WO2016051800A2

WO2016051800A2 - Optical transcevier and a method to assemble the same

Info

Publication number: WO2016051800A2
Application number: PCT/JP2015/004990
Authority: WO
Inventors: Kazushige Oki
Original assignee: Sumitomo Electric Industries, Ltd.
Priority date: 2014-10-01
Filing date: 2015-09-30
Publication date: 2016-04-07
Also published as: WO2016051800A3

Abstract

A process to assembly a coherent optical transceiver is disclosed. The process includes steps of: (i) assembling the modulator unit outside of the housing; (ii) installing the source unit within the housing; (iii) installing the modulator unit within the housing; (iv) mounting the receiver unit on the top surface of the mother board outside of the housing; (v) installing the mother board that mounts the receiver unit within the housing so as to expose the back surface of the mother board; (vi) mounting the PMC on the back surface of the mother board; (vii) coupling the source unit with the PMC, the PMC with the modulator unit, and the PMC with the receiver unit by respective polarization maintaining fibers (PMFs) and polarization maintaining connectors (PM connectors), and (viii) coupling the optical receptacle with the modulator unit and the receiver unit by respective single mode fibers (SMFs).

Description

OPTICAL TRANSCEVIER AND A METHOD TO ASSEMBLE THE SAME

The present application relates to an optical transceiver, in particular, the optical transceiver applicable to the optical coherent communication, and a method to assemble the optical transceiver.

Efforts to enhance the transmission capacity have been paid in the optical communication system. The coherent system seems to be one of solutions for enhance the transmission capacity. In the coherent system, an additional modulation except for the amplitude modulation is concurrently carried out to enhance the transmission capacity. The polarization mode modulation, the phase modulation, and a combination of these modulations have been used as the additional modulation. In particular, a system which utilizes two phases of 0°and 90° of optical signals for both the X- and Y- polarizations called as DP-QPSK (Dual Polarization Quadrature Phase Shift Keying), secures the transmission capacity four (4) times greater than conventional systems. An optical transceiver applicable to the DP-QPSK system inevitably implements an optical source for four signal channels, a multi-channel optical modulator, a multi-channel optical receiver, and some or many signal processing circuits within a housing of the optical transceiver. Accordingly, such an optical transceiver must assemble those optical and electrical components in a particular order.

An aspect of the present application relates to a method to assemble an optical transceiver applicable to the optical coherent system. The optical transceiver comprises a source unit that generates continuous wave (CW) light, a polarization maintaining coupler (PMC) that splits the CW light into two portions, a modulator unit that modulates one of the CW light split by the PMC, a receiver unit that receives another of the CW light, and a housing that encloses the modulator unit, the source unit, the receiver unit and the PMC therein. The method includes steps of: (a) assembling the modulator unit outside of the housing; (b) installing the source unit within the housing; (c) installing the modulator unit within the housing; (d) mounting the receiver unit on a top surface of a mother board outside of the housing; (e) installing the mother board mounting the receiver unit within the housing as exposing a back surface of the mother board opposite to the top surface thereof; (f) mounting the PMC on the back surface of the mother board; (g) coupling the source unit with the PMC by a first inner fiber, a second inner fiber, and a first polarization maintaining connector; (h) coupling the PMC with the modulator unit by a third inner fiber, a fourth inner fiber, and a second polarization maintaining connector; (i) coupling the PMC with the receiver unit by a fifth inner fiber and a third polarization maintaining connector; and (j) coupling an optical receptacle provided in the housing with the modulator unit and the receiver unit by respective single mode fibers (SMFs). A feature of the method of the present application is that the steps from (g) to (i) may be performed in no particular order.

[Fig. 1] Fig. 1 is a perspective view of an optical transceiver according to the present embodiment.
[Fig. 2] Fig. 2 is an exploded view of the top, bottom housings and the front panel.
[Fig. 3] Fig. 3 shows an inside of the optical transceiver viewed from the top by removing the top housing from the bottom housing.
[Fig. 4] Fig. 4 also shows the inside of the optical transceiver viewed from the bottom by removing the bottom housing.
[Fig. 5] Fig. 5 schematically illustrates a functional block diagram of the optical transceiver primarily relating to the optical coupling system thereof.
[Fig. 6] Fig. 6(a) is a perspective view of the optical source, and Fig. 6(b) schematically shows the inside of the LD module.
[Fig. 7] Fig. 7 shows an outer appearance of the ICR.
[Fig. 8] Fig. 8(a) schematically illustrates the inside of the ICR, and Fig. 8(b) is a functional block diagram of the ICR.
[Fig. 9] Fig. 9 illustrates the arrangement of the components and the wiring of the inner fibers within the optical transceiver.
[Fig. 10] Fig. 10(a) is a perspective view magnifying a rear portion of the top housing and Fig. 10(b) is a plan view thereof.
[Fig. 11] Fig. 11 is an exploded view of the modulation unit.
[Fig. 12] Fig. 12 is a perspective view of the optical modulator that couples with the SMF in the output port thereof and the PMF in the input port.
[Fig. 13] Fig. 13(a) to 13(d) illustrate the processes to assemble the bias board and the cover with the optical modulator.
[Fig. 14] Fig. 14(a) shows the modulation unit viewed from the rear bottom; and Fig. 14(b) shows a cross section taken along the line XVIB-XVIB indicated in Fig. 14(a).
[Fig. 15] Fig. 15(a) shows the shield viewed from the top, while, Fig. 15(b) shows the shield viewed from the bottom.
[Fig. 16] Fig. 16 is a perspective view showing a process to install the source unit within the housing.
[Fig. 17] Fig. 17 shows a process to install the modulation unit into the housing.
[Fig. 18] Fig. 18(a) is a perspective view of the top surface of the mother board, and Fig. 18(b) is also a perspective view but the back surface of the mother board.
[Fig. 19] Fig. 19 is an exploded view of the receiver unit.
[Fig. 20] Fig. 20 is a front view of the receiver unit on the mother board.
[Fig. 21] Fig. 21 shows a process to install the mother board that mounts the ICR and so on in the top housing that already installs the modulation unit and the source unit.
[Fig. 22] Fig. 22 schematically illustrates the wiring of the inner fibers.
[Fig. 23] Fig. 23 is a perspective view of the bottom housing.

Next, some preferable embodiments according to the present application will be described. In the description of drawings, numerals or symbols same with or similar to each other will refer to elements same with or similar to each other without duplicated explanations.

Fig. 1 is a perspective view of an optical transceiver 1 according to the present embodiment. The optical transceiver 1 follows one of the multi-source agreements (MSAs) concerning to an optical transceiver called as Centium Form factor Pluggable (CFP), and includes a top housing 2, a bottom housing 3, two fastening screws 4, and a front panel 5. The description below assumes that a direction like “front” or “forward” corresponds to a side where the front panel 5 is provided, while, “rear” corresponds to a side opposite to the front, and a direction from the front to the rear will be called as the longitudinal direction. However, these descriptions are only for explanation sakes and do not restrict the scope of the present invention.

