on the substrate by etching the stacked semiconductor layers.

On the entire surface of the substrate, an interlayer insulating film 116 such as SiON, SiOx, or the like is formed to cover a bottom portion, a side portion, and a portion of top portion of the post P. In the interlayer insulating film 116 at a top portion of the post P, a circular contact hole 116 a for exposing the contact layer 114 is formed. At a top portion of the post P, a p-side electrode 118 made of a conductive material is formed. The p-side electrode 118 is ohmic-connected to the contact layer 114 through the contact hole 116 a . In a center portion of the p-side electrode 118 , a circular emission window for emitting laser light is formed. In this example, the emission window exposes the contact layer 114 ; however, the emission window may be protected with a dielectric film, alternatively.

The lower DBR 106 and the upper DBR 112 function as a resonator structure, and therebetween, the active region 108 and the current confining layer 110 are interposed. The current confining layer 110 includes an oxidized region formed by selectively oxidizing from a side surface of the post P and a circular conductive region surrounded by the oxidized region, and confines current and light in the conductive region. The center portion of the conductive region coincides with the center of the emission window in the p-side electrode 118 . When a forward driving current is injected to the p-side electrode 118 and the n-side electrode 102 , laser light of 850 nm, for example, may be emitted from the emission window of the post P perpendicularly to the substrate. The emitted laser light passes through the flat glass plate 20 b.

The IC chip 16 includes a silicon substrate on which an integrated circuit is formed. The integrated circuit may include a component such as a driving circuit for driving the VCSEL 18 as described later, and a switching circuit for switching a driving current to the VCSEL. On the silicon substrate, a light receiving element 16 a is formed adjacent to the region on which the VCSEL 18 is mounted. The light receiving element 16 a receives a portion of laser light emitted from the VCSEL 18 , in other words, the light reflected from the flat glass plate 20 b , and outputs it as a signal for monitoring optical output.

On a surface of the IC chip 16 , an electrode or the like for connecting the n-side electrode 102 of the VCSEL 18 may be formed. The n-side electrode 102 may be electrically connected to the electrode by means of a conductive adhesive, soldering, or other connecting means. In this example, the VCSEL is mounted on the IC chip 16 ; however, it is not necessarily limited to this example, and the IC chip 16 and a VCSEL chip may be disposed on a submount next to each other.

For the submount 14 , preferably, an insulating ceramic substrate (for example, aluminum nitride) having a high thermal conductivity and an excellent dissipation is used. On a surface of the submount 14 , a thin film pattern of Ti/Pt/Au or the like may be formed, and the IC chip 16 may be mounted thereon.

In the stem 12 , through holes (not shown) for inserting the plural lead terminals 22 are formed. The inner wall of each through hole is covered with glass, which electrically insulates the lead terminals from the stem 12 . One end of the lead terminal 22 extends in the internal space, and is electrically connected to a corresponding wiring or electrode of the IC chip 16 or the VCSEL 18 using a connecting medium such as a bonding wire or the like in the internal space.

FIG. 3 is a block diagram illustrating a functional configuration of an optical transmission module. The IC chip 16 includes a modulation current providing unit 40 for driving and modulating the VCSEL 18 , a burn-in current providing unit 42 for providing a burn-in driving current to the VCSEL 18 when a burn-in test of the VCSEL is performed, a switching unit 44 , and a defect determination unit 46 .

To lead terminals 22 a and 22 b of the optical transmission module 10 , a driving signal D and a control signal C are applied, respectively. From a lead terminal 22 c , a defect determination signal H determined by the defect determination unit 46 is outputted. The control signal C controls operations of the modulation driving current providing unit 40 , the burn-in current providing unit 42 , the switching unit 44 , and the defect determination unit 46 , as described later. Each of the lead terminals 22 a , 22 b , and 22 c is electrically connected to a corresponding electrode of the IC chip.

The modulation driving current providing unit 40 becomes active responsive to the control signal C, and outputs a modulation driving current that corresponds to the driving signal D inputted from the lead terminal 22 a to an input 44 a of the switching unit 44 . The burn-in current providing unit 42 becomes active responsive to the control signal C, and outputs a burn-in driving signal to an input 44 b of the switching unit 44 . The defect determination unit 46 becomes active responsive to the control signal C, and outputs a measuring signal for defect determination to an input 44 c of the switching unit 44 . The defect determination unit 46 further determines whether the VCSEL has a failure or not by analyzing the measuring signal, and outputs the defect determination signal H to the lead terminal 22 c as a result of the analysis.

