JP2014203895A - Photodiode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photodiode capable of improving photosensitivity without increasing an element size.SOLUTION: A photodiode 100 includes: a connection waveguide 101 for introducing signal light to the photodiode; and a light absorption region 102, connected to the connection waveguide 101, and having a circumferential part formed so as to circulate the signal light in a circumferential direction.

Description

  The present invention relates to a photodiode (PD) for monitoring the intensity of signal light.

  In order to increase the capacity of a WDM optical communication system, it is useful to increase the transmission rate per wavelength. For example, when the symbol rate transmitted to the optical transmission line is increased without changing the modulation method, the allowable residual dispersion amount is inversely proportional to the square of the symbol rate, so that the chromatic dispersion tolerance of the optical transmission line decreases. was there. Further, in this case, since it is necessary to execute electric signal processing at a high speed, there is a problem that the cost of the analog electric component increases.

  In order to avoid such a problem, in recent years, research has been actively conducted to increase the capacity of a system by increasing the signal multiplicity per symbol without increasing the symbol rate. As a method for increasing the signal multiplicity, for example, a QPSK method that doubles the transmission capacity by allocating 2 bits (multiplicity 2) to one symbol, and transmission by allocating 4 bits (multiplicity 4) to 1 symbol. Multi-level modulation schemes such as 16QAM scheme and 16APSK scheme for quadrupling the capacity are known.

  In general, when such a multilevel modulation method is adopted, an I / Q modulator is used as an optical modulator. The I / Q modulator is also called a quadrature modulator, and is a modulator that can independently generate orthogonal optical electric field components (I channel and Q channel). This I / Q modulator has a special configuration formed by connecting Mach-Zehnder (MZ) modulators in parallel.

  For example, when executing QPSK modulation, a dual parallel MZ modulator (DPMZM: Dual Parallel Mach-Zehnder Modulator) in which two MZ modulators are connected in parallel is used. For example, when performing 16QAM modulation, a method of driving DPMZM with an electrical multilevel signal is used.

  FIG. 6 is a schematic diagram showing the configuration of a conventional DPMZM600.

  In FIG. 6, a DPMZM 600 includes an input waveguide 601, an optical branch circuit 602, connection waveguides 603a and 603b, Mach-Zehnder modulators 610a and 610b, phase adjustment region portions 607a and 607b, and an optical confluence circuit 608. And an output waveguide 609.

  The Mach-Zehnder modulator 610a includes an optical branching circuit 604a, phase modulators 605a and 605b, and an optical combining circuit 606a. The Mach-Zehnder modulator 610b includes an optical branch circuit 604b, phase modulators 605c and 605d, and an optical merge circuit 606b.

  In FIG. 6, two Mach-Zehnder modulators 610a and 610b are arranged in parallel. The Mach-Zehnder modulators 610a and 610b are incorporated in a part of the arm. That is, a parent Mach-Zehnder interferometer is formed. In general, such a configuration is referred to as a Dual Parallel MZ Modulator (DPMZM).

  In the DPMZM 600, the signal light first enters from the input waveguide 601 and is branched into two by the optical branch circuit 602, and is guided to the Mach-Zehnder modulators 610a and 610b via the connection waveguides 603a and 603b. Both the Mach-Zehnder modulators 610a and 610b operate by push-pull drive.

  The signal light modulated by the Mach-Zehnder modulators 610a and 610b is incident on the phase adjustment regions 607a and 607b, and after the phase relationship is adjusted, the signal light is combined by the optical combining circuit 608 and output from the output waveguide 609. . In the phase adjustment areas 607a and 607b in this case, the phases are relatively adjusted so that the phase difference between both signal lights when combined by the optical combining circuit 608 is π / 2.

  Next, constellation will be described with reference to FIGS. 6 and 7 by taking, for example, quaternary QPSK modulation as an example of optical multilevel modulation.

  7A and 7B are diagrams for explaining the constellation of the optical modulator, in which FIG. 7A is a locus of an optical electric field modulated by the push-pull drive Mach-Zehnder modulator 610a, and FIG. 7B is a constellation of DPMZM. Is shown.

In the Mach-Zehnder modulator 610a, the phase modulators 605a and 605b are driven by differential signals. In the case of a phase modulator, the signal light has a constant intensity and the phase angle changes according to the applied voltage. For example, considering the case of a QPSK signal, the electrical signal is a binary signal having an amplitude of half-wave voltage _Vπ . Therefore, the electric field of the signal light emitted from the phase modulator 605a traces the upper half of the semicircle shown in FIG. 7A according to the electric signal between the I ₀ point and the I ₁ point. Make a round trip.