The inner space of the housing 2 formed by the top and bottom housings, 2 and 3, installs all optical and electrical components. Accordingly, the procedure to install those components within the inner space becomes a key factor to assemble the optical transceiver 1. Also, the CFP agreement has ruled that only one type of the voltages supplied from the host system, namely, the host system provides the supply voltage of 3.3 V through the electrical plug of the optical transceiver 1. However, the optical transceiver 1, in particular, electronic circuits therein inevitably uses various types of supply voltages. Various type of DC to DC converters (DC/DC-C) are necessary to be installed, which widens the circuit board or increases the number of the circuit boards electrically connected by, for instance, flexible printed circuit (FPC) boards to each other. The procedures to wire the FPC boards become also primitive or substantial for the assembly of the optical transceiver 1. Furthermore, the coherent communication system sometimes installs an electrical de-modulator generally constructed by a digital signal processor (DSP). The DSP, whose operational speed exceeds 10 Gbps or sometimes reaches 25 or 40 Gbps, generates large heat and the power consumption thereof exceeds 30 W. This requests an optical transceiver 1, in particular, a coherent optical transceiver, to have effective heat dissipating mechanism.

The optical transceiver 1 like the present embodiment applicable to the coherent communication system provides many inner fibers to couple optical components within the optical transceiver 1, and some of the inner fibers and the optical components inevitably require the function of maintain the polarization of light propagating therein.

Fig. 2 is an exploded view of the top, bottom housings, 2 and 3, and the front panel 5. The top and bottom housings, 2 and 3, which are made of metal die casting, have a longitudinal length of 144 mm from the front panel 5 to the rear end 3c, and a width of 82 mm in the front panel 5. Respective sides of the front panel 5 provide pockets 2p extending from the front end to the rear end, into which the fastening screws 4 to latch the optical transceiver 1 with the host system are housed. The top housing 2 provides in the front thereof a protrusion 2a that secures an extra space S1. The extra space S1 is covered with the lid 3d of the bottom housing 3. The front panel 5 provides a square opening 5a through which the protrusion 2a passes and exposes an optical receptacle 18.

Fig. 3 shows an inside of the optical transceiver 1 viewed from the top by removing the top housing 2 from the bottom housing 3, and Fig. 4 also shows the inside of the optical transceiver 1, which is viewed from the bottom by removing the bottom housing 3. As shown in Figs. 2 to 4, the fastening screws 4 are set in the pockets 3a formed by the top and bottom housings, 2 and 3, in respective sides of the optical transceiver 1.

The rear end 4a of the fastening screw 4 extrudes in respective outer sides of the electrical plug 6 which provides terminals of over 100 counts for radio frequency (RF) signals and those for DC signals with a pitch of 0.8 mm. Mating the rear end 4a of the fastening screws 4 with female holes provided in respective sides of the optical connector in the host system, the optical transceiver 1 may be securely and precisely set in the host system and electrically communicate with the host system.

Inner space formed by the top and bottom housings, 2 and 3, installs two drivers 11 to generate modulation signals, an optical modulator 12, an optical source 13, a polarization maintaining coupler (PMC) 14, an integrated coherent receiver (ICR) 15 as an optical module, a digital signal processor (DSP) 16, semi-rigid cables 17, and an optical receptacle 18. Some of those elements are mounted on a mother board 7. In the present embodiment, the electrical plug 6 is independent of the mother board 7.

The optical modulator 12, which has an extended and slim rectangular housing, is placed in a side along the driver 11. Four (4) semi-rigid cables 17 electrically connect the driver 11 with the optical modulator 12. The semi-rigid cable 17 is a type of a co-axial cable sheathed with, for instance, copper so as to be flexibly and freely bent and to maintain the bent shape. Accordingly, the semi-rigid cable 17 enhances the flexibility of the arrangements of respective components within the inner space. The optical transceiver 1 further provides inner fibers, F1 to F7, to couple the optical components of the optical modulator 12, the optical source 13, the PMC 14, the ICR 15, and the optical receptacle 18. Details of the wiring of the inner fibers, F1 to F7, will be described later. Four optical signals modulated by the optical modulator 12 and multiplexed thereby is output through the optical connector C1 to be set in the optical receptacle 18. Also, an external optical signal is input to the other optical connector C2.

The optical receptacle 18 is exposed from the front panel 5. The optical source 13, which includes a wavelength tunable laser diode (told) to emit CW light for the optical modulator 12 and the ICR 15, is placed in the extra space S1. The wavelength of the CW light output from the optical source 13 is tunable in a range around 1.55 μm, specifically, 1.53 to 1.57 μm. The PMC 14 is set in the rear of the optical receptacle 18 and in side by side to the driver 11. The DSP 16 is placed in the rear of the ICR 15. The front panel 5 in the opening 5a thereof exposes not only the optical source 13 but the optical receptacle 18 as receiving the optical connectors, C1 and C2. Thus, the optical transceiver 1 may perform the full-duplex optical communication.

Fig. 5 schematically illustrates a functional block diagram of the optical transceiver 1 primarily relating to the optical coupling system thereof. The optical system in the optical transceiver 1 inevitably disposes polarization maintaining fibers (PMFs) from the optical source 13 to the optical modulator 12 through the PMC 14 and from the PMC 14 to the ICR 15. That is, the optical modulator 12, the optical source 13, and the ICR 15 are required to maintain the polarization of the optical signals concerning thereto. Accordingly, the PMFs are disposed from those optical components, 12 to 15, by the pig-tailed arrangement permanently fixed thereto. When an arrangement of the receptacle type for the PMFs, which is pluggably attached to those components, 12 to 15; the polarization of the optical signals is secured no longer.

Another technique to maintain the polarization of the optical signal is the fusion splicing to connect optical fibers. However, the fusion splicing requires extra lengths of several centimeters in front and behind the spliced portion to reinforce the spliced portion. As described, the optical transceiver 1 of the embodiment only provides a limited inner space to dispose or wire the inner fibers, F1 to F7. The fusion splicing is hard to be applicable to the optical transceiver 1. The optical transceiver 1 of the embodiment, substituting from the fusion splicing, applies polarization maintaining (PM) connectors, P1 to P3, each comprising male members, P11 to P31, and female members, P12 to P32, to couple the optical components, 12 to 15.

The inner fiber F1, the first PMF, extracted from the optical source 13 couples with the inner fiber F2 extracted from the PMC 14 through the first PM connector P1. The inner fiber F3, the third PMF, extracted from the PMC 14 couples with the inner fiber F4, the fourth PMF, extracted out from the optical modulator 12 through the second PM connector P2. The inner fiber F5, the fifth PMF, extracted from the PMC 14 directly couples with the ICR 15 without interposing any PMFs through the third PM connector P3 which is integrated with the ICR 15. The rest of the inner fibers, F6 and F7, are not concerned with the polarization, because, the optical modulator 12 multiplexes two optical signals having respective polarizations perpendicular to each other. Accordingly, the optical signal output from the optical modulator 12 is carried on a single mode fiber (SMF). The inner fiber F6, which is extracted forward from the optical modulator 12, then turned rearward, and couples with the optical receptacle 18. Also, the optical signal externally received through the optical receptacle 18 is unsecured in the polarization thereof, or inherently including two components of the polarization perpendicular to each other. Accordingly, the inner fiber F7 connecting the optical receptacle 18 to the ICR 15 is also the type of the SMF. The optical signal is carried from the optical receptacle 18 to the ICR 15 on the inner fiber F7, which extends rearward from the optical receptacle 18 and turns in the inner space of the optical transceiver 1.

