JP2006126274A - Optical receiving module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical receiving module meeting the need for high-speed optical communication. <P>SOLUTION: The optical receiving module 10 is equipped with a spherical tip fiber 11 which emits, from an outgoing end face 11a formed by a radius of curvature R, a light beam that has entered; a photodetector 12 including a light-absorbing layer for converting a received light beam to a current signal; a sub-mount 13 for mounting the photodetector 12; a fiber-fixing part 14 for fixing the spherical tip fiber 11; a preamplifier IC 15, which converts the current signal outputted from the photodetector 12 into a voltage signal and amplifies it; a bonding wire 16 for transmitting electrical signals; a package 17 for sealing each component; and a terminal 18 for outputting the electrical signals. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical receiving module that receives an optical signal and converts it into an electrical signal.

A conventional optical receiving module includes a short optical fiber that emits an optical signal from an end face that is cut in a plane, and a light receiving element that receives the emitted light, and the light receiving element is sufficient to receive a spread light beam. A wide light receiving portion (see, for example, Patent Document 1).
JP 2001-33667 A (page 3-4, FIG. 1)

  The light receiving area of the light receiving element in the optical receiving module becomes smaller as the optical communication speed increases, and the spread light beam cannot receive light efficiently, so that there is a problem that the characteristics required for high speed optical communication cannot be sufficiently satisfied. .

  In high-speed optical communication exceeding gigabit / second, it is necessary to reduce the capacity of the light receiving area, that is, to reduce the area of the light receiving section due to the restriction of the high cut-off frequency. For example, when the optical communication speed is 10 gigabits / second, the radius of the light receiving unit is about 10 μm, so the light beam spreading radius must also be suppressed to 10 μm or less, and a conventional optical receiver module uses a lens. With this, the spread of the light beam was suppressed, and high-speed optical communication exceeding 10 gigabits / second was supported.

  The present invention has been made to solve such a problem, and can efficiently collect an optical signal on a light receiving element without using a lens, and can cope with an increase in optical communication speed. An optical receiver module is provided.

  An optical receiver module according to claim 1 of the present invention is provided with an optical fiber and a light receiving element that is opposed to the optical output end face of the optical fiber at a predetermined interval and converts light into an electrical signal. The optical fiber is a tip spherical fiber formed in a spherical shape with the light emitting end face having a predetermined radius of curvature, and the position of the beam waist of the light emitted from the tip spherical fiber is the light receiving portion of the light receiving element. It has the structure characterized by being nearer than.

  According to a second aspect of the present invention, there is provided the optical receiver module according to the first aspect, wherein the light receiving element is an avalanche photodiode (APD).

  Furthermore, an optical receiver module according to claim 3 of the present invention is the optical receiver module according to claim 1 or 2, wherein the light receiving element is of a back-illuminated type.

  With these configurations, the optical receiver module of the present invention emits light from a short optical fiber having a predetermined radius of curvature and miniaturizes the spread radius of the light beam at the light receiving portion of the light receiving element. Therefore, it is possible to condense light efficiently and cope with high speed optical communication.

  The present invention can provide an optical receiver module capable of efficiently condensing an optical signal on a light receiving element and having an effect of being able to cope with high speed optical communication.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  First, the configuration of the optical receiver module according to the embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a plan view showing an outline of the internal structure of the optical receiver module according to the present embodiment, and FIG. 2 is an explanatory diagram of the configuration and positional relationship of the main part of the optical receiver module.

  As shown in FIG. 1, the optical receiver module 10 of the present embodiment includes an optical fiber (hereinafter referred to as “front-end fiber”) 11 that emits light from a light emitting end face 11 a formed with a radius of curvature R, and light reception. Light receiving element 12 including a light absorption layer for converting the converted light into a current signal, a submount 13 for mounting the light receiving element 12, a fiber fixing portion 14 for fixing the tip fiber 11, and a current output from the light receiving element 12. A preamplifier IC 15 that converts a signal into a voltage signal, a bonding wire 16 that transmits an electrical signal, a package 17 that seals each component, and a terminal 18 that outputs the electrical signal are provided.

  The tip fiber 11 is formed of a material such as glass or plastic, and transmits an incident optical signal to be emitted from the light emitting end face 11a to the light receiving element 12.

  The light receiving element 12 is configured by, for example, a mesa avalanche photodiode (hereinafter referred to as “APD”) in which a plurality of semiconductor layers are stacked in a trapezoidal shape.

