Abstract:
Methods, systems, and apparatus, for optical communication. One optical assembly includes a Fabry-Perot (FP) laser diode; a first polarization controller (PC) coupled to the FP laser diode; a circulator having four ports, a first port coupled to the first PC; an optical fiber coupled at a first end to a second port of the circulator; a second PC coupled to a third port of the circulator; an optical amplifier coupled to the second PC and a fourth port of the circulator; a wavelength division multiplexer (WDM) filter coupled to the second end of the optical fiber; a splitter having at least three ends coupled at a first end to the WDM; and a Faraday rotator mirror (FRM) coupled directly or indirectly to a second end of the splitter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of the filing date of U.S. Provisional Patent Application 61/821,948, for Compact WDM Optical Modules, which was filed on May 10, 2013, and which is incorporated here by reference. 
    
    
     BACKGROUND 
     This specification relates to optical communication. 
     Wavelength Division Multiplexing (WDM) technology has been widely used in optical fiber communications to increase the transmission capacity for point to point data transmission through a single optical fiber. Various conventional WDM laser technologies have been developed. Solutions suitable for Dense WDM (DWDM) applications and with high speed modulation performance at 10 Gigabit/second or above are typically desired. 
     Conventional or proposed WDM laser solutions include fixed wavelength externally modulated laser (EML), wavelength tunable lasers, externally seeded injection locked Fabry-Perot (FP) lasers, and reflective optical amplifier (RSOA) lasers. 
     Other conventional architectures can include self-seeding mechanisms in which drop fibers connecting the light sources to the passive distribution node serve as part of the laser cavity, and a partial reflector on the transmission side of the WDM multiplexer (MUX) at the passive node together with the WDM MUX serves as the wavelength selective mirror in the laser cavity to lock the laser wavelength to the channel defined by the WDM MUX. Though such architectures greatly simplify a passive optical network WDM (WDM-PON) system, it is nevertheless challenging to manage a long external cavity laser for stable and high performance transmission at high data rates and long distance. 
     SUMMARY 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in optical assemblies that include a Fabry-Perot (FP) laser diode; a first polarization controller (PC) coupled to the FP laser diode; a circulator having four ports, a first port coupled to the first PC; an optical fiber coupled at a first end to a second port of the circulator; a second PC coupled to a third port of the circulator; an optical amplifier coupled to the second PC and a fourth port of the circulator; a wavelength division multiplexer (WDM) filter coupled to the second end of the optical fiber; a splitter having at least three ends coupled at a first end to the WDM; and a Faraday rotator mirror (FRM) coupled directly or indirectly to a second end of the splitter, wherein an optical signal output by the FP laser diode passes through the circulator, through the optical fiber, through the WDM filter, through the splitter, and onto the FRM or leaves the optical assembly as an output signal through a third end of the splitter; and wherein the optical signal reflected by the FRM then passes through the WDM filter, through the optical fiber, through the circulator, through the second PC such that the polarization of the optical output of the second PC is substantially matched with the polarization of the optical amplifier, through the optical amplifier, back through the circulator again, through the first PC such that the polarization of the optical output of the first PC is substantially matched with the polarization of the FP laser diode, and then injected back into the FP laser diode. Other embodiments of this aspect include corresponding methods. 