Fig. 6(a) is a perspective view of the optical source 13. The optical source 13 includes a laser diode (LD) module 13A that generates the CW light, a base 13B, a circuit board 13C, and a connector 13D. An FPC board 7e coupled with the connector 13D electrically connects the optical source 13 with circuits mounted on the mother board 7. Fig. 6(b) schematically shows the inside of the LD module 13A, which installs a wavelength tunable LD (t-LD) 13a, a wavelength detector 13b including an etalon filter 13c, two monitor photodiodes (mPD), 13d and 13e, and so on. The t-LD 13a may generate the CW light with a line width thereof substantially equal to or narrower than 100 kHz. The wavelength band around 1.55 μm corresponds to the oscillation frequency of about 1.95 THz. Accordingly, the line width of around 100 kHz is equivalent to the stability of about 5×10^-8.

The PMC 14 splits the CW light generated by the optical source 13 into two portions, one of which is provided to the optical modulator 12 and the other is provided to the ICR 15. The optical modulator 12 modulates the CW light provided from the PMC 14 by the driving signals output from the drivers 11. The driving signal has a speed of 10 GHz, 25 GHz, and sometimes 40 GHz. Four semi-rigid cables 17 carry the driving signals corresponding to QX, QY, IX, and IY, where “X” and “Y” denote the X-polarization and the Y-polarization, respectively, and “Q” and “I” means the quadrature and in-phase, respectively. That is, the signals, QX or QY, and the signals, IX or IY, correspond to “Imaginary” and “Real” components of the signals having a phase difference of π/2.

Fig. 7 shows an outer appearance of the ICR 15. The ICR 15 has a box-shaped housing 15B with DC leads 15C in respective sides of the housing and RF leads 15D in the rear. The DC leads 15C include ground GND leads. Fig. 8(a) schematically illustrates the inside of the ICR 15, and Fig. 8(b) is a functional block diagram of the ICR 15. The ICR 15, which receives the other portion of the CW light generated by the optical source 13 and splits by the PMC 14, may extract the phase information of the input optical signal provided from the optical receptacle 18 by performing the interference of the input optical signal with the CW light.

As shown in Figs. 8(a) and 8(b), the ICR 15 includes a variable optical attenuator (VOA) 15a; two optical paths 15b each corresponding to respective polarizations and including the in-phase signal (I) and the quadrature phase signal (Q); two 90° hybrids 15c to perform the interference of the two beams; two pre-amplifier 15d; and some optical components such as a polarization beam splitter (PBS), a beam splitter (BS) and lenses. The ICR 15 further provides a half-wave plate (λ/2 plate) 15A to rotate the polarization of the optical signal in the path from the signal side to the local side, while, the local beam provided from the polarization maintaining fiber (PMF) is kept in the polarization thereof until respective hybrids 15c.

Specifically, referring to Fig. 8(b), the ICR 15 receives the local signal from the optical source 13 through the PMF and the optical signal from the optical connector C2 of the optical receptacle 18 through the SMF. Each beam is split into two beams by the BS and the PBS. One of 90° hybrids interferes one of the signal beams split by the PBS with one of the local beams split but by the BS to generate the in-phase and the quadrature phase signals for the X-polarization, IX and QX, respectively. The other 90° hybrids 15c interferes one of signal beams but passing through the λ/2 plate 15A with one of the local beams to generate the in-phase and the quadrature phase signals for the Y-polarization, IY and QY, respectively. Because the optical components set in the paths 15b for the local beam and the signal beam except for the λ/2 plate 15A maintain the polarization, the respective 90° hybrids may exactly generate the signals for two polarizations. Four generated signals, IX to QY, are provided to the DSP 16 to recover information contained in the input optical signal. The DSP 16 provides the information thus recovered to the host system through the electrical plug 6.

The optical modulator 12, the optical source 13, and/or the ICR 15 are necessary to be provided with a lot of DC biases for the stable operations thereof. For instance, the optical modulator 12 needs, in addition to the driving signals, biases to compensate the phases of the optical beams, to balance respective power of the optical outputs, and/or to monitor respective optical outputs. The t-LD 13a requires, in addition to the bias current to generate the CW light, to control the wavelength of the CW light in the target one, to monitor the power of the output CW light, and so on. Also, the ICR 15 is necessary to be provided with biases for PDs and pre-amplifiers installed therein, commands to adjust the gains of the pre-amplifiers, and so on. The optical transceiver 1 provides such many biases by respective FPCs from the mother board 7. An optical transceiver 1 for the coherent communication system is inevitably requested to enclose those electrical and optical components within a housing whose outer dimensions are precisely determined in MSAs. Next, details of the optical transceiver 1 of the present embodiment will be described.

Fig. 9 illustrates the arrangement of the components and the wiring of the inner fibers, F1 to F7, within the optical transceiver 1. The front extra space S1 installs the front portion of the optical modulator 12. Accordingly, even when the optical modulator 12 in the dimensions thereof, in particular, the longitudinal length thereof, is longer than the longitudinal length of the optical transceiver 1 whose outer dimensions follows the CFP standard, the optical transceiver 1 may build an optical modulator of the MZ type primarily made of dielectric material such as lithium niobate (LiNbO3). Because of smaller electro-optical interactive co-efficient of dielectric materials, an optical modulator made of such dielectric material requires a length to show a substantial modulation degree. Without the front extra space S1, no optical modulator of the MZ type made of dielectric material is available to be installed within the optical transceiver following the CFP standard. Moreover, the front extra space S1, or the front protrusion 2a, does not interfere with the function of the optical transceiver 1 to be plugged within the host system and communicate therewith. That is, the CFP standard is silent for the arrangement of the front panel, only sets the limitation that the optical connector provided in a CFP transceiver is to have the type of the LC connector. Accordingly, the optical transceiver 1 of the present embodiment is an exclusive solution for an optical transceiver to install an optical modulator with the MZ type primarily made of dielectric materials.

The top housing 2 provides in a rear end thereof a rear wall 2c and an eaves 2e extending outwardly from the rear wall 2c. The rear wall 2c faces the rear end 3c of the bottom housing 3 as shown in Fig. 2. That is, the rear wall 2c, and the top and bottom housings, 2 and 3, form the inner space to install the components therein. Referring to Fig. 10(a) and Fig. 10(b), where Fig. 10(a) is a perspective view magnifying a rear portion of the top housing and Fig. 10(b) is a plan view thereof, which the rear wall 2c sets the electrical plug 6 thereon. As described later, the electrical plug 6 does not interfere with the wiring of the inner fiber F4 extracted outward from the rear wall 2c and returning back into the inner space of the optical transceiver 1.

