Patent Description:
Digital-signal processing techniques including digital coherent have been introduced in optical fiber communication systems, and backbone network transmission techniques achieving <NUM> Gbps per wavelength have been established. Currently, high-speed transmission of <NUM> to <NUM> Gbps per wavelength has reached a practical level.

In a <NUM> digital coherent system in the early stage, as illustrated in <FIG>, each part (ICs, photonic IC (PIC)) had an individual package, and for example, each part was mounted on a printed circuit board (PCB).

In a conventional <NUM> digital coherent system in <FIG>, a DSP package substrate <NUM> is mounted on a PCB board substrate <NUM> and is electrically connected to it with a ball grid array (BGA) <NUM>. On the DSP package substrate <NUM> is mounted a DSP-ASIC <NUM> chip.

Electrical input and output of the DSP package substrate <NUM> are connected via printed line on the PCB board substrate <NUM> and surface-mounted lead pins <NUM> to a driver/TIA <NUM>, then connected via this driver/TIA <NUM> to an optical modulation (optical reception) module <NUM>.

The optical modulation (optical reception) module <NUM> receives modulated electrical signals, performs optical modulation on them, and outputs the modulated light to optical fiber <NUM>. The optical modulation (optical reception) module <NUM> receives signal lights from the optical fiber <NUM>, converts the signal lights into electrical signals, and sends the electrical signals to the DSP package substrate <NUM>, where the DSP-ASIC <NUM> processes received signals.

In the case of systems exceeding <NUM>, analog parts are required to be adapted to a wider bandwidth (for example, a modulation bandwidth <NUM> or more). Hence, for the purposes of reducing high-frequency losses and downsizing, configurations as illustrated in <FIG> are attracting attention: a configuration on the transmission side in which an RF driver and an optical modulator are integrated into one package and mounted (coherent driver modulator: CDM), and a configuration on the reception side in which a transimpedance amplifier TIA and a light receiver PD are integrated into one package and mounted (integrated coherent receiver: ICR). (In the following description, both configurations are referred to as CDM configurations.

Although description of the same functions as in <FIG> is omitted, a conventional <NUM> digital coherent system in <FIG> includes an integrally mounted optical modulation (optical reception) module <NUM> in which a driver/TIA and an optical modulation (optical reception) module are integrally mounted.

In addition, in order to reduce deterioration in high-frequency characteristics resulting from packaging and mounting, attempts as illustrated in <FIG> are also being made to mount all the high-frequency analog ICs on the package substrate the same as the one on which the DSP is mounted (DSP co-package mounting). In <FIG>, although description of the same functions as in <FIG> and <FIG> is omitted, an integrally mounted optical modulation (optical reception) module in which all the high-frequency analog ICs are mounted on the same package substrate <NUM> as the one on which a DSP is mounted (DSP co-package mounting) is used.

In this case, a DSP-ASIC that generates heat in the order of watts and an optical transmission-reception device are disposed on one and the same package substrate so as to be close to each other, and thus for the optical transmission-reception device, one that has less characteristic fluctuation against temperature changes or rises (less temperature dependence) is desired.

For materials for the optical transmission-reception device, instead of conventional lithium niobate (LN) optical modulators, semiconductor-based optical modulators are attracting attention in the viewpoint of downsizing and cost reduction. In particular, for higher-speed modulation operation, compound semiconductors using compounds typified by InP are mainly used, and for systems having importance on further downsizing and cost reduction, research and development are concentrated on Si-based optical devices.

Semiconductor optical modulators also have advantages and disadvantage unique to their materials. For example, for InP optical modulators, it is thought that in order to control band-edge absorption effects, temperature control with a controller is indispensable in modulation operation. Si modulators have an advantage that temperature control is not necessary, but their electro-optical effects are smaller than those of other material-based modulators. Thus, in the case of Si modulator, the electric-light interaction length needs to be longer, and this may result in an increase in high-frequency losses. Hence, there are many issues to achieve higher speed (wider bandwidth). To make conventional optical transmitter-receivers as illustrated in <FIG> operate higher speed, it is important not only to speed up ICs (for example, Si-CMOS or the like) and PICs (for example, optical modulation devices, optical reception devices, and the like) but also to make packaging and high-frequency line adapted to higher speed (lower RF losses) and to make smaller the losses in the electrical connections between components (lower reflection). In other words, it can be said that from the viewpoint of achieving higher speed related to mounting, the configuration in <FIG> having a higher degree of integration and the co-package configuration in <FIG> including a plurality of chips are more advantageous for achieving higher speed than the individual-package configuration illustrated in <FIG>.