     The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. The WDM filter is a dense wavelength division multiplexing filter. The optical amplifier is positioned between the third port and the fourth port of the circulator. The optical amplifier is a semiconductor optical amplifier. The semiconductor optical amplifier cancels a modulation of the optical signal that passes through the semiconductor optical amplifier. 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in optical assemblies that include a Fabry-Perot (FP) laser diode; a reflective semiconductor optical amplifier (RSOA); a first polarization controller (PC) coupled to an optical output of the FP laser diode; a circulator having four ports, a first port coupled to an optical output of the RSOA and a second port coupled to an optical output of the first PC; a second PC coupled to a third port of the circulator; a third PC coupled to a fourth port of the circulator; a polarization beam splitter (PBS) coupled on a first end to the second PC and coupled on a second end to the third PC; an optical fiber coupled at a first end to a third end of the PBS; a wavelength division multiplexer (WDM) filter coupled to the second end of the optical fiber; a splitter having at least three ends coupled at a first end to the WDM; and a Faraday rotator mirror (FRM) coupled directly or indirectly to a second end of the splitter, wherein an optical signal output by the FP laser diode passes through the circulator, through the second PC such that optical signal entering the PBS after leaving the second PC is substantially matched with the polarization of the PBS, through the PBS, through the optical fiber, through the WDM filter, through the splitter, and onto the FRM or leaves the optical assembly as an output signal through a third end of the splitter; and wherein the optical signal reflected by the FRM passes through the WDM filter, through the optical fiber, through the PBS, through the third PC such that the polarization of the output signal of the third PC is substantially matched with the polarization of the RSOA, enters the circulator through the fourth port and out of the first port into the RSOA, is reflected back into the circulator through the first port, and exits the circulator through the second port, through the first PC such that the polarization of the output signal is substantially matched with the FP laser diode, and is injected back into the FP laser diode. Other embodiments of this aspect include corresponding methods. 
     The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. The WDM filter is a dense wavelength division multiplexing filter. The optical amplifier is a reflective semiconductor optical amplifier. The reflective semiconductor optical amplifier cancels a modulation of the optical signal that passes through the reflective semiconductor optical amplifier. 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in optical assemblies that include a Fabry-Perot (FP) laser diode; a first splitter coupled to receive an optical signal from FP laser diode and separate the optical signal into two output optical signals; a first photodiode coupled to receive a first output optical signal from the first splitter; a polarization beam splitter (PBS) coupled to receive a second optical signal from the first splitter; an optical fiber coupled at a first end to receive a first optical output from the PBS; a wavelength division multiplexer (WDM) filter coupled to receive the optical output from a second end of the optical fiber; a second splitter coupled to receive the optical signal from the WDM filter and separates the optical signal into at least two output optical signals; a Faraday rotator mirror (FRM) coupled directly or indirectly to a first output of the second splitter; a third splitter coupled to receive the optical signal from a second optical output from the PBS; a second photodiode coupled to receive a first output optical signal from the third splitter; a half wave plate coupled to receive a second output optical signal from the third splitter; an optical amplifier coupled on a first end to receive the output optical signal from the half wave plate and coupled on a second end to transmit an optical signal to the FP laser diode, wherein the FRM reflects the optical signal back to the PBS such that the reflected optical signal has a polarization that is rotated by substantially 90 degrees relative to the optical signal received by the FRM, wherein the reflected optical signal is separated by the PBS from a forward going optical signal outputted from the FP laser diode, wherein the reflected optical signal is then outputted to the third splitter such that part of the reflected optical signal is sent to the second photodiode to be monitored, and wherein the forward going optical signal is outputted to the first splitter such that part of the forward going optical signal is sent to the first photodiode to be monitored. Other embodiments of this aspect include corresponding methods. 
     The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. The half wave plate rotates the polarization of the reflected optical signal by substantially 90 degrees. The WDM filter is a dense wavelength division multiplexing filter. The optical amplifier is a semiconductor optical amplifier. 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in optical assemblies that include a Fabry-Perot (FP) laser diode; a first collimating lens coupled to an optical output end of the FP laser diode; a polarization beam splitter (PBS) coupled at a first end to receive the collimated optical signal with matched polarization from the first lens; a second collimating lens coupled to the optical output from a first end of the PBS and to focus the optical output into a first end of an optical fiber positioned at a second end of the second collimating lens; a first mirror coupled to receive an optical output from the PBS; a half wave plate coupled to receive an optical output from the first mirror; a second mirror coupled to receive an optical output from the half wave plate; a third mirror coupled to receive an optical output from the second mirror; a third collimating lens coupled to receive an optical output from the third mirror; an optical amplifier coupled on a first end to receive an focused optical output from the third collimating lens and configured to output from a second end an optical signal to the FP laser diode, wherein an optical input received at the second collimating lens from the optical fiber is a reflected optical signal from the FP laser diode that passes through the second collimating lens, has a polarization such that it is reflected by the PBS by substantially 90 degrees, is folded by substantially 90 degrees by the first mirror, passes through the half wave plate such that the signal is rotated to match the polarization of the optical amplifier, is folded by the second mirror by substantially 90 degrees, is further folded by the third mirror by substantially 90 degrees, passes through the third collimating lens, passes through the optical amplifier, and then is injected into the rear facet of the FP laser diode. Other embodiments of this aspect include corresponding methods. 