The rear wall 2c also provides a groove 2b on a top thereof into which a gasket is set to shield the inner space, and two slits, 2g and 2d, in a center and a side thereof, respectively. The side slit 2d is formed in a position just behind the optical modulator 12. Referring to Fig. 9, the inner fiber F4 passes these slits, 2d and 2g. Specifically, the inner fiber F4 extracted from the rear end of the optical modulator 12 passes the rear wall 2c through the side slit 2d, turns in the rear extra space S2, returns back to the inner space passing through the center slit 2g, and reaches the PMC 14 from the rear after running along the optical modulator 12 frontward, turned backward in the front extra area S1, passing the PM connector P2, and turned again frontward. Another inner fiber F1 extracted from the optical source 13 rearward reaches the PM connector P1 from the front by rounding twice the optical modulator 12. The inner fiber F2 extracted from the PMC 14 turns in the rear of the inner space and couples with the PM connector P1 from the rear.

The inner fiber F5, which extends from the PMC 14 rearward, crosses laterally in the rear end of the inner space, runs frontward between the optical modulator 12 and one of the side walls, turns rearward in the front extra space S1, and finally reaches the PM connector P3 provided integrally in the front wall of the ICR 15. The inner fiber F7, extracted rearward from the optical connector C2, turns in the inner space and reaches the other connector C3 also provided in the front wall of the ICR 15.

The last inner fiber F6, which is extracted rearward from the other optical port 18a of the optical receptacle 18, reaches the optical modulator 12 from the front, turned in the rear of the inner space, running in the center thereof, and turned again rearward in the front extra space S1. That is, the inner fiber F6 reaches the optical modulator 12 from the port 18a as shaping an S-character. Two inner fibers, F2 and F3, which are coupled with the PMC 14, provide respective PM connectors, P1 and P2. Moreover, the inner fiber F5, which is also coupled with the PMC 14, has the plug P31 in the end to the ICR 14 to maintain the polarization thereof. Thus, the PMC 14 may be replaced by detaching respective connectors.

The optical transceiver 1 of the embodiment further provides a cover 20 to cover the rear extra space S2 into which the inner fiber F4 is set. The inner fiber F4, which passes the rear wall 2c through the side slit 2d behind the optical modulator 12, turns along the periphery of the hollow 2j in the rear extra space S2 and returns the inner space as passing through the center slit 2g. The cover 20 is assembled with the top housing 2 by engaging three latches 20f with three holes, 2f provided in the eaves 2e of the top housing 2.

The eaves 2e of the top housing 2 provides the hollow 2j corresponding to the shape of the rear extra space S2. The hollow 2j has a diameter greater than 15 mm, which is a smallest bending diameter allowable for an ordinary single mode fiber. Setting the inner fiber F4 along the periphery of the rear extra space S2, the round diameter of the inner fiber F4 automatically becomes greater than 15 mm. The bent loss of the inner fiber F4 may be thus suppressed.

The CW light output from the source unit 13 is split by the PMC 14 as maintaining the polarization thereof. The polarization of the local beam is in parallel to the active layer of the t-LD 13a, that is, because the t-LD 13a enclosed within the housing of the LD module 13A is assembled substantially in parallel to the bottom thereof, the polarization of the CW light output from the LD module 13A is kept in substantially in parallel to the bottom of the housing. The optical modulator 12, which has a type of the LN modulator comprised of lithium niobate, modulates thus provided one of local beams based on the modulation signals provided from the driver 11 through the semi-rigid cables 17. The modulation signals may have a frequency exceeding 10 GHz, sometimes reaching 40 GHz. The modulation signals thus provided correspond to IX, IY, QX, and QY, each containing one information unit, where I and Q mean the in-phase and quadrature, respectively; while, x and y correspond to the polarizations. Thus, the optical modulator 12 may be applicable to the DP-QPSK system. In the present embodiment, the PMC 14 splits the CW light by the ratio of 7:3, that is, 70% of the CW light is for the optical modulator 12, and the rest (30 %) is for the ICR 15. However, the split ratio of the PMC 14 is not restricted to those values. The split ratio of the PMC 14 is optional.

Next, a method to assemble the optical transceiver 1 will be described. The method first assembles the modulation unit U1 in outside of the optical transceiver.

a: Assemble of Modulation Unit outside of Housing
Fig. 11 is an exploded view of the modulation unit U1. As shown in Fig. 11, the modulation unit U1 includes the optical modulator 12, a bias board 21 to mount circuits to supply biases to the optical modulator 12, an FPC 23 connected to the bias board 21, a shield 25 and a spacer 24. Fig. 12 is a perspective view of the optical modulator 12 that couples with the SMF F6 in the output port 12A thereof and the PMF F4 in the input port 12B. The SMF F6 in an end thereof not coupled to the output port 12A provides the output port 18a in the optical receptacle 18, while, the PMF F4 provides the PM connector P2 in the end opposite to the input port 12B. The optical modulator 12 has a housing extending longitudinally and having a rectangular cross section with a top 12a and a side 12b perpendicular to the top 12a. The input port 12B is provided in one end of the housing, while, the output port 12A is provided in the other end of the longitudinal housing. The side 12b provides four AC terminals 12c in a rear side, while, DC terminals 12d in a front side. The AC terminals 12c are coupled with the semi-rigid cables 17. The DC terminals 12d are grouped into three portions each containing six terminals. The DC terminals 12d receive biases for adjusting phases of the optical signal, output monitored intensities of the input optical signal and the output optical signal, and receive the ground. The bias board 21 mounts the circuits that generate biases for the optical modulator 12.

The process first prepares the FPC 23, which is to be connected to the bias board 21, includes one end portion 23a, a connecting portion 23c, and another end portion 23b, where the connecting portion 23c shows flexibility. Referring to Fig. 12, pads in the end portion 23a, which are connected to the DC terminals 12d of the optical modulator 12, has a pitch to the next pad and a space between groups correspond to the pitch and the space of the DC terminals 12d. On the other hand, pads in the other end portion 23b are arranged by a pitch smaller than the pitch of the pads in the end portion 23a. Accordingly, the other end portion 23b has a width smaller than a width of the end portion 23a. Interconnections in the FPC 23 extend between the pads in the end portion 23a and the pads in the other end portion 23b. Soldering the pads in the end portion 23a with the DC terminals 12d of the optical modulator 12, the FPC 12 extends along the side 23b of the optical modulator 12.

Next, the bias board 21 shown in Fig. 13A is prepared. The bias board 21 provides top and back surfaces, 21a and 21b, where the top surface 21a mounts a circuit to be connected to the DC terminals 12d of the optical modulator 12. The back surface 21b, which mounts no electronic components, provides a ground pattern, exactly, a signal ground pattern in all over the surface. The pads provided in the other end portion 23b of the FPC board 23 are soldered to the pads in the back surface 21b of the bias board 21, as shown in Fig. 13(a). Thus, the DC terminals 12d of the optical modulator 12 are connected to the bias board 21 through the FPC 23.