From the above background, for Si-based optical modulators having less temperature dependence, more highly integrated DSP co-package configurations are being studied, and for InP-based optical modulators having high temperature dependence, configurations in which, separately from a DSP that generates much heat, only a high-frequency amplification device (driver IC) is mounted in the same package (for example, CDM) are often employed. Note that an optical modulation device in this case is, in general, mounted on a temperature controller (TEC), and its temperature is controlled (to be constant).

Patent Literature <NUM>: <CIT>
Further prior art documents are <CIT> showing an optical module and optical transmission/reception device and <CIT> showing an optical transceiver module and optical cable module.

As described above, mounting configurations of conventional semiconductor optical modulators are mainly classified into the CDM configuration illustrated in <FIG> (the receiver side is also referred to as ICR, and in the case of a transmitter-receiver integrated package, this is also referred to as an integrated coherent transmitter and receiver optical sub-assembly (IC-TROSA)) and the DSP co-package configuration illustrated in <FIG>.

Here, in order to achieve higher speed in the entire optical transmitter (receiver), it is necessary to speed up each IC and the PIC and to make line connecting those and everything about the packaging and mounting adapted to higher speed (wider bandwidth). However, each of the foregoing two conventional mounting configurations has a problem that prevents achieving wider bandwidth, as below.

For example, high-speed analog electrical signals outputted from a digital/analog conversion circuit (DAC) provided in the DSP-ASIC are transmitted in the order of the ASIC → the DSP package substrate -<NUM> the PCB board substrate → the optical modulation module, where the electrical signals are converted into optical signals. For the electrical interfaces, for example, the surface mount technology (SMT), flexible printed circuits (FPCs), and flexible printed line boards are used.

In this case, it is necessary to transmit electrical signals through multiple high-frequency circuit substrates of different types, and thus the length of electrical line is elongated, increasing electrical losses.

In addition, for connection between substrates, especially for the ball grid array (BGA) connection portion between the DSP package and the PCB board, solder balls having diameters of a hundred to several hundred micro meters are used for connection. Thus, for high-frequency signals of <NUM> or more, electrical reflection caused by impedance mismatch at solder ball connecting portions is a factor that degrades high-frequency characteristics largely.

This high-frequency characteristic deterioration was not cited as a serious problem in conventional <NUM> systems (<NUM> baud rate as a modulation drive baud rate, about <NUM> as a required bandwidth), but it is a big obstacle to achieve next-generation <NUM> or 1T systems (required bandwidth > <NUM>).

Hence, even if an optical modulation module including an InP modulation device having a modulation bandwidth of <NUM> or more is used, it is difficult to achieve necessary bandwidth characteristics as the entire optical transmitter (receiver).

Besides the above problem, in the structure for the case in which a low-loss FPC is used for a high-frequency interface for the optical module, as illustrated in a sectional side view of an example in which an FPC is used for a conventional CDM mounting system in <FIG>, the FPC is connected from an optical module terrace having a different height to a portion on the PCB board. Thus, the FPC <NUM> need to be bent sharply when it is mounted.

If the FPC is bent, it increases mounting stress at the connection portions of the FPC, causing reliability problems. In addition, there are also concerns about change in high-frequency characteristics caused by the bending (change in characteristic impedances) and increase in electrical losses due to the longer line length.

A widely-known method for solving the above problem is the DSP co-package mounting configuration also illustrated in <FIG>. In this configuration, as illustrated in <FIG>, not only the DSP-ASIC <NUM> but also the driver (TIA) <NUM> and the optical modulator (optical receiver) PIC <NUM> are mounted on the package substrate <NUM>, and thus high-frequency electrical signals can be electrically supplied to the optical modulator through the shortest line without passing through solder balls or the like.