On the other hand, considering the phase modulator 605b, since the Mach-Zehnder modulator 610a is driven by push-pull, the phase modulator 605b is driven by the differential signal of the phase modulator 605a. Therefore, the electric field of the signal light emitted from the phase modulator 605b in the form of tracing the lower half of the semi-circular as shown in FIG. 7 (a), the response to an electrical signal between the ₀ point and the I ₁ point I Go back and forth.

Since the electric field of the signal light output from the optical converging circuit 606a is a combination of the outputs from the phase modulators 605a and 605b, in FIG. 7A, in a straight line passing through the origin 0, the points I ₀ and I ₁ It reciprocates between points according to the electric signal.

  Similarly, since the Mach-Zehnder modulator 610b is also driven by a pull-pull drive using a differential signal, the output signal trace is a straight line passing through the origin 0. The signal lights output from the two Mach-Zehnder modulators 610 a and 610 b are adjusted so that the relative phase difference between them is π / 2 in each phase adjustment region 607 a and 607 b, and added by the optical combining circuit 608. Combined. Therefore, the trace of the electric field of the signal light output from the Mach-Zehnder modulator 610a and the trace of the electric field of the signal light output from the Mach-Zehnder modulator 610b are added in an orthogonal manner. Therefore, the final constellation of the signal light output from the output waveguide 609 is four points that are 90 degrees apart from each other as shown in FIG. 7B. This is a QPSK constellation that is quaternary modulation. When the relative phase difference is adjusted to be π / 2 and added by the optical combining circuit 608, the principle loss is 3 dB.

  In order to perform the operation as described above, the operation point or bias point of the Mach-Zehnder modulators 610a and 610b, and the relative phase difference when they are added by the optical converging circuit 608 are π / 2. Therefore, it is necessary to take out a part of the light output and monitor it. For example, in the case of a modulator using a semiconductor material, a monitor photodiode (monitor PD) for monitoring an operation state can be monolithically integrated on the same substrate as the modulator (for example, Non-Patent Document 1). .

  FIG. 8 is a diagram schematically showing the configuration of a Mach-Zehnder modulator 800 with a power monitor PD in Non-Patent Document 1.

  In FIG. 8, a Mach-Zehnder modulator 800 includes an input waveguide 801, an optical branch circuit 802, connection waveguides 803a and 803b, phase modulators 804a and 804b, connection waveguides 805a and 805b, and an optical confluence circuit. 806, an output waveguide 807, and a monitor PD 808.

  The Mach-Zehnder modulator 800 can be considered as a monitor PD 808 added to the Mach-Zehnder modulator 610a shown in FIG. On the other hand, unlike the one shown in FIG. 6, the optical merge circuit 806 of the Mach-Zehnder modulator 800 is a 2-input 2-output merge circuit.

  In the case of a Mach-Zehnder modulator using a 2-input 2-output junction circuit, it is possible to configure a port from which a main signal is output and a port from which a signal having an intensity inverted from that of the main signal is output. The main signal and the inverted signal are connected to the monitor PD 808 and monitored, so that the state of the main signal can be estimated.

  The monitor PD 808 has a layer that absorbs propagating light in a part of the core or clad of the waveguide, and the waveguide has a linear shape.

M. G. Boudreau, 4 others, “An Integrated Waveguide Detector for Power Control in an InP Mach-Zehnder Modulator based 10 Gb / s Transmitter”, IPRM (International Conference on Indium Phosphide and Related Materials) 2006, WP23

  Since the Mach-Zehnder modulator described in Non-Patent Document 1 includes a linear waveguide, it is necessary to increase the waveguide length in order to increase the light receiving sensitivity of the monitor PD. Therefore, there has been a problem that the element size becomes large.

  The present invention has been made under such circumstances, and an object thereof is to provide a photodiode capable of improving the light receiving sensitivity without increasing the element size.

  A photodiode for solving the above-described problems includes a waveguide for guiding signal light to the photodiode, and a circumference connected to the waveguide and configured to circulate the signal light in a circumferential direction. A light absorption region portion having a portion.

  Here, the waveguide may be connected to the light absorption region portion so as to coincide with a tangential direction of the circumferential portion.

  The circumferential portion may be circular or donut shaped.

  The photodiode may be integrated in a waveguide type optical device, and the optical signal may be input light or output light of the waveguide type optical device.

  According to the photodiode of the present invention, the light receiving sensitivity can be improved without increasing the element size.