     The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. The optical assembly further includes a wavelength division multiplexer (WDM) filter coupled to receive the optical output from a second end of the optical fiber; and a Faraday rotator mirror (FRM) coupled directly or indirectly to an output end of the WDM, wherein an output optical signal of the WDM filter is partially reflected by the FRM such that the polarization of a reflected beam is rotated by substantially 90 degrees after transmission through and reflection by the FRM, and wherein the reflected optical signal passes through the WDM filter. The WDM filter is a dense wavelength division multiplexing filter. The optical amplifier is a semiconductor optical amplifier. The semiconductor optical amplifier cancels a modulation of the optical signal that passes through the semiconductor optical amplifier. The optical assembly further includes an optical isolator optically coupled to the PBS and the optical amplifier such that an enabling beam direction of the optical isolator points to the first end of the optical amplifier. The optical assembly further includes a steering lens optically coupled to the PBS and the third collimating lens. The first mirror is coated with partial transmission reflection film such that an optical signal from the PBS can be partially transmitted through it, the optical assembly further includes: a first integrated photodiode and mirror device configured to receive and focus the partially transmitted optical signal from the first mirror; a beam splitter coupled on a receiving end to the first lens and on an output end to the PBS such that an optical signal from the FP laser diode is partially reflected; and a second integrated photodiode and mirror device configured to receive and focus the partially reflected optical signal from the beam splitter. The optical assembly further includes a wire bonding connecting the anode and cathode bonding pad of each photodiode. All the components of the optical assembly are placed on top of a substrate. The substrate is formed from silicon or ceramic. The optical assembly further includes a C/L band filter coupled on a first end to the PBS; and a receiver module coupled to a second end of the C/L band filter, wherein the C/L band filter is configured to separate an incoming optical signal into a C-band wavelength output signal and a L-band wavelength signal such that a first output signal follows the previous optical path while a second output signal is reflected. The C/L band filter separates an incoming L-band signal from the PBS such that a L-band signal is reflected onto the receiver while a C-band signal transmits through the C/L band filter into the WDM. All the components of the optical assembly are placed on top of a substrate. The substrate is formed from of silicon or ceramic. The optical assembly is placed in a thermoelectric (TEC) cooler. The optical assembly is contained in a package case with lead pins located on two sides of the package case such that a modulation current going into the FP laser diode or a received AC signal output from the receiver can exit the package case. 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in an apparatus that include a Fabry-Perot laser diode assembly coupled to a first port of a circulator; an optical amplifier coupled to a second port of the circulator; a wavelength division multiplexer (WDM) filter coupled to a third port of the circulator; and a Faraday rotator mirror coupled to the WDM filter. 
     Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. FP lasers, directly or externally modulated, are used as the light source to improve the basic laser characteristics including RIN noise quality, spectral quality, and modulation speed performance compared to a semiconductor optical amplifier. Additionally, the architectures described in the specification improve the stability of the laser and its performance over prior architectures. 
     The details of one or more embodiments of the subject matter of this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  show examples of an external cavity coupled FP laser structure with a circulator and an optical amplifier in between the FP laser and a DWDM filter. 
         FIG. 2  shows an example of an external cavity coupled FP laser structure with an FP laser positioned between a polarization beam splitter (PBS) and an optical amplifier. 
         FIG. 3  shows an example of a compact WDM laser transmitter optical subassembly (TOSA). 
         FIG. 4  shows an example of a compact WDM laser transmitter optical subassembly (TOSA) with an isolator. 
         FIG. 5  shows an example of a compact WDM laser transmitter optical subassembly (TOSA) with a steering lens. 
         FIG. 6  shows an example of a compact WDM laser transmitter optical subassembly (TOSA) with monitoring photodiodes. 
         FIG. 7  shows an example of a compact WDM laser transmitter optical subassembly (TOSA) with optical system placed and aligned on a carrier. 