Next, the process arranges a spacer 24 that covers a portion of the top surface 12a of the optical modulator 12, as shown in Fig. 13(b). The spacer 24, which is a U-shaped slab with an inner space 24a surrounded by respective bars of the U-shape, is set apart from the other end portion 23b of the FPC 23 by setting the FPC 23 in the inner space 24a. Then, the FPC 23 is bent in the connecting portion 23c thereof such that the other end portion 23b of the FPC 23 is set within the inner space as shown in Fig. 13(c). This arrangement of the spacer 24 and the FPC 23 may set the pads in the other end portion 23b apart from the side 12b of the optical modulator 12 by a thickness of the spacer 24, the DC terminals 12d or the pads in the other end portion 23b do not make a short circuit even the optical module 12 has a metal housing. Also, bending the FPC 23 in almost a right angle, the bias board 21 extends in parallel to the DC terminals 12d of the optical modulator 12 and the bias board 21 is fixed to the housing of the optical modulator 12 as putting the spacer 24 therebetween. The spacer 24 may be made of copper slab with a thickness of 2 to 3 mm and coated with gold (Au) or nickel (Ni); or the spacer 24 may be made of stainless steel instead of copper. Because the optical modulator 12 generates lesser heat, the spacer 24 may be optionally selected in a material thereof.

Next, the process solders another FPC 27 to an end portion 21d of the bias board 21 as shown in Fig. 13(d). The other FPC 27 electrically connects the bias board 21 to the mother board 7 of the optical transceiver 1. Then, the shield 25 is assembled with the optical modulator 12. The shield 25, which covers the top surface 21a of the bias board 21 and the DC terminals 12d of the optical modulator 12, may be formed only by folding a metal sheet so as to form a U-shaped side cross section. The bias board 21 is put in a space surrounded by respective bars of U-character. The other FPC 27 is pulled out from the bias board 21 through the slit 25e provided in the end 25d of the shield 25. The back surface 25b of the shield 25 is bent outward so as to form a protecting surface 25c that protects the end portions 23a of the FPC 23 connected to the DC terminals 12d of the optical modulator 12. The protecting surface 25c also has a function to electrically shield the DC terminals 12d. The protecting portion 25c is apart from the end portion 23a by bending respective sides 25g thereof toward the optical modulator 12. The bias board 21, the spacer 24, and the shield 25 have respective holes, 21f, 24f, and 25f, each aligned with screw holes 12f on the top surface 12a of the optical modulator 12. Screwing those members, 21, 24 and 25, on the housing of the optical modulator 12 by screws 26, those members, 21, 24 and 25, are assembled with each other.

Fig. 14(a) shows the modulation unit U1 viewed from the rear bottom; and Fig. 14(b) shows a cross section taken along the line XVIB-XVIB indicated in Fig. 14(a). Fig. 15(a) shows the shield 25 viewed from the top, while, Fig. 15(b) shows the shield 25 viewed from the bottom. As shown in Fig. 14(b), the FPC 23 is bent such that one end portion 23b extends above the top surface 12a of the optical modulator 12, while, the other end portion 23a is set along the side 12b. The bias board 21 faces the top surface 12a of the optical modulator 12 as putting the spacer 24 therebetween that defines a space between the optical modulator 12 and the bias board 21. The bias board 21 provides a portion 21c above the optical modulator 12 and another portion 21d not overlapping with the optical modulator 12. The shield 25 has the top surface 25a covering the top surface 21a of the bias board 21, and the back surface 25b covering an area of the bias board 21 corresponding to the portion 21d, and the protecting surface 25c covering the DC terminals 12d of the optical modulator 12.

Further specifically, the top surface 25a shapes a metal slab extending along the top surface 21a of the bias board 21. The back surface 25b also shapes a metal slab extending along the back surface 21b of the bias board 21. Finally, the protecting surface 25c shapes a metal slab extending along the side 12b of the optical modulator 12. The top and back surfaces, 25a and 25b, extend in parallel with a gap and connected by the end 25d to form the U-shaped cross section. The back surface 25b has a length about half of a length of the top surface 25a. The end 25d provides the slit 25e through which the other FPC 27 is extracted. The back surface 25b makes a right angle against the protecting surface 25c. Thus, the shield 25 may be formed only by cutting and bending a metal plate. Also, the legs 25g provided in respective sides of the protecting surface 25c stick out from the top and back surfaces, 25a and 25b, and slightly bent toward the side 12b of the optical modulator 12 so as to form a gap against the side 12b, which may effectively protect the protecting surface 25c from being in contact with the DC terminals 12d.

Finally, four semi-rigid cables 17 are connected to the connector 12c of the optical modulator 12. The semi-rigid cables 17 provide the modulation signals corresponding to the components of IX, QX, IY, and QY. The semi-rigid cable 17 shows the specific transmission impedance with a center core and a conductive sheath with an insulating filler therebetween. The conductive sheath may be made of flexible metal. The semi-rigid cables 17, which are preferably pre-formed as shown in Fig. 3, are coupled with the optical modulator 12 as keeping the pre-formed shape thereof. Thus, the step to assemble the modulator unit U1 is competed.

b: Installation of Source Unit
Next, the process to install the source unit U2 within the top housing 2 will be described. Fig. 16 is a perspective view showing a process to install the source unit U2 within the housing. Referring to Fig. 6(a), the source unit U2 includes the LD module 13A and a circuit board 13C that mounts circuits to determine the wavelength of the CW light output from the LD module 13A. The LD module 13A extracts the inner fiber F1 by the pig-tailed arrangement, that is, the inner fiber F1 is permanently fixed to the LD module 13A. The inner fiber F1 provides the PMC P1 in the other end thereof. Referring back to Fig. 16, the source unit U2 is installed in the front extra space S1 of the protrusion 2a in the top housing 2. Installing the source unit U2 in the front extra space S1, the inner fiber F1 is extended rearward, then wired in the next step. Thus, the step to install the source unit U2 is completed.

c: Installation of Modulation Unit
The process next install the modulation unit U1, which is assembled out of the top housing 2 in the process (a) described above, within the top housing 2 that already mounts the source unit U2. Fig. 17 shows a process to install the modulation unit U1 into the housing. As shown in Fig. 17, four semi-rigid cables 17 are set within respective grooves 2i formed in the inner surface of the top housing 2, and four connectors 17a are positioned above the second PM connector P2. Because the semi-rigid cables 17 are pre-formed so as to trace the shape of the grooves 2i, the connectors 17a may be automatically set in respective positions. Then, the grooves 2i are covered with the metal cover 44, which is not illustrate in Fig. 17, by screwing the metal cover 44 to holes provided in the terraces 2h that form the grooves 2i therebetween. The process next wires some of the inner fibers.

The third inner fiber F4 coupling the optical modulator 12 to the PMC 14 is first wired. The inner fiber F4, extracted outward from the rear end of the optical modulator 12 by passing through the cut 2d provided in the rear wall 2c. Then, bending along the edge of the hollow 2j formed in the eaves 2e, the inner fiber F4 is drawn inside of the top housing 2 again as passing through the center cut 2g. The hollow 2j forms the rear extra space S2. After drawing the inner fiber F4 within the housing, the hollow 2f is covered with the cover 20 to protect the inner fiber F4. The cover 20 provides three latch tabs 20a in respective rear sides and a center of the front edge, and two cover tabs 20b in respective sides of the front edge. Fitting the latch tabs 20a with the holes 2f formed in the eaves 2e, the cover 20 may be securely assembled with the top housing 2. The cover tabs 20b cover the cuts, 2d and 2g. The inner fiber F4 back to the inside of the top housing 2 through the center cut 2g extends frontward, turns backward in the front extra space S1, and finally couples the PM connector P2 secured in the end of the inner fiber F4 at a center of the top housing 2. The PM connector P2 is to be coupled with the other inner fiber F3 coming from the PMC 14.