It is the schematic diagram which showed the structural example of the photodiode which concerns on 1st Embodiment. FIG. 2 is a diagram for explaining an example of a cross-sectional structure at A-A ′ of the photodiode of FIG. 1. It is the schematic diagram which showed the structural example of the photodiode which concerns on 2nd Embodiment. It is a figure for demonstrating an example of the cross-section in B-B 'of the photodiode of FIG. In the modification of 2nd Embodiment, it is a figure for demonstrating an example of the cross-section in B-B 'of FIG. It is the schematic diagram which showed the structure of conventional DPMZM. It is a figure for demonstrating the constellation of the conventional optical modulator. 3 is a schematic diagram illustrating a configuration of a Mach-Zehnder modulator of Non-Patent Document 1. FIG.

<First Embodiment>
Hereinafter, an embodiment of a photodiode of the present invention will be described with reference to the drawings. The photodiode of this embodiment is, for example, an integrated monitor photodiode.

  FIG. 1 is a schematic diagram illustrating a configuration example of a photodiode according to the first embodiment.

  A photodiode 100 shown in FIG. 1 includes a connection waveguide (waveguide) 101 and a PD 102.

  The PD 102 of this embodiment is a light absorption region portion that is connected to the connection waveguide 101 and has, for example, a circular circumferential portion formed so as to circulate signal light in the circumferential direction. For example, the PD 102 can be used as a monitor for input light or output light.

  In FIG. 1, in the connection portion 103 between the connection waveguide 101 and the PD 102, the connection waveguide 101 is connected to the PD 102 so as to coincide with the tangential direction of the circumferential portion of the PD 102. As a result, the signal light propagates in the circumferential direction of the circumferential portion of the PD 102, and as a result, the waveguide length of the photodiode 100 increases.

  FIG. 2 is a diagram showing an example of a cross-sectional structure taken along line A-A ′ in FIG. 1.

  2, the photodiode 100 includes an n-InP substrate 201, a light absorption layer 202, an upper p-InP clad 203, an upper electrode 204, and a lower electrode 205.

  The n-InP substrate 201 constitutes the lower clad of the waveguide. The light absorption layer 202 also has a waveguide core layer having a non-doped multiple quantum well structure having an InGaAsP well layer and an InP barrier layer.

  In this photodiode 100, the signal light incident on the circular PD 102 having the light absorption region from the connection waveguide 101 circulates around the light absorption layer 202 of the PD 102 along the circumference of the circular circumferential portion. . As a result, the signal light can propagate a distance that is equal to or greater than the circumference of the circumference, and thus the light absorption efficiency is increased.

  Further, even when a material system in which the absorption coefficient is reduced in the waveguide core layer by circulating the signal light inside the element of the PD 102, the waveguide is as in the case of the linear waveguide structure of Non-Patent Document 1. It is not necessary to increase the element size of the PD 102 in order to increase the length. In the case of the straight waveguide structure of Non-Patent Document 1, if the waveguide length is increased, the area of the waveguide defined by (waveguide width) × (waveguide length) increases. However, in the case of the photodiode 100 of the present embodiment, it is not necessary to change the element size of the PD 102, so that the area of the circle in the circumferential portion of the PD 102 does not increase. Therefore, an increase in the element capacity of the PD 102 can be prevented and the absorption efficiency can be improved.

  The photodiode 100 of this embodiment has a simple configuration, low cost, and easy manufacture. Furthermore, the photodiode 100 can operate at high speed with high sensitivity and low capacitance.

Second Embodiment
The photodiode 300 according to the second embodiment is different from that of the first embodiment in that the shape of the PD is a donut shape.

  FIG. 3 is a schematic diagram illustrating a configuration example of the photodiode 300 according to the present embodiment.

  In FIG. 3, the photodiode 100 includes a connection waveguide 301 and a PD 302.

  The PD 302 of this embodiment is a light absorption region portion that is connected to the connection waveguide 301 and has, for example, a donut-shaped circumferential portion that is formed to circulate signal light in the circumferential direction. For example, the PD 302 can be used as a monitor for input light or output light.

  In FIG. 3, in the connection portion 303 between the connection waveguide 301 and the PD 302, the connection waveguide 301 is connected to the PD 302 so as to coincide with the tangential direction of the donut-shaped circumferential portion of the PD 302. As a result, the signal light propagates in the circumferential direction of the donut-shaped circumferential portion (ring waveguide) of the PD 302, and as a result, the waveguide length of the photodiode 300 increases.

  FIG. 4 is a diagram showing an example of a cross-sectional structure taken along B-B ′ in FIG. 3.