The switching unit 44 may select any of the inputs 44 a , 44 b , and 44 c , for example, depending on the voltage level of the control signal C, and connects the selected input to an output 44 d . The output 44 d is connected to the p-side electrode 118 on an anode side of the VCSEL 18 .

The switching unit 44 selects the input 44 a when the control signal C is 0 V. By this selection, the driving current outputted from the modulation current providing unit 40 is provided to the VCSEL 18 , and the VCSEL 18 emits light in a normal operation mode.

When the control signal C is 1.2 V, the switching unit 44 selects the input 44 b . By this selection, the burn-in driving current outputted from the burn-in current providing unit 42 is provided to the VCSEL 18 , and a burn-in test of the VCSEL 18 is performed. In the burn-in test, the optical transmission module 10 is held in an oven at a high temperature of about 120 degrees centigrade in a state where the IC chip 16 and the VCSEL 18 are mounted in the optical transmission module 10 as shown in FIG. 1 . The VCSEL 18 is driven for a constant time period by a burn-in test driving current that is about four times the driving current in the normal operation mode. During the burn-in test, the modulation current providing unit 40 is electrically disconnected by the switching unit 44 , which prevents the burn-in driving current from leaking into the modulation current providing unit 44 . Therefore, the VCSEL can be tested under an accurate test condition. In addition, the modulation current providing unit 40 is protected from the burn-in driving current.

When the control signal C is 3.3 V, the switching unit 44 selects the input 44 c . By this selection, the measuring signal of the defect determination unit 46 is provided to the VCSEL 18 . The measuring signal includes any of a forward current, a forward voltage, or a reverse current, and compares operating conditions of the VCSEL at an initial state and operating conditions of the VCSEL after the burn-in test, and determines whether the VCSEL has a failure or not according to predetermined defect determination conditions. For example, the current value in a case where a minute forward voltage is applied, or the current value in a case where a reverse voltage is applied may be measured. Although the current is a minute current of equal to or less than 1 μA, the switching unit 44 electrically disconnects the modulation current providing unit 40 and the burn-in current providing unit 42 when a defect determination is performed, and therefore leakage of the measuring signal can be prevented and degradation in S/N ratio of the measuring signal can be prevented, and a defect of the VCSEL can be accurately determined. In addition, by combining with an output signal of the light receiving element 16 a formed in the IC chip, for example, the optical output at an oscillation threshold current value or at a constant current value can be measured for use in the defect determination.

As described above, by inputting the control signal C from outside of the optical transmission module to the lead terminal 22 b , the optical transmission module can select a desired operation mode from plural operation modes. In other words, any mode can be selected from: the normal mode in which the VCSEL 18 is driven by a driving current from the modulation driving current providing unit 40 ; the test mode in which the VCSEL 18 is driven by a burn-in driving current from the burn-in driving current providing unit 42 ; or the defect determination mode in which the VCSEL 18 is driven by a measuring signal from the defect determination unit 46 for determining a defect of the VCSEL.

The defect determination mode is preferably performed after the test mode; however, it is not limited to the particular order. For example, during the normal mode, by intermittently switching to the defect determination mode for performing a defect determination, an advance notice of degradation and a defect of the VCSEL can be provided.

FIG. 4 illustrates a detailed circuit configuration example of the IC chip shown in FIG. 3 . Formed in the IC chip 16 are: input terminals 50 a and 50 b for inputting a differential input signal being a driving signal, an input terminal 52 for inputting the control signal C, an output terminal 54 for outputting the defect determination signal H, an anode output terminal 56 connected to the p-side electrode of the VCSEL, a cathode output terminal 58 connected to the n-side electrode, a Vdd terminal for providing a power source, and a GND terminal for grounding. These input and output terminals are electrically connected to corresponding lead terminals 22 ( 22 a , 22 b , and 22 c ).

A modulation current providing circuit 40 A includes an amplifier 60 for amplifying differential input signals inputted from the input terminals 50 a and 50 b , a temperature correction table 62 that stores temperature correction data for performing temperature compensation of the VCSEL, a temperature correction circuit 64 for controlling the amplifier 60 based on the temperature correction data in the temperature correction table 62 , and a light amount correction circuit 66 for correcting the amount of light based on the optical output received by the light receiving element 16 a . The amplifier 60 is a multi-stage amplifier including an input amplifier 60 a for amplifying the differential input signal, a preamplifier 60 b , and an output amplifier 60 c.