However, since Si-based modulators having less temperature dependence are mainly used as optical modulators in the current situation, for achieving higher speed (wider bandwidth) as described above, improvement in characteristics of optical modulation devices themselves is cited as a serious issue.

In general, the bandwidth and the modulation efficiency (related to drive voltage Vπ, modulation-output optical intensity, and the like) in optical modulators are in a trade-off relationship, and thus, a design only prioritizing expansion of the bandwidth, on the contrary, leads to deterioration in the signal-to-noise ratio (SNR) of modulated light, resulting in signal quality deterioration.

In addition, aiming to compensate for the deterioration in the SNR, if a compound semiconductor optical amplification device such as an SOA is mounted in addition to the Si modulation device, problems raised are cost increase due to temperature control for this amplification device itself and increase in the number of mounted parts, and increase in power consumption.

In addition, if an InP modulator, instead of an Si modulator, is mounted in the same package as the DSP, as DSP co-packaging, the composition of the InP modulator core needs to be changed (to reduce the band-edge absorption in the material). In that case, there is also a problem that the modulation efficiency of the InP modulator itself decreases (decrease in quantum-confined Stark effect (QCSE)), leading to deterioration in the SNR.

To solve the foregoing problems, in the present invention, an InP optical modulator having excellent properties in high speed operation is mounted on an optical modulation module, and a flexible printed circuit (FPC) is used as a high-frequency interface for directly connecting a DSP package substrate and the optical modulation module.

For improvement in the modulation efficiency and long-term stable operation of the InP modulation device, the optical modulation module may desirably have a temperature controller (TEC). In addition, it is desirable to make the inside of the module airtight and put inert gas in it for long term stability of the optical characteristics.

In addition, a structure is employed in which a high-frequency line pattern on the DSP package substrate and a mechanism for connecting to the FPC board are provided on the DSP package substrate not having a metal lid, and high-frequency signals are electrically supplied directly to the optical transmission module via the FPC (not via the PCB board substrate).

Employing the structure as above makes it possible to prevent high-frequency losses resulting from part-mounting between the IC and the PIC and to drive the InP optical modulator having excellent wide bandwidth properties, using high-speed and high-quality electrical signals. The present invention plays an important role, in particular, in building next generation <NUM> Gbps or <NUM> Tbps (per wavelength) systems in which the required bandwidth of the optical transmitter-receiver is <NUM> or more (<NUM> baud rate as the modulation baud rate (symbol rate)).

Embodiments of the present invention are defined by the claims.

In an optical transmission-reception apparatus having a digital-signal processing circuit and including an optical transmitter (optical modulator) and an optical receiver, it is possible to connect, by using flexible printed circuits, a package substrate of the digital-signal processing circuit and optical modulation and optical reception modules in a wide bandwidth while preventing high-frequency losses, and this makes it possible to achieve a high-speed optical transmission-reception apparatus.

Hereinafter, examples of the present invention will be described in detail.

<FIG> is a sectional side view of an optical transmission-reception apparatus, illustrating an outline configuration of the present invention. In the present invention, as illustrated in <FIG>, a DSP package substrate <NUM> is mounted on an upper left portion of a PCB board substrate <NUM> and connected to it with a BGA <NUM>, and a DSP-ASIC <NUM> is mounted on the DSP package substrate <NUM>.

An optical modulation (reception) module <NUM> on the right is mounted such that the height difference between the height of a package terrace (the portion having a shelf-like surface at the middle level of the package height) of the module and the height of the upper surface of the DSP package substrate <NUM> is less than or equal to <NUM>. The optical modulation (reception) module <NUM> is directly connected to the DSP package substrate <NUM> with a flexible printed circuit (FPC <NUM>) serving as a high-frequency interface, and through which the optical modulation (reception) module <NUM> is supplied with electrical signals.

The DSP package substrate <NUM> may be mounted with the height of the upper surface being adjusted such that the height difference is less than or equal to <NUM>. In short, the FPC <NUM> only needs to be reliably connected without having a sharp bend.

Although in this example, the FPC <NUM> has a structure having at least <NUM> layers, a base film (upper layer) and a copper foil (lower layer), the structure is not limited to this example. The same applies to the following description.