         FIG. 8  shows an example of a compact bidirectional WDM optical module. 
         FIG. 9  shows an example of a compact bidirectional WDM optical module with a thermoelectric (TEC) cooler. 
         FIG. 10  shows an example of SFP type transceiver built with proposed bidirectional WDM optical module. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     This specification describes WDM laser architectures based on an external cavity Fabry-Perot (FP) laser diode. Additionally, this specification describes implementations of the WDM laser architectures in a small form factor transceiver. The transceiver can be configured to operate at up to 10 Gigabits per second (Gbit/s) to provide a so-called “colorless enhanced small form-factor pluggable (SFP+)” transceiver. A small form-factor pluggable (SFP) is a compact, hot-pluggable transceiver used for both telecommunication and data communications applications. SFP+ is an enhanced version of the SFP that supports data rates up to 10 Gbit/s. 
       FIGS. 1A and 1B  show examples of an external cavity coupled FP laser structure with a circulator and an optical amplifier in between the FP laser and a DWDM filter. 
     In particular,  FIG. 1A  shows a FP laser structure  100 . The FP laser structure  100  includes an FP laser diode  102 . The FP laser diode  102  is a particular type of laser diode. A laser diode is an electrically pumped semiconductor laser. In the FP laser diode, a gain region is surrounded with an optical cavity to form a laser. In the simplest form of a laser diode, an optical waveguide is made on a crystal surface, such that the light is confined to a relatively narrow line. The two ends of the crystal are cleaved to form parallel edges, providing a Fabry-Pérot resonator. Photons emitted into the waveguide will travel along the waveguide and be reflected several times from each end face before they are emitted. As a light wave passes through the cavity, it is amplified by stimulated emission, but light is also lost due to absorption and by incomplete reflection from the end facets. If there is more amplification than loss, the diode begins to “lase.” 
     The FP laser diode  102  is coupled to a first polarization controller (PC)  104 , which is coupled to a circulator  106 . Thus, the first PC  104  is positioned between the FP laser diode  102  and the circulator  106 . The polarization controller  104  is an optical device configured to modify the polarization state of received optical signals. 
     The circulator  106  is a four port optical circulator such that light signals entering the first port exit a second port; light signals entering the second port exit a third port; and light signals entering the third port exit the fourth port, etc. The first port of the circulator  106  is coupled to the first PC  104 . 
     The second port of the circulator  106  is coupled to a first end of an optical fiber  108 . A second end of the optical fiber  108  is coupled to a WDM filter  110 . The WDM filter  110  can be a DWDM filter. A WDM filter is configured to pass particular bands of wavelengths while blocking others. Thus, an optical signal can be configured to transmit signals having particular wavelengths without passing other wavelengths. 
     The WDM filter  110  is coupled to a splitter  112 , which separates incoming light into two paths. A first path from the splitter  112  is coupled to a Faraday rotator mirror (FRM)  114  and the second path from the splitter  112  is couple to an output port  116 . A FRM is an optical device that rotates the polarization of incident light signals based on an applied magnetic field and then reflects the light having the rotated polarization. 
     A third port of the circulator  106  is coupled to a second PC  118 . The second PC  118  is coupled to an optical amplifier  120 . The optical amplifier  120  can be a semiconductor optical amplifier (SOA). Semiconductor optical amplifiers are amplifiers that use a semiconductor to provide a gain medium. An output of the optical amplifier  120  is coupled to a fourth port of the circulator  106  forming a loop between the third port and the fourth port of the circulator  106 . 
     In operation, an optical signal output by the FP laser diode  102  enters the first port of the circulator  106  and is output into the optical fiber  108 . The optical signal then passes through the WDM filter  110  to the splitter  112 . The splitter  112  splits the optical signal such that a portion of the optical signal passes to the output port  116  and another portion of the optical signal is directed to the FRM  114 . The optical signal reflected by the FRM  114 , which becomes a feedback signal, passes back through the WDM filter  110  and optical fiber  108  to the second port of the circulator  106 . 