  The submount 13 is made of, for example, an alumina substrate having a wiring pattern so that the light receiving element 12 can be flip-chip mounted. Specifically, the submount 13 has bumps made of, for example, Au / Sn in order to alleviate stress generated during bonding mounting with the light receiving element 12 and to ensure strength during mounting. The bumps are provided on an n electrode and a p electrode, which will be described later, so that a predetermined voltage is applied to the light receiving element 12 and a signal from the light receiving element 12 is transferred.

  The connection from the light receiving element 12 to the submount 13 and the connection by the bonding wire 16 from the submount 13 to the preamplifier IC 15 are designed to minimize parasitic inductance and stray capacitance.

  The fiber fixing portion 14 is fixed to the package 17 and has, for example, a groove for holding and fixing the front-end fiber 11.

  The preamplifier IC 15 includes a transimpedance amplifier (hereinafter referred to as “TIA”) that converts a current signal output from the light receiving element 12 into a voltage, amplifies the signal, and outputs the voltage signal.

The tip fiber 11 and the light receiving element 12 have the configuration and positional relationship as shown in FIG. That is, the tip fiber 11 includes a core portion 11b having a radius w ₀ for transmitting an optical signal and a cladding portion 11c provided on the outer periphery of the core portion 11b. The light emitting end surface 11a of the core portion 11b is obtained by polishing, for example. The formed spherical surface has a radius of curvature R.

The light receiving element 12 includes a semiconductor substrate 12a having a thickness of _{d 2,} and a light receiving portion 12b formed on the semiconductor substrate 12a. The semiconductor substrate 12a has a light incident surface 12c on which an optical signal from the tip fiber 11 is incident, and a light receiving portion forming surface 12d facing the light incident surface 12c.

  Although not shown, when the light receiving element 12 is formed of a mesa APD, for example, a buffer layer made of n + type InP is formed on the light receiving part forming surface 12d of the semiconductor substrate 12a made of n + type InP. , Made of i-type InGaAs, which absorbs incident light and generates electron-hole pairs as carriers, an electric field relaxation layer made of n + -type InP, and a multiplication made of p-type InP A layer and a contact layer made of p + -type InGaAs are stacked in the mesa portion by epitaxial growth. An n electrode is provided on the light receiving portion forming surface 12d, and a p electrode is provided in contact with the contact layer.

In FIG. 2, the light beam emitted from the apex of the light emitting end face 11a of the front-end fiber 11 is d _{0 from} the beam waist position to the beam waist position (A surface) where the minimum beam spot radius w ₁ is reached. the distance to the light incident surface 12c and _{d 1.} Further, the distance from the apex of the light emitting end surface 11a to the light incident surface 12c is z.

The beam spot radius at the beam waist position is w ₁ , and the beam spot radius at the light receiving unit 12b is w ₂ . The beam spot radius w _2, when made to correspond to the optical communication speed of, for example, 10 Gbit / s, is set to less than about 10 [mu] m.

  Here, the basic circuit configuration of the optical receiver module will be described with reference to FIG. FIG. 3 shows an equivalent circuit of APD and TIA.

As shown in FIG. 3, APD can be represented by a parallel equivalent circuit of the current source and the capacitance and the internal resistance R _1. The electrostatic capacitance includes a parasitic capacitance C ₀ generated when mounted on the submount 13 and an element capacitance C ₁ resulting from the structure of the light receiving portion 12b.

Further, TIA is composed of an amplifier having a feedback resistor R _f, so that the amplification factor of the amplifier is adjusted by the value of the feedback resistor R _f. The input impedance R _{0 of the} TIA is proportional to the feedback resistance R _f .

  In order to make the optical receiver module 10 compatible with high-speed optical communication, it is necessary to improve the frequency characteristics and the signal-to-noise ratio (hereinafter referred to as “SN ratio”) of the optical receiver module 10.

First, the frequency characteristics of the optical receiving module 10 are determined by the parasitic capacitance C ₀ , the element capacitance C ₁ , the input impedance R _0, and the internal resistance R ₁ . The high cut-off frequency f _{3 dB} that determines this frequency characteristic is expressed by the following equation.

f _{3 dB} = 1 / (2π (C ₀ + C ₁ ) (R ₀ + R ₁ )) (1)
On the other hand, in order to improve the SN ratio of the optical receiving module 10, it is necessary to increase the feedback resistance _Rf in the TIA.