Another inner fiber F1, which extends from the LD module 13A to the PMC 14, secures the PM connector P1 in the end thereof opposite to a side coupled with the LD module 13A. The inner fiber F1 is extracted from the LD module 13A rearward, turns in the rear of the top housing 2 without being extracted externally, extends frontward along the optical modulator 12, bends so as to form an S-character in the front extra space S1, extends rearward again, couples with the PM connector P1 at the center of the top housing 2.

The present embodiment installs the modulation unit U1 only by two screws 32 mating with the screw holes 21f of the bias board 21. Specifically, referring to Fig. 14(a), the bias board 21 provides two holes 21f in portions not overlapping with the optical modulator 12. Screwing the bias board 21 with the top housing 2 by these two holes 21f, the modulation unit U2 is fixed to the top housing 2 as the housing of the optical modulator 12 is physically apart from the top housing 2. This means that the housing of the optical modulator 12 is connected to the signal ground but floated from the chassis ground, namely, the top and bottom housings, 2 and 3. The isolation of the signal ground from the chassis ground may enhance the noise tolerance of the optical transceiver 1.

As explained later, the top of the drivers 11 come in contact with the metal cover 21, which enhances the heat dissipation from the drivers 11 to the top housing 2. The drivers 11 each provide two connectors 45 to be mated with the connectors 17a secured in the end of the semi-rigid cables 17. After the set of the metal cover 44, the modulation unit U1 is assembled with the top housing 2. Thus, the process to install the modulation unit U1 within the housing is completed.

d: Mounting Receiver Unit on Mother Board
Next, the forth step for mounting the receiver unit U3 on the top surface 7a of the mother board 7 in the outside of the housing will be described. Fig. 18(a) is a perspective view of the top surface 7a of the mother board 7, and Fig. 18(b) is also a perspective view but the back surface 7b of the mother board 7. The top surface 7a mounts circuits for processing RF signals. Specifically, the top surface 7a mounts the DSP 16 in one side thereof and two drivers 11 behind four connectors 45 in a center of the top surface 7a.

The present embodiment further provides a daughter board 8 independent of the mother board 7. The daughter board 8 mounts circuit relating to power processing, for instance, DC to DC converters (DC/DC-C), bias generators, and so on. Because a DC/DC-C integrates a large inductor, such a DC/DC-C often has a large height, which sometimes becomes greater than 10 mm. The present optical transceiver 1, in particular, the outer dimensions thereof, follows the MSA; and the height of the housing thereof is about 10 mm maximum. Moreover, the mother board 7 of the embodiment mounts components in top and back surfaces, 7a and 7b, which further makes the mother board 7 hard to mount DC/DC-Cs thereon.

Accordingly, the present optical transceiver 1 provides the daughter board 8 independent of the mother board 7 and a horizontal level of the daughter board 8 is set lower than that of the mother board 7. Thus, the daughter board 8 may mount components with greater heights. The daughter board 8 is fixed to the mother board 7 as putting conductive spacers 47 therebetween, and electrically connected thereto through an FPC board 48. Because the daughter board 8 mounts the power circuit that processes large currents, the conductive spacer 47 may flow this large current. Moreover, the back surface 8b of the daughter board 8 in all over thereof is grounded. Setting the back surface 8b directly in contact to the bottom housing 3, exactly a lid 3a in the bottom housing 3, the heat dissipating efficiency from the power circuit on the daughter board 3 to the bottom housing 3 may be enhanced. Although not illustrated in figures, the bottom housing 3 in a portion coming in contact to the back surface 8b of the daughter board 8 is formed relatively thin, which may further gives a room to mount components with greater height.

Next, arrangements around the receiver unit U3, namely, the ICR 15 and three FPCs, 15c to 15e, will be described as referring to Figs. 19 and 20, where Fig. 19 is an exploded view of the receiver unit, and Fig. 20 is front view of the receiver unit on the mother board. Referring to Fig. 7, the ICR 15 has a box-shaped housing 15B providing DC terminals 15C in respective sides 15a and RF terminals 15D in the rear. Fig. 19 omits the RF terminals 15D and a portion of the DC terminals 15C in the hidden side. Two FPCs, 15c and 15d, which electrically connect the DC terminals 15C of the ICR 15 to circuits mounted on the mother board 7, are devised in the arrangement from a viewpoint how to install the ICR 15 within the densely packed housing of the optical transceiver 1 as soldering the FPCs, 15c and 15d, with pads, 7c and 7d, on the mother board 7. The ICR 15 is mounted on the mother board 7 through the holder 19.

Specifically, referring to Fig. 20, the DC-FPC 15d, which is connected to the DC terminals 15C in the side 15a facing the DSP 16, namely, the center side 15a, extends downward from the DC terminals 15b, bent along the outer edge of the holder 19, passes a gap between the holder 19 and the mother board 7, bent upward in the outer side of the ICR 15, folded so as to form a U-shape, bent again in the outer side of the housing, extends in the gap between the holder 19 and the mother board 7, and finally connected to the pads 7c. The pads 7c are provided in a center of the mother board 7.

The other DC-FPC 15c, which is soldered to the DC terminals 15C in the outer side of the housing of the ICR 15, extends upward from the DC terminals 15b, folded downward so as to wrap the other DC-FPC 15d, bent in a right angle at the edge of the outer side of the ICR 15, extends in the gap between the holder 19 and the mother board 7, and finally connected to the pads 7d in an outer side on the mother board 7. The pads, 7c and 7d, are formed beneath the holder 19, and the DC-FPC 15d is set upper of the other DC-FPC 15c in the gap between the holder 19 and the mother board 7. The curvature of the U-shaped fold of the former DC-FPC 15d is greater than that of the other DC-FPC 15c.

As described above, the pads, 7c and 7d, on the top surface 7a, and/or the soldering of the DC-FPCs, 15c and 15d, to the pads, 7c and 7d, are unable to be inspected after the holder 19 or the ICR 15 is mounted on the mother board 7 because these pads, 7c and 7d, are provided beneath the holder 19. Accordingly, the embodiment of the present invention first solders the DC-FPCs, 15c and 15d, to the pads, 7c and 7d, then mounts the ICR 15 on the holder 19 as folding the DC-FPCs, 15c and 15d. Specifically, the ICR 15 in the DC terminals 15b thereof is soldered with the DC-FPCs, 15c and 15d, to form an intermediate assembly. Then, the DC-FPC 15c is soldered to the pad 7c and the other DC-FPC 15d is soldered to the other pad 7d. Finally, the ICR 15 is set on the holder 19 such that two DC-FPCs, 15c and 15d, are folded to each other. After mounting the ICR 15 on the holder 19, the RF-FPC 15e, which is connected to the RF terminals 15D, are soldered to pads also provided on the top surface 7a of the mother board 7. Because two DC-FPCs, 15c and 15d, carry on DC signals, or at least signals containing only low frequencies, the quality of the signals carried thereon is not degraded even when the DC-FPCs, 15c and 15d, have extended lengths. The arrangement of the receiver unit U3 thus described, in particular, the arrangement of two DC-FPCs, 15c and 15d, may complete an optical transceiver 1 with a lot of optical and electrical components each strictly restricted in respective installation thereof.