  4, the photodiode 300 includes an n-InP substrate 401, a light absorption layer 402, an upper p-InP clad 403, an upper electrode 404, and a lower electrode 405.

  The n-InP substrate 401 constitutes the lower clad of the waveguide. The light absorption layer 402 also has a waveguide core layer having a non-doped multiple quantum well structure having an InGaAsP well layer and an InP barrier layer.

  In FIG. 4, the outer peripheral side wall of the donut-shaped waveguide in which the ring waveguide of the PD 302 is formed has a so-called high mesa structure etched deeper than the waveguide core layer. Further, the side wall inside the donut-shaped waveguide of the PD 302 has a so-called ridge structure in which the upper cladding layer 404 is partially left on the core layer by being etched partway through the upper cladding layer 404.

  The operation of the photodiode 300 is substantially the same as that of the first embodiment. That is, the signal light that has entered the donut-shaped PD 302 having the light absorption region from the connection waveguide 101 circulates around the light absorption layer 302 of the PD 302 along the circumference of the donut-shaped circumferential portion. As a result, the signal light propagates a distance longer than the circumference of the circumference of the PD 302, and the light absorption efficiency is increased. Thereby, also about this photodiode 300, the effect | action and effect similar to the thing of 1st Embodiment are acquired.

  In addition, the example of a change of each said embodiment is demonstrated.

(Modification 1)
The waveguide structure constituting the PDs 102 and 302 may be, for example, a buried structure, a ridge structure, a high mesa structure, or the like as long as it is a waveguide structure through which signal light can propagate. Even if it does in this way, the effect similar to PD102,302 of embodiment mentioned above can be acquired.

(Modification 2)
The PD 301 shown in FIG. 4 may be configured as shown in FIG. 5, for example. FIG. 5 is a diagram for explaining an example of a cross-sectional structure taken along the line BB ′ of FIG.

  5, the photodiode 300 </ b> A of Modification 2 includes an n-InP substrate 501, a light absorption layer 502, an upper p-InP cladding 503, an upper electrode 504, and a lower electrode 505.

  The n-InP substrate 501 constitutes the lower clad of the waveguide. The light absorption layer 502 also has a waveguide core layer having a non-doped multiple quantum well structure having an InGaAsP well layer and an InP barrier layer.

  In FIG. 5, both side walls of the waveguide are etched to the lower cladding 501 beyond the core layer. Even if it does in this way, the effect similar to embodiment mentioned above can be anticipated.

  The side walls on both sides of the waveguide may have a ridge structure or a buried structure.

(Modification 3)
There are no particular restrictions on the structure and material of the photodiodes 100, 300, and 300A described above. For example, a compound semiconductor material typified by an InP system or a GaAs system, a semiconductor material such as Si, or any material system that can be normally used as a photodiode may be applied.

(Modification 4)
The above-described photodiodes 100, 300, and 300A have been described by taking the monitor photodiode as an example. For example, the photodiode 100, 300, and 300A can be used for an optical modulator that converts an electrical signal into an optical signal and transmits the optical signal, and an optical modulation method. .

(Modification 5)
The photodiodes 100, 300, and 300A described above are integrated in a waveguide type optical device, and may receive input light or output light of the waveguide type optical device.

101,301 Connecting waveguide 102,302 PD
103, 303 Connection waveguide and PD connection portion 201, 401, 501 n-InP substrate 202, 402, 502 Light absorption layer 203, 403, 503 p-InP cladding 204, 404, 504 Upper electrode 205, 405, 505 Lower Electrode 601 Input waveguide 602 Optical branch circuit 603a, 603b Connection waveguide 604a, b Optical branch circuit 605a, 605b, 605c, 605d Phase modulator 606a, 606b Optical merge circuit 607a, 607b Phase adjustment area 608 Optical merge circuit 609 Output guidance Waveguide 610a, 610b Mach-Zehnder modulator

Claims (4)

A photodiode,
A waveguide for guiding signal light to the photodiode;
And a light absorption region portion having a circumferential portion connected to the waveguide and formed to circulate the signal light in a circumferential direction.   The photodiode according to claim 1, wherein the waveguide is connected to the light absorption region portion so as to coincide with a tangential direction of the circumferential portion.   The photodiode according to claim 1, wherein the circumferential portion has a circular shape or a donut shape.   4. The photodiode according to claim 1, wherein the photodiode is integrated in a waveguide type optical device, and the optical signal is input light or output light of the waveguide type optical device. The photodiode described in 1.

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