VCSELs have a characteristic that the optical output decreases as the operating temperature rises, and the optical output increases as the operating temperature lowers. Therefore, the temperature correction circuit 64 reads the temperature correction data that corresponds to the operating temperature from the temperature correction table 62 , and adjusts the gain of the output amplifier 60 C depending on the temperature correction data. The light amount correction circuit 66 feedback-controls the output amplifier 60 C to the optical output monitored by the light receiving element 16 a such that the optical output of the VCSEL is kept constant.

A control circuit 70 generates internal control signals C 1 , C 2 , C 3 , and C 4 responsive to the control signal C inputted from the input terminal 52 . The internal control signal C 1 is provided to the amplifier 60 . When the internal control signal C 1 is H level, the amplifier 60 becomes active and outputs a driving current responsive to the differential input signal. When the internal control signal C 1 is L level, the amplifier 60 becomes inactive, and stops providing the driving current.

The internal control signal C 2 is provided to a burn-in driving circuit 42 A. When the internal control signal C 2 is H level, the burn-in driving circuit 42 A becomes active and outputs a burn-in driving current. When the internal control signal C 2 is L level, the burn-in driving circuit 42 A becomes inactive and stops providing the burn-in driving current.

The internal control signal C 3 is provided to a defect determination circuit 46 A. When the internal control signal C 3 is H level, the defect determination circuit 46 A becomes active and outputs a measuring signal. The defect determination circuit 46 A further analyzes the current values or the like of the measuring signal, determines whether the VCSEL has a defect, and provides the defect determination signal H as a result of the determination to the output terminal 54 . When the internal control signal C 3 is L level, the defect determination circuit 46 A becomes inactive and stops providing the measuring signal. FIG. 5 illustrates the relation between the internal control signals C 1 , C 2 , C 3 , and the operation modes.

The internal control signal C 4 is provided to a switching circuit 44 A. The internal control signal C 4 controls the switching operations of the switching circuit 44 A, depending on the voltage level of the control signal C described above and corresponding to each operation mode. In the normal mode, an input that receives the driving current outputted from the amplifier 60 is connected to outputs 44 d - 1 and 44 d - 2 . In the test mode, an input that receives the driving current outputted from the burn-in driving circuit 42 A is connected to the outputs 44 d - 1 and 44 d - 2 . In the defect determination mode, an input that receives the measuring signal outputted from the defect determination circuit 46 A is connected to the output 44 d - 1 . The output 44 d - 1 of the switching circuit 44 A is connected to the anode output terminal 56 , and the output 44 d - 2 is connected to the cathode output terminal 58 . The VCSEL 18 is driven by the voltage and current provided from the anode output terminal 56 and the cathode output terminal 58 .

By screening the optical transmission module configured as described above using the burn-in test and the defect determination prior to shipment, generation of early failures associated with the VCSEL can be reduced.

The process for testing early failures is preferably performed in a finished form when possible. This is because, if the burn-in test is performed at a halfway of the manufacturing, the VCSEL may be damaged in a subsequent process, especially in handing the VCSEL chip cut out from the semiconductor substrate, and an early failure caused by the damage cannot be eliminated.

In general, when a driving circuit and a VCSEL are implemented in a package, if the driving circuit and the VCSEL are kept electrically connected each other, a test of a finished form cannot be performed accurately, and thus the VCSEL needs to be tested before the VCSEL is packaged in the finished form. In this case, early failures due to the factors as described above cannot be eliminated, and resulting that it is difficult to reduce early failures caused after shipping.

In the examples of the invention, the driving circuit and the VCSEL can be disconnected by the switching unit.

A second example of the present invention will be now described. FIG. 6 is a block diagram illustrating a functional configuration of an optical transmission module according to the second example. Same reference numerals have been retained for similar configurations to those of the first example, and explanation thereof are not described herein. The second example differs from the first example in that an IC chip 16 A does not include a defect determination unit. An optical transmission module 10 A includes a lead terminal 22 c for inputting a measuring signal externally provided from the defect determination unit 46 . When a defect determination of the VCSEL is performed, the switching unit 44 connects the input 44 c to the output 44 d responsive to the control signal C. By this switching, the measuring signal inputted from the lead terminal 22 c is provided to the VCSEL 18 . The defect determination unit 46 analyzes the current value or the like of the measuring signal flowing through the lead terminal 22 c for determining whether the VCSEL has a failure.