The holder 19, as described above, provides short legs 19a each having a U-shaped cross section, namely, bent so as to form the U-shape, in respective corners of the primary surface 19b. The short legs 19a in respective ends thereof abut against the top surface 7a to form the gap between the holder 19 and the mother board 7. The holder 19 also provides extensions 19c bent upward from the primary surface 19b thereof. The extensions 19c each provides a hook in a top end thereof. The hook may secure the ICR 15 set on the primary surface 19b. The holder 19 also provides a long leg 19d bent downward in a front edge of the primary surface 19b. The long leg 19d provides a hooked end into which the PMC 14 is to be secured against the back surface 7b of the mother board 7.

The process of assembling the optical transceiver 1 may couple the inner fiber F5 with the third PM connector P3 already assembled with the ICR 15. That is, the inner fiber F5 extended from the PMC 14 by the pig-tailed arrangement may mate the male connector P31 thereof with the female connector P32 provided in the ICR 15. Moreover, the process may apply gels to enhance the heat dissipation on respective tops of integrated circuits (ICs) mounted on the mother board 7. Because the operational speed of the optical transceiver 1 like the present embodiment exceeds 10 Gbps, or sometimes reaches 40 Gbps, the ICs are necessary to secure heat conducting paths to the top and bottom housings, 2 and 3. Thermal gels or heat-dissipating gels may enhance the heat conductance to the top and bottom housings, 2 and 3. Specifically, the gels may be applied on the metal cover 44, the inner surface of the top housing 2 facing the DSP 16, the inner surface facing the DC/DC-Cs 46 mounted on the daughter board 8, and the inner surface to be in contact to a top of the ICR 15. Also, the top of the driver 11, that of the DSP 16, that of the DC/DC-Cs 46, and that of the ICR 15 are to be applied with the gels.

e: Installation of Mother Board within Housing
Next, the fifth step to install the mother board 7 within the housing will be described. Fig. 21 shows a process to install the mother board 7 that mounts the ICR 15 and so on, in the top housing 2 that already installs the modulation unit U1 and the source unit U2. As shown in Fig. 21, the mother board 7 is installed within the top housing 2 as exposing the back surface 7b thereof and the back surface 8b of the daughter board 8 such that the DC/DC-C 46 mounted on the daughter board 8 in the top surface thereof is to be in thermally contact with the housing of the optical modulator 12. The daughter board 8 does not interfere with the shield 25 in the modulation unit U1, that is, the daughter board 8 is apart from the shield 25 in the rear of the shield 25. Thus, the mother board 7, which mounts the ICR 15, the driver 11, and the DSP 16, is installed within the top housing 2.

Installation of PMC
Next, the PMC 14 is mounted on the back surface 7b of the mother board 7. As illustrated in Fig. 21, the mother board 7 provides a cut 7f in the side thereof, through which the long leg 19d of the holder 19 exposes. Mounting respective sides of the PMC 14 on the back surface 7b as setting the center thereof above the cut 7f, the PMC 14 is inserted within the hooked long leg 19d of the holder 19, which secures the PMC 14 against the back surface 7b of the mother board 7. Thus, the PMC 14 may be physically assembled with the mother board 7.

f: Wiring of Inner Fibers
Next, the inner fibers, F1 to F7, are to be wired with respect to the PM connectors, P1 and P2. Fig. 22 schematically illustrates the wiring of the inner fibers, F1 to F7. Referring to Fig. 22, the process to couple the source unit U2 with the PMC 14 by the inner fibers, F1 and F2, and the PM connector P1 is described. In Fig. 22, the positions of the optical source 13, the PMC 14, the optical modulator 12, the ICR 15, and the optical receptacle 18 roughly reflect the practical arrangement thereof within the top housing 12 viewed from the bottom. The inner fiber F1 extracted from the optical source 13 extends to the rear of the top housing 2, and turns there without being externally guided to the rear extra space S2. The turned inner fiber F1 extends to the front extra space S1 and turns again there, and couples with the PM connector P1 in a center of the top housing 2.

The second inner fiber F2 is extracted from the first PM connector P1 extends to the rear of the top housing 2, and turns clockwise there so as to round the optical modulator 12. The turned inner fiber F2 extends outside of the optical modulator 12 toward the front extra space S1 and turns again there toward the center of the top housing 2 as passing the side of the optical source 13 and the rear of the optical receptacle 18. The inner fiber F2 couples with the PMC 14 in the center of the top housing 2. Thus, the first wiring step for the inner fibers, F1 and F2, is completed.

Next, the inner fibers, F3 and F4, and the second PM connector P2 couple the PMC 14 with the optical modulator 14. Specifically, the third inner fiber F3 is extracted from the PMC 14 rearward, turns in the rear of the top housing 2, and couples with the second PM connector P2. The fourth inner fiber F4, which is extracted from the second PM connector P2, extends frontward as passing the side of the optical source 13, turns in the front extra space S1, extends rearward as passing the side of the optical modulator 12, passes the rear wall 2c of the top housing 2 outward to the rear extra space S2, turns again there, returns the inside of the top housing 2 as passing through the side cut 2d in the rear wall 2c, and finally couples with the rear port of the optical modulator 12 just in front of the rear wall 2c. Thus, the process to extend the infer fibers, F3 and F4, is completed.

Next, the inner fiber F5 and the third PM connector P3 couples the PMC 14 with the IRC 15. Specifically, the inner fiber F5, which is extracted rearward from the PMC 14, turns in the rear of the top housing 2 without being extracted outward to the rear extra space S2, extends frontward outside of the optical modulator 12, and bends toward the optical receptacle 18 in front of the front extra space S1 without entering the front extra space S1. The inner fiber F5 in almost whole portion thereof exposes in the back surface 7b of the mother board 7. Accordingly, the inner fiber F5 is secured with a tab 25g provided in the top surface 25a of the shield 25. Thus, the process to extend the fifth inner fiber F5 is completed. The processes to extend the inner fibers, F1 to F5, may be optional in the order thereof. For instance, the process to extend the infer fiber F5 may be carried out in advance to the process for installing the mother board 7 within the top housing 2.