<FIG> is a plan view of an optical transmission-reception apparatus for digital coherent communication, according to Example <NUM> of the present invention. In <FIG>, a DSP package substrate <NUM> having a DSP-ASIC <NUM> on it is mounted on a left portion of a PCB board substrate <NUM>, and low-speed signal interfaces <NUM> are connected to the DSP package substrate <NUM> at the left end or the upper and lower ends of the PCB board substrate <NUM>. Low-speed electrical signals are converted by the DSP-ASIC <NUM> into high-frequency signals, which are inputted to or outputted from FPC interfaces <NUM> via expansion line <NUM> and connection PADs <NUM> on the right side of the DSP-ASIC. The connection PADs <NUM> may be replaced with FPC connectors.

The FPC interfaces <NUM> are connected to an optical transmission module CDM <NUM> and an optical reception module ICR <NUM>, which perform photoelectric conversion, and transmit and receive optical signals to and from optical fibers <NUM> at the right end. The FPC connection may be at least one of between the DSP package substrate <NUM> and the transmission (modulation) module CDM <NUM> or between the DSP package substrate <NUM> and the optical reception module ICR <NUM>. The optical transmission module CDM and the optical reception module ICR may also receive connection of low-speed electrical signals as necessary.

In the assumption here, the optical transmission-reception apparatus of Example <NUM> in <FIG> employs a polarization multiplexing IQ optical modulation method, and the high-frequency signals are inputted to and outputted from the DSP-ASIC <NUM> through four channels for each of the input and output. (X polarization I channel/X polarization Q channel/Y polarization I channel/Y polarization Q channel).

Because one channel in general has a differential pair of electrical signals, the number of signal lines for high-frequency line is in total <NUM>, <NUM> lines for each of optical transmission and optical reception (<NUM> differential pn lines × <NUM> channels). The electrical signals are transmitted through FPCs <NUM> between the optical transmission-reception modules (CDM <NUM> and ICR <NUM>) and the DSP-ASIC <NUM>. The transmission length and the off-set length of the FPC <NUM> are determined by considering the mounting spaces for the optical transmission module (CDM <NUM>) and the optical reception module (ICR <NUM>). In this example, the line length of the FPCs is <NUM> in consideration of assembling workability of each part, but it is clear that the line length does not affect the usefulness of the invention. (Note that high-frequency signals in each channel may be single-phase signals instead of differential signals).

<FIG> is a sectional side view of the optical transmission (modulation) module CDM <NUM> side of the optical transmission-reception apparatus according to Example <NUM> of the present invention. The optical reception module ICR <NUM> side also has approximately the same structure, and thus description of it is omitted. At least one of the optical transmission (modulation) module CDM <NUM> or the optical reception module ICR <NUM> may have FPC connection.

As illustrated in <FIG>, the optical transmission (modulation) module CDM <NUM> is placed on a PCB substrate <NUM>. On a temperature controller (TEC) <NUM> is placed a subcarrier (optical-device base) <NUM>, on which are placed an optical modulator PIC <NUM>, a chip condensing lens <NUM> (first lens), and a fiber condensing lens <NUM> (second lens), through which transmission light (modulated light) is outputted to optical fiber <NUM>.

For the optical modulator PIC <NUM>, an InP-based IQ optical modulation device having excellent wide bandwidth properties is employed in this example. The optical modulation device employs an InP substrate and has at least two or more Mach-Zehnder optical interference waveguides.

On the input side of the optical modulator PIC <NUM>, a module-line substrate base <NUM> and a module package wall <NUM> are disposed as the package left wall of the optical transmission (modulation) module CDM <NUM>. The module-line substrate base <NUM> and the module package wall <NUM> are formed of, for example, ceramic members having different thicknesses, and the step formed by the difference of the thicknesses forms a package terrace. The metal line pattern (lower layer) of an FPC <NUM> is connected to high-frequency line <NUM> on the upper surface of the module-line substrate base <NUM>.

The high-frequency line <NUM> on the upper surface of the module-line substrate base <NUM> passes through the ceramic wall face between the module-line substrate base <NUM> and the module package wall <NUM> and inputs modulated electrical signals to the optical modulator PIC <NUM> via gold-wire wiring <NUM>.

The module-line substrate base <NUM> may be formed as part of the FPC connector, or may be combined with the module package wall <NUM> into a connector composed of one or two integrated ceramic parts.