FIG. 7 is a block diagram illustrating a detailed circuit configuration example of the IC chip shown in FIG. 6 . As shown in FIG. 7 , the IC chip 16 A has connection terminals 54 a and 54 b being connected to a defect determination circuit 46 A. When the defect determination mode is performed, a measuring signal is provided to the VCSEL 18 through the connection terminals 54 a and 54 b , and the switching circuit 44 A.

A third example of the present invention will be now described. FIG. 8 is a block diagram illustrating a functional configuration of an optical transmission module according to the third example. In an optical transmission module 10 B according to the third example, an IC chip 16 B includes the modulation current providing unit 40 and the switching unit 44 . The burn-in current providing unit 42 and the defect determination unit 46 are connected to the lead terminal 22 c of the optical transmission module.

When the normal mode is selected by the control signal C, a driving current outputted from the modulation current providing unit 40 is provided to the VCSEL through the switching unit 44 . In other modes than the normal mode, the switching unit 44 disconnects the input 44 a from the output 44 d , and connects the input 44 b to the output 44 d . When a burn-in test is performed, a burn-in driving current outputted from the burn-in current providing unit 42 is externally provided to the VCSEL 18 through the lead terminal 22 c and the switching unit 44 . When a defect determination is performed, a measuring signal outputted from the defect determination unit 46 is externally provided to the VCSEL 18 through the lead terminal 22 c and the switching unit 44 . As described above, it is prevented that the burn-in driving current and the measuring signal from leaking into the modulation current providing unit 40 during a burn-in test or a defect determination. FIG. 9 illustrates a detailed circuit configuration example of the IC chip shown in FIG. 8 . Although the burn-in driving circuit 42 A and the defect determination circuit 46 A are not shown, the outputs from these circuits are connected to the connection terminal 54 of the IC chip 16 B.

FIG. 10 is a block diagram illustrating of a functional configuration of an optical transmission module according to a fourth example of the present invention. In an optical transmission module 10 C according to the fourth example, the switching unit 44 is not formed in an IC chip 16 C, and formed isolated from the IC chip 16 C. The switching unit 44 may be disposed on the submount shown in FIG. 1 and spaced from the IC chip 16 , and may use an electromagnetic relay that performs a switching operation responsive to the control signal C, for example. Other arrangement is same as in the third example.

FIG. 11 is a block diagram illustrating a functional configuration of an optical transmission module according to a fifth example of the present invention. In an optical transmission module 10 D according to the fifth example, a burn-in driving current or a measuring signal provided from the lead terminal 22 c is not inputted to the switching unit 44 , and connected directly to an anode electrode (p-side electrode) of the VCSEL 18 . When the normal mode is performed in the optical transmission module 10 D, the driving signal D is inputted to the modulation current providing unit 40 through the lead terminal 22 a , and the modulation current providing unit 40 outputs a driving current. Responsive to a control signal C inputted from the lead terminal 22 c , the switching unit 44 connects the input 44 a that receives a driving current outputted from the modulation current providing unit 40 to the output 44 d , which drives the VCSEL 18 . In other modes than the normal mode, the switching unit 44 disconnects the input 44 a connected to the output of the modulation current providing unit 40 from the output 44 d . This prevents the burn-in driving current or the measuring signal from electrically interfering with the modulation current providing unit 40 during a burn-in test or a defect determination.

FIG. 12 is a block diagram illustrating a functional configuration of an optical transmission module according to a sixth example of the present invention. In an optical transmission module 10 E according to the sixth example, the IC chip 16 C does not include the switching unit as in the case of the fourth example. Also, the optical transmission module 10 E does not include the lead terminal 22 b for inputting the control signal C. The switching unit 44 may be made of, for example, a jumper switch, a DIP switch or the like, and is implemented in the optical transmission module 10 E. When a burn-in test or a defect determination is performed, the switching operation of the switching unit 44 is performed from outside of the optical transmission module 10 E such that the input 44 b connected to the lead terminal 22 c is connected to the output 44 d.

While exemplary embodiments of the present invention have been described in detail, the present invention is not intended to be limited to specific examples and various changes and modifications can be made within the scope of the invention set forth in the appended claims.

An optical transmission module according to the present invention can be applied to a transmission module used in various fields such as optical transmission.