     The optical signal is output at the third port of the circulator  106  and passes through the second PC  118  to the optical amplifier  120 . The optical signal passes through the second PC  118  such that the polarization of the optical output of the second PC  118  is substantially matched with the polarization of the optical amplifier  120 . In some implementations, the optical amplifier  120  is configured to cancel a modulation of the optical signal passing through it. From the optical amplifier  120 , the, now amplified, optical signal enters the fourth port of the circulator  106  and exits at the first port of the circulator  106 . From the circulator  106 , the optical signal passes through the first PC  104  such that the polarization of the optical signal output from the first PC  104  is substantially matched with the polarization of the FP laser diode  102 . The optical signal exiting the first PC  104  is injected back into the FP laser. 
       FIG. 1B  shows FP laser structure  101 . The FP laser structure  101  includes an FP laser diode  130 . The FP laser diode  130  is coupled to a first polarization controller (PC)  132 , which is coupled to a circulator  134 . Thus, the first PC  132  is positioned between the FP laser diode  130  and the circulator  134 . 
     The circulator  134  is a four port optical circulator, e.g., similar to circulator  106  of  FIG. 1A . The first port of the circulator  134  is coupled to the first PC  132 . The second port of the circulator  134  is coupled to a second PC  136 . The second PC  136  is further coupled to a polarization beam splitter (PBS)  138 . 
     A third port of the circulator  134  is coupled to a third PC  140 . The third PC  140  is coupled to the PBS  138 . The PBS  138  is coupled to a first end of an optical fiber  142 . A second end of the optical fiber  142  is coupled to a WDM filter  144 . The WDM filter  144  can be a DWDM filter. 
     The WDM filter  144  is coupled to a splitter  146  which separates incoming light into two paths. A first path from the splitter  146  is coupled to a Faraday rotator mirror (FRM)  148  and a second path from the splitter  146  is coupled to an output port  150 . 
     A fourth port of the circulator  134  is coupled to an optical amplifier  152 . The optical amplifier  152  can be a reflective optical amplifier (RSOA). 
     In operation, an optical signal output by the FP laser diode  130  enters the first port of the circulator  134  and is output from the second port of the circulator  134 . The optical signal passes through the second PC  136  to the PBS  138 . The polarization of the optical signal exiting the second PC  136  is substantially matched with the polarization of the PBS  138 . The optical signal passes through the PBS  138  and into the optical fiber  142  to the WDM filter  144 . The optical signal passes through the WDM filter  144  to the splitter  146 . 
     The splitter  146  splits the optical signal such that a portion of the optical signal passes to the output port  150  and another portion of the optical signal is directed to the FRM  148 . The optical signal reflected by the FRM  148 , which becomes a feedback signal, passes back through the WDM filter  144  and optical fiber  142  to the PBS  138 . The optical signal passes through the PBS  138  and the third PC  140  to the third port of the circulator  134 . The optical signal output through the third PC  140  has a polarization that is substantially matched with the polarization of the optical amplifier  152 . 
     The optical signal exits the fourth port of the circulator  134  and into the optical amplifier  152 . In some implementations, the optical amplifier  120  is configured to cancel a modulation of the optical signal passing through it. The amplified optical signal is output from the optical amplifier  152  and enters the fourth port of the circulator  132 . In particular, when the optical amplifier  152  is a RSOA, the optical signal is reflected back out of the optical amplifier  152  as an amplified optical signal. The amplified optical signal exits the first port of the circulator  132  and passes through the first PC  132  such that the polarization of the output amplified optical signal is substantially matched with the FP laser diode  130 . The amplified optical signal is then injected back into the FP laser diode  130 . 
       FIG. 2  shows an example of an external cavity coupled FP laser structure  200  with an FP laser positioned between a polarization beam splitter (PBS) and an optical amplifier. The FP laser structure  200  includes an FP laser diode  202 . The FP laser diode  202  is coupled to a first splitter  204 . The first splitter  204  separates an incoming optical signal from the FP laser diode  202  into two output optical paths. The first optical path from the splitter  204  is coupled to a first photodiode  206 . The second optical path from the splitter  204  is coupled to a PBS  208 . 