The wall surface continuing from the package terrace of the ceramic part connector may have a through hole which the high-frequency line <NUM> passes through, and the cross section of the through hole may be formed such that the upper portion of the high-frequency line <NUM> are like a cavity in the form of a tunnel.

This cavity may be formed such that the height of this cavity gradually decreases from a height larger than the thickness of the FPC <NUM> (to a height smaller than at least the thickness of the FPC <NUM>), from the entrance of through hole at the wall face toward the inside of the module.

If the through hole is formed like this, only inserting an end of the FPC <NUM> into the cavity of the through hole generates pressing force between the metal line pattern of the lower layer of the FPC <NUM> and the high-frequency line <NUM>, forming electrical connection.

Note that for long-term stability of the optical lenses, inert gas such as Ar or N<NUM> may be putted inside the module, and the module may be sealed hermetically.

A driver-IC-integrated optical transmission module as illustrated in <FIG> may be used as Example <NUM> of the present invention. The different point between the driver-IC-integrated optical transmission module <NUM> in <FIG> and the optical transmission module CDM <NUM> in <FIG> is that a high-frequency amplification IC <NUM> is disposed as a driver IC between a module-line substrate base <NUM> and an optical modulator PIC <NUM>, and hence description of other portions will be omitted.

One advantage of the driver integration is that compensation for losses of high-frequency electrical signals resulting from line, modules, and the like and increase in modulation efficiency resulting from the signal gain can be expected. Another advantage is that configurations in which an InP modulator and a driver IC are integrally mounted are already widely recognized as CDM (as established techniques).

As in Example <NUM>, the module-line substrate base <NUM> may be formed as part of the FPC connector and combined with a module package wall <NUM> into a connector composed of one or two integrated ceramic parts.

In these examples, differential signal line traces for the number of channels (high-frequency line traces, for example, G-S-S-G/channel) are provided on the DSP package substrate. Expansion line for changing the pitch of the high-frequency line traces into a pitch that matches the pitch of the connection channels of the FPCs (the expansion wiring <NUM> in <FIG>) is provided as an expansion substrate. The FPCs are connected and fixed to the electrical-signal supply pads provided on the DSP package substrate with solder.

The FPC can be connected to the DSP package substrate by fixing an end of the FPC to the DSP package substrate by using connectors (connection mechanisms) such as FPC connectors for high-frequency transmission. Connector connection has advantages that heat damage to the DSP side can be avoided because connector connection does not give heat unlike solder fixation and that reduction in assembly cost can be expected.

Note that the interface for low-speed signals including DC signals can be connected to peripheral circuits via a higher-density BGA or the like as in conventional techniques. For the optical transmission-reception module side, any of SMT, FPC, and BGA can be used without any problem. In this example, an SMT type is used. The type of low-speed signal interface does not affect the effectiveness of the present invention.

Lastly, as Example <NUM> of the present invention, <FIG> illustrates a configuration example in which an IC-TROSA module <NUM> into which optical transmission and optical reception are integrated (also including functional devices such as lasers) is used for the optical transmission-reception module. For the FPC connection between a DSP package substrate <NUM> and the IC-TROSA module <NUM>, a pair of connectors can be used.

Claim 1:
A high-speed optical transmission-reception apparatus comprising:
a digital-signal processing circuit (<NUM>);
at least one of an optical modulation module (<NUM>) having at least a driver and an optical modulation device or an optical reception module (<NUM>) having at least a transimpedance amplifier and an optical reception device;
a flexible printed circuit (<NUM>, <NUM>) being used as a high-frequency interface for the at least one of the optical modulation module and the optical reception module;
wherein a package substrate of the digital signal processing circuit and the at least one of the optical modulation module or the optical reception module are connected by the flexible printed circuit,
characterized in that,
the apparatus comprises a mechanism for connecting (<NUM>) a high-frequency line pattern to the flexible printed circuit, the mechanism being provided on the package substrate of the digital-signal processing circuit, the high-frequency line pattern being formed on said the package substrate,
wherein said package substrate and the at least one of the optical modulation module or the optical reception module connected by the flexible printed circuit are mounted on a common printing circuit board.