However, the feedback resistor R _f is proportional to the input impedance R _0, and increasing the feedback resistor R _f to increase the S / N ratio increases the input impedance R ₀ and decreases the high-frequency cutoff frequency f _{3 dB.} Therefore, it is difficult to increase the feedback resistance _Rf more than necessary.

Therefore, in order to increase the high frequency cutoff frequency f _{3 dB} , the parasitic capacitance C ₀ and the element capacitance C ₁ must be reduced. Since the parasitic capacitance C ₀ can be reduced by controlling variations in pattern design and mounting of the submount 13, it is necessary to reduce the element capacitance C ₁ resulting from the APD structure. That is, by reducing the device capacitance C ₁ by reducing the light receiving area of the light receiving portion 12b, and an optical receiver module 10 can correspond to high-speed optical communication.

For example, 10 when made to correspond to the optical communication gigabit / second speed, according to the study results, it is necessary to the device capacitance C ₁ of the light receiving portion 12b to approximately 250 fF, the light receiving portion 12b in this case is common receiving sensitivity Considering the elongation (length) of the depletion layer of the light receiving element having a radius of about 10 μm, a value can be obtained. Therefore, should the beam spot radius w ₂ of the optical signals incident on the light receiving portion 12b below about 10 [mu] m.

Next, to the beam spot radius w ₂ below about 10 [mu] m, will be described setting of the radius of curvature R and the distance z of the light emitting end face 11a.

4 is a graph showing a relation between the beam spot radius _{w 2} in the radius of curvature R and the light receiving portion 12b of the light emitting end face 11a, in FIG. _{2, w} 0 = 4 [mu] _m, and d 2 = 100 [mu] m, the distance z parameters This is the calculation result.

As shown in FIG. 4, the beam spot radius w ₂ To 10μm or less, the distance z to 40μm or less, the radius of curvature R of the light-emitting end face 11a of about 15μm or more, preferably it is desirable to set the above 20μm .

FIG. 5 is a graph comparing the tip ball fiber 11 of the present invention and a conventional flat-cut flat fiber with respect to the relationship between the distance z and the beam spot radius w _{2. In} FIG. 2, w ₀ = This is a calculation result when 4 μm and d ₂ = 100 μm.

As shown in FIG. 5, if you set the radius of curvature R of the light-emitting end face 11a to 20 [mu] m, with the distance z decreases, towards the beam spot radius w ₂ by hemispherically fiber 11 of the present invention according to conventional flat fiber It tends to be smaller than things. In order to set the beam spot radius w ₂ to about 10 μm or less, the distance z may be set within a range of about 0 to 45 μm, and the distance z is set to about 20 μm in consideration of component variations and manufacturing workability. Is desirable.

  Next, a method for manufacturing the optical receiver module 10 of the present embodiment will be described with reference to FIGS.

  First, the preamplifier IC 15 is fixed to a predetermined position of the package 17, and the electrodes provided on the preamplifier IC 15 and the package 17 are electrically connected by the bonding wire 16. Next, the submount 13 on which the light receiving element 12 is mounted is fixed to a positioning member (not shown) provided in the package 17, for example. The light receiving element 12 is previously positioned at a predetermined position of the submount 13, and is fixed by, for example, soldering.

  Further, the electrodes provided on the preamplifier IC 15 and the submount 13 are electrically connected by bonding wires 16.

  Next, the tip spherical fiber 11 is positioned and fixed on the fiber fixing portion 14 fixed to the package 17. The front-end fiber 11 is held by an XY position adjusting jig that moves in the X-axis and Y-axis directions, and the optical axis with respect to the light receiving unit 12b is adjusted. The X axis represents the direction perpendicular to the Z axis in contact with the upper surface of the fiber fixing portion 14, the Y axis represents the direction toward the paper surface orthogonal to the X axis, and the Z axis represents the longitudinal direction of the tip fiber 11.

  Subsequently, a power supply device that drives the light receiving module 10 and a measuring device that monitors the output of the light receiving module 10 are connected to the terminal 18, and light is incident on the front-end fiber 11. Next, the position of the tip spherical fiber 11 is adjusted by the XY position adjusting jig and fixed to the fiber fixing portion 14. Specifically, with respect to the Z-axis direction, the distance z from the apex of the light emitting end surface 11a to the light incident surface 12c is set to about 20 μm, and the output level from the light receiving element 12 is set with respect to the X-axis and Y-axis directions. The position is set to a predetermined value and fixed. The upper surface opening is sealed and fixed by a sealing cover (not shown).