     An output of the PBS  208  is coupled to a first end of an optical fiber  210 . A second end of the optical fiber  210  is coupled to a WDM filter  212 . The WDM filter  212  can be a DWDM filter. The WDM filter  212  is coupled to a second splitter  214 , where a first path from the splitter  214  is coupled to a Faraday rotator mirror (FRM)  216  and the second path from the splitter  214  is coupled to an output port  218 . 
     The PBS  208  includes a second optical output coupled to a third splitter  220 . A second photodiode  222  is coupled to a first output path of the third splitter  220 . A half wave plate  224  is coupled to a second output path of the third splitter  220 . An optical amplifier  222 , e.g., an SOA, is coupled between the half wave plate  220  and a rear facet of the FP laser diode  202 . 
     In operation, an optical signal output by the FP laser diode  202  enters splitter  204  where it is separated into two output optical signals. One output optical signal is routed to the first photodiode  206  for power monitoring. This first photodiode  206  monitors the signal power emitted by the FP laser diode  202 . The second output optical signal is routed to the PBS  208 . 
     The optical signal passes through the PBS  208  and into the optical fiber  210  to the WDM filter  212 . The optical signal passes through the WDM filter  212  to the splitter  214 . The splitter  214  splits the optical signal such that a portion of the optical signal passes to the output port  218  and another portion of the optical signal is directed to the FRM  216 . The optical signal reflected by the FRM  216 , which becomes a feedback signal, passes back through the WDM filter  212  and optical fiber  210  to the PBS  208 . The FRM  216  reflects the optical signal back to the PBS  208  such that the optical signal reflected by the FRM  216  has a polarization that is rotated by substantially 90 degrees relative to the optical signal received by the FRM  216 . Because of the rotation of polarization, the optical signal is directed to the second optical output port of the PBS  208 . The FP laser structure  200  takes advantage of the fact that the reflected optical signal from the FRM  216  has an orthogonal polarization to the incoming optical signal. 
     The optical signal exits the second optical output of the PBS  208  and enters the third splitter  220 . The third splitter  220  separates the optical signal into two output optical signals. One output optical signal is routed to the second photodiode  222  for power monitoring. In particular, the second photodiode  222  monitors the feedback signal power. The monitored power can be compared with the signal power monitored by the first photodiode  206  to measure a cavity loss of the external cavity, which can be used to align the FP laser diode  202  for optimum performance. 
     The second output optical signal is routed to the half wave plate  224 . The half wave plate  224  rotates the polarization of the optical signal by substantially 90 degrees relative to the optical signal received by the half wave plate  224  such that the optical signal is substantially matched with the polarization of the optical amplifier  226 . The optical signal exits the half wave plate  224  and passes into the optical amplifier  226 . In some implementations, the optical amplifier  226  is configured to cancel a modulation of the optical signal before coupling the amplified optical signal into a rear facet of the FP laser diode  202 . The optical amplifier  226  can be a SOA. The amplified optical signal is output from the optical amplifier  226  and enters the rear facet of the FP laser diode  202 , which locks the FP laser&#39;s lasing wavelength. The FP laser structure  200  eliminates the use of an optical circulator and provides for the implementation of a compact transmitter optical subassembly for building SFP+ transceivers. 
       FIG. 3  shows an example of a compact WDM laser transmitter optical subassembly (TOSA)  300 . The TOSA  300  can provide an integrated, monolithically or as a hybrid, and mutually coupled FP+SOA device used as a light source with the FP laser being directly modulated for data transmission while the SOA is used to amplify and cancel modulation of a self-seeding feedback signal. 
     The TOSA  300  includes an FP laser diode  302 . An optical output end of the FP laser diode  302  is coupled to a first collimating lens  304 . The first collimating lens  304  is coupled to a PBS  306  such that the first collimating lens  304  is positioned between the PBS  306  and the FP laser diode  302 . The PBS  306  has a first output that is coupled to a second collimating lens  308 , which passes collimated light to a fiber  310 , e.g., through a receptacle assembly. This fiber  310  can be coupled to a WDM filter and FRM assembly, not shown, for example as described above with respect to  FIGS. 1 and 2 , and can return a feedback signal. 
     A second optical output of the PBS  306  is coupled to a first mirror  312 . The first mirror  312  is also coupled to a half wave plate  314  such that the first mirror  312  is positioned between the PBS  306  and the half wave plate  314 . The half wave plate is coupled to a second mirror  316 , which is coupled to a third mirror  318 . The third mirror is coupled to a third collimating lens  320 . 