Next, the optical modulator 12 and the ICR 15 are coupled with the optical receptacle 18 by the inner fibers, F6 and F7, respectively, that is, the inner fiber F6 couples with the optical output port 18a, while, the other inner fiber F7 is coupled with the optical input port 18b. Specifically, the inner fiber F6, which is extracted frontward from the optical modulator 12, extends to the front extra space S1 and turns rearward there so as to round the source unit U2. Then, extending rearward as passing the rear of the optical source 13 and the side of the optical modulator 12, the infer fiber F6 turns frontward in the rear of the top housing 2 corresponding to the position of the optical output port 18a, and finally couples with the optical output port 18a. On the other hand, the other inner fiber F7, which carries the optical input signal and is extracted from the optical input port 18b rearward, passes rearward in the center side of the ICR 15, turns frontward in the rear of the top housing 2, extends frontward in the outer side of the optical modulator 12, bends toward the optical receptacle 18 in the front of the optical modulator 12, passes the rear of the optical receptacle 18, and finally couples with the port C3 in the ICR 15. Thus, the process to extend the inner fibers, F6 and F7, where the former couples the optical modulator 12 with the optical output port 18a, while, the latter couples with the optical input port 18a with the ICR 15, is completed. After the process of extending the inner fibers, F1 to F7, two inner fibers, F1 and F4, are hidden under the mother board 7, while, rest of inner fibers, F2, F3, and F5 to F7, are exposed from the mother board 7.

Subsequent to the process of extending the inner fibers, F1 to F7, the modulation unit U1 and the source unit U2 are electrically connected with respective circuits mounted on the mother and daughter boards. Specifically, the source unit U2 is coupled with the circuits on the mother board 7 by mating a connector provided in the end of the FPC 7e with the connector 13D on the base 13B of the source unit U2. Then, the FPC 27, which is extended from the bias board 21, is to be soldered in the other end thereof with the mother board 7.

Finally, the bottom housing 3 is to be assembled with the top housing 2 as installing the units, U1 to U3, and the mother and daughter boards, 7 and 8, in the space formed by two housings, 2 and 3. Specifically, the process first sets the fastening screws in respective sides of the top housing 2. Fig. 23 is a perspective view of the bottom housing 3. As shown in Fig. 23, the bottom housing 3 provides a square opening 3b in a portion facing the daughter board 8. The square opening 3b is covered with a metal lid 3a with a thickness relatively thin compared with any other portions of the bottom housing 3. The lid 3a is fitted from the inside of the bottom housing 3. When the bottom housing 3 is made of die-casting of aluminum, the bottom housing 3 is necessary to have a thickness thereof at least 0.5 mm. As explained above, the DC/DC-Cs 46 mounted on the daughter board 8 often has greater thicknesses thereof. When the daughter board 8 and the DC/DC-Cs 46 are simply mounted on the bottom housing 3, a total height or a summed height sometimes becomes greater than a dimension ruled by the MSA for the optical transceiver 1. Accordingly, the bottom housing 3 of the present embodiment provides the square opening 3b covered by the lid 3a with a thinned thickness to secure a room for the components with relatively greater thickness. The lid 3a of the embodiment may be made of austenitic stainless steel with a thickness of 0.2 mm, which has stiffness comparable to a die-casted aluminum (Al). The austenitic stainless steel has thermal conductivity comparable to that of aluminum (Al).

While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.

Claims

A method to assemble an optical transceiver for an optical coherent system, the optical transceiver comprising a source unit for generating continuous wave (CW) light, a polarization maintaining coupler (PMC) for splitting the CW light into two portions, a modulator unit for modulating one of the CW light split by the PMC, a receiver unit for receiving another of the CW light, and a housing for enclosing the modulator unit, the source unit, the receiver unit and the PMC therein, the method comprising steps of:
(a) assembling the modulator unit outside of the housing;
(b) installing the source unit within the housing;
(c) installing the modulator unit within the housing;
(d) mounting the receiver unit on a top surface of a mother board outside of the housing;
(e) installing the mother board mounting the receiver unit within the housing as exposing a back surface of the mother board opposite to the top surface thereof;
(f) mounting the PMC on the back surface of the mother board;
(g) coupling the source unit with the PMC by a first inner fiber, a second inner fiber, and a first polarization maintaining connector;
(h) coupling the PMC with the modulator unit by a third inner fiber, a fourth inner fiber, and a second polarization maintaining connector;
(i) coupling the PMC with the receiver unit by a fifth inner fiber and a third polarization maintaining connector; and
(j) coupling an optical receptacle provided in the housing with the modulator unit and the receiver unit by respective single mode fibers (SMFs),
wherein the steps from (g) to (i) are performed in no particular order.
The method of claim 1,
wherein the step (a) includes steps of:
connecting an optical modulator to a bias board with a flexible printed circuit (FPC) board, the bias board mounting circuits for providing biases to the optical modulator,
assembling the bias board with the optical modulator as putting a spacer between the bias board and the optical modulator,
covering the bias board by a shield assembled to the optical modulator as extracting another FPC board connected to the bias board from a slit provided in the shield, and
connecting cables to the optical modulator, the cables being pre-formed so as to follow an inner structure of the housing.
The method of claim 2,
wherein the housing includes grooves and terraces forming the grooves,
wherein the step (c) includes steps of:
setting the pre-formed cables into respective grooves,
screwing a plate on the terraces such that the plate covers the respective grooves,
screwing on the bias board to the housing such that the optical modulator assembled with the bias board is apart from the housing.
The method of claim 2,
wherein the step (c) includes a step of installing the optical modulator in an extra space provided in the housing, the extra space being protruding from a front wall where the optical receptacle is provided.
The method of claim 4,
wherein the step (b) includes a step of installing the source unit in the extra space.
The method of claim 1,
wherein the optical receptacle provides an optical input port coupled with the receiver unit and an optical output port coupled with the modulator unit by the respective SMFs.
The method of claim 1,
wherein the receiver unit includes an integrated coherent receiver (ICR) that interferes the another of the CW light provided through the fifth inner fiber with an input optical signal provided from the optical receptacle through one of the sign,
wherein the third polarization maintain connector coupled with the fifth inner fiber is integrated with the ICR.
The method of claim 1,
wherein the steps (g) and (h) includes a step of coupling the first inner fiber with the second inner fiber by the first PM connector on the back surface of the mother board, and a step of coupling the third inner fiber with the fourth inner fiber by the second PM connector on the back surface of the mother board.
The method of claim 8,
wherein the step (h) includes a step of,
extending the fourth inner fiber from the optical modulator to an outside of the housing as passing through a cut provided in the housing, turning the fourth inner fiber in the outside of the housing, and returning the fourth inner fiber to an inside of the housing as passing through another cut in the housing.
The method of claim 9,
wherein the step (h) further includes a step of covering a portion of the fourth inner fiber in the outside of the housing by a cover, the cover covering the cut and the another cut of the housing.
The method of claim 1,
further comprising steps of, after the step (j),
connecting the source unit to the mother board by an FPC, and
connecting the modulating unit to the mother board by another FPC.
The method of claim 1,
wherein the mother board accompanies with a daughter board for mounting integrated circuits processing DC signals, and the housing includes a top housing and a bottom housing forming a space therein,
wherein the method further includes a step of, after the step (i), enclosing the source unit, the modulator unit, the receiver unit, the mother board with the daughter board, the inner fibers, the PMC, and the PM connectors in the space such that the daughter board is in contact with a lid that covers the square opening of the bottom housing.