  Next, the operation of the optical receiver module 10 according to the present embodiment will be described with reference to FIGS.

  First, the optical signal incident on the tip fiber 11 is transmitted through the core portion 11b, reaches the light emitting end face 11a, and is emitted from the light emitting end face 11a.

The light beam emitted from the light emitting end face 11a, the beam diameter after being emitted is gradually reduced, and the minimum beam radius w ₁ at the beam waist position. From the beam waist position, the beam diameter gradually increases and enters the light incident surface 12 c of the light receiving element 12. Then, while spreading within the semiconductor substrate 12a at an angle satisfying the Snell's law reaches the light receiving portion 12b, a beam spot radius w _2.

  Subsequently, the light incident on the light receiving portion 12b is absorbed by the light absorption layer, and a pair of electrons and holes is generated and moves to the electrode of the submount 13 respectively. Specifically, electrons move to the electrode of the submount 13 through the n electrode of the light receiving element 12, and the holes move to the multiplication layer of the light receiving element 12 and are multiplied to move to the p electrode. After that, it moves to the electrode of the submount 13.

  Next, a current signal is output from the light receiving element 12 and input to the preamplifier IC 15 by the holes of the main carrier that has moved to the electrode of the submount 13. Then, the current signal output from the light receiving element 12 is voltage-converted and amplified by the preamplifier IC 15 and output from the terminal 18 via the bonding wire 16.

As described above, according to the optical receiving module 10 of the present embodiment, hemispherically fiber 11 has a radius of curvature R, the configuration to narrow a beam spot radius w ₂ of the light beam at the light receiving portion 12b to about 10μm Therefore, the optical signal can be efficiently collected on the light receiving unit 12b, and the optical communication can be speeded up.

  In the present embodiment, the light receiving element 12 is described as an example of a mesa APD. However, the present invention is not limited to this example. For example, the light receiving element 12 is a planar pin type. A similar effect can be obtained even when the semiconductor light receiving element is used.

  In the present embodiment, the back-illuminated light receiving element in which the light receiving portion 12b is provided on the light receiving portion forming surface 12d facing the light incident surface 12c on which the optical signal is incident has been described as an example. The invention is not limited to this, and the same effect can be obtained even when the light receiving portion 12b is provided on the light incident surface 12c, for example.

  As described above, the optical receiver module according to the present invention has an effect that the optical signal can be efficiently collected on the light receiving element, and the optical communication speed can be increased, and the optical signal is received. Thus, it is useful as an optical receiver module that converts an electrical signal.

The top view which shows the outline | summary of the internal structure of the optical receiver module which concerns on embodiment of this invention Explanatory drawing of a structure and positional relationship of the principal part of the optical receiver module which concerns on embodiment of this invention The figure which shows the equivalent circuit of APD and TIA of the optical receiver module which concerns on embodiment of this invention Diagram showing the relationship between the radius of curvature R and the beam spot radius w ₂ in the optical receiver module according to the embodiment of the present invention Shows the relationship between the distance z and the beam spot radius w ₂ in the optical receiver module according to the embodiment of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Optical receiving module 11 Tip spherical fiber 11a Light emission end surface 11b Core part 11c Clad part 12 Light receiving element 12a Semiconductor substrate 12b Light receiving part 12c Light incident surface 12d Light receiving part formation surface 13 Submount 14 Fiber fixing part 15 Preamplifier IC
16 Bonding wire 17 Package 18 Terminal

Claims (3)

In an optical receiver module comprising: an optical fiber (11); and a light receiving element (12) that is disposed to face the light emitting end face (11a) of the optical fiber at a predetermined interval and converts light into an electrical signal.
The optical fiber is a front-end fiber formed in a spherical shape with a light-emitting end face having a predetermined radius of curvature, and the position of the beam waist of the light emitted from the front-end fiber is the light-receiving portion (12b) of the light-receiving element. An optical receiver module which is on the near side of the above.   The optical receiver module according to claim 1, wherein the light receiving element is an avalanche photodiode (APD).   3. The optical receiver module according to claim 1, wherein the light receiving element is a back-illuminated type.

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