     The third collimating lens  320  is coupled to an optical amplifier  322 . The optical amplifier  322  can be an SOA. The optical amplifier  322  is configured to receive a focused optical signal from the third collimating lens  320  and output an amplified optical signal to the FP laser diode  302 . The FP laser diode  302  and the optical amplifier  322  can be positioned together on a submount  324 . 
     Furthermore, the components of the TOSA  300  can be placed an aligned on a substrate and positioned within a package case  326  for providing a compact optical module. The substrate can be silicon or ceramic. The substrate can be used as a carrier to have precise dimension control and alignment features, e.g., alignment marks or fillister for passive alignment of optical components. As a result, optical component alignment and repair can be performed more easily. 
     In operation, an optical signal output by the FP laser diode  302  enters the first collimating lens  304 . The resulting collimated light, having a matched polarization, passes through the PBS  306 . The output light from the first output of the PBS  306  passes through the second collimating lens  310  and exits the package  326  and into the optical fiber  310 . 
     A feedback optical signal is returned from the optical fiber  310 . The feedback optical signal can be provided from, for example, a FRM as described above with respect to  FIGS. 1 and 2 . The feedback optical signal is rotated to be orthogonal to the outgoing light of the FP laser diode  302 . Consequently, after being collimated by the second lens  308 , the feedback optical signal is reflected by the PBS  306  because of the polarization and exits the second output of the PBS  306 . The exiting feedback optical signal is incident on the first mirror  312 , half wave plate  314 , second mirror  316 , and third mirror  318 . To provide a compact TOSA, each mirror folds the feedback optical signal by 90 degrees. The half wave plate  314  rotates the feedback optical signal such that the polarization matches the optical amplifier  322 . After being reflected by the third mirror  318 , the feedback optical signal passes through third collimating lens  320  and then into a rear facet of the optical amplifier  322 . The optical amplifier  322  amplified the feedback optical signal and couples it back into the rear facet of the FP laser diode  302  to lock the lasing wavelength of the FP laser diode  302 . 
       FIG. 4  shows an example of a compact WDM laser transmitter optical subassembly (TOSA)  400  with an isolator  402 . The TOSA  400  provides an integrated, monolithically or as a hybrid, and mutually coupled FP+SOA device used as a light source with the FP laser being directly modulated for data transmission while the SOA is used to amplify and cancel modulation of a self-seeding feedback signal. 
     The TOSA  400  includes the FP laser diode  302 . An optical output end of the FP laser diode  302  is coupled to the first collimating lens  304 . The first collimating lens  304  is coupled to the PBS  306  such that the first collimating lens  304  is positioned between the PBS  306  and the FP laser diode  302 . The PBS  306  has a first output that is coupled to the second collimating lens  308 , which passes collimated light to the fiber  310 , e.g., through a receptacle assembly. This fiber  310  can be coupled to a WDM filter and FRM assembly, not shown, for example as described above with respect to  FIGS. 1 and 2 . 
     The second optical output of the PBS  306  is coupled to the first mirror  312 . The first mirror  312  is also coupled to the isolator  402  such that the first mirror  312  is positioned between the PBS  306  and the isolator  402 . The isolator  402  is coupled to the half wave plate  314 . The half wave plate is coupled to the second mirror  316 , which is coupled to the third mirror  318 . The third mirror is coupled to the third collimating lens  320 . 
     The third collimating lens  320  is coupled to the optical amplifier  322 . The optical amplifier  322  can be an SOA. The optical amplifier  322  is configured to receive a focused optical signal from the third collimating lens  320  and output an amplified optical signal to the FP laser diode  302 . The FP laser diode  302  and the optical amplifier  322  can be positioned together on submount  324 . 
     Furthermore, the components of the TOSA  400  can be placed an aligned on a substrate and positioned within a package case  326  for providing a compact optical module. The substrate can be silicon or ceramic. The substrate can be used as a carrier to have precise dimension control and alignment features, e.g., alignment marks or fillister for passive alignment of optical components. As a result, optical component alignment and repair can be performed more easily. 
     In operation, an optical signal output by the FP laser diode  302  enters the first collimating lens  304 . The resulting collimated light, having a matched polarization, passes through the PBS  306 . The output light from the first output of the PBS  306  passes through the second collimating lens  310  and exits the package  326  and into optical fiber  310 . 
     A feedback optical signal is returned from the optical fiber  310 . The feedback optical signal can be provided from a FRM as described above with respect to  FIGS. 1 and 2 . The feedback optical signal is rotated to be orthogonal to the outgoing light of the FP laser diode  302 . Consequently, after being collimated by the second lens  308 , the feedback optical signal is reflected by the PBS  306 . The exiting feedback optical signal is incident on the first mirror  312 , isolator  402 , half wave plate  314 , second mirror  316 , and third mirror  318 . 
     To provide a compact TOSA, each mirror folds the feedback optical signal by 90 degrees. The half wave plate rotates the feedback optical signal such that the polarization matches the optical amplifier  322 . After being reflected by the third mirror  318 , the feedback optical signal passes through the third collimating lens  320  and then into a rear facet of the optical amplifier  322 . The optical amplifier  322  amplified the feedback optical signal and couples it back into the FP laser diode  302  through its rear facet to lock the lasing wavelength of the FP laser diode  302 . The isolator  402  is used to block any reflected optical signals in the opposite direction, for example, light emitted from the rear facet of the optical amplifier  322 . In some alternative implementations, the isolator  402  can be positioned at any point in the optical path between the PBS  306  and the rear facet of the optical amplifier  322 . 
       FIG. 5  shows an example of a compact WDM laser transmitter optical subassembly (TOSA)  500  with a steering lens  502 . The TOSA  500  provides an integrated, monolithically or as a hybrid, and mutually coupled FP+SOA device used as a light source with the FP laser being directly modulated for data transmission while the SOA is used to amplify and cancel modulation of a self-seeding feedback signal. 
     In particular, the TOSA  500  includes the same structure as the TOSA  400  of  FIG. 4  with the addition of the steering lens  502 . The steering lens  502  is positioned between the first mirror  312  and the isolator  402 . While the steering lens  502  is shown in this position, in alternative implementations the steering lens can be positioned at any suitable location in the optical path between the PBS  306  and the third collimating lens  320 . The steering lens  502  is positioned in the feedback path to improve backward light coupling to the optical amplifier  322  by correcting possible beam shifting and tilting induced by assembly misalignment of optical components. The steering lens  502  can also increase the tolerance of alignment in coupling the light into the optical amplifier  322 . 
       FIG. 6  shows an example of a compact WDM laser transmitter optical subassembly (TOSA)  600  with monitoring photodiodes. 
     The TOSA  600  includes the FP laser diode  302 . An optical output end of the FP laser diode  302  is coupled to the first collimating lens  304 . The first collimating lens  304  is coupled to a beam splitter  602 . The beam splitter  602  can have a partial reflection coating that reflects a portion of an incident light along a first output path and passing a portion of the incident light along a second output path. The beam splitter  602  is coupled on the first output path to a first photodiode  606  having an integrated mirror lens  604 . The mirror lens  604  reflects and focuses light on the first photodiode  606 . The photodiode  606  can include a bonding pad that is wire bonded to a lead pin along the package case  326  to provide for power monitoring electric signal access. 
     The beam splitter  602  is coupled on the second output path to the PBS  306  such that the beam splitter  602  is positioned between the PBS  306  and the first collimating lens  304 . The PBS  306  has a first output that is coupled to the second collimating lens  308 , which passes collimated light to the fiber  310 , e.g., through a receptacle assembly. This fiber  310  can be coupled to a WDM filter and FRM assembly, not shown, for example as described above with respect to  FIGS. 1 and 2 . 
     A second optical output of the PBS  306  is coupled to a first mirror  608 . The first mirror  608  is a partial transmission mirror instead of a full reflection mirror. The first mirror  608  is optically coupled, along a transmission path, to a second photodiode  612  integrated with a mirror lens  610 . The mirror lens  610  reflects and focuses light on the second photodiode  612 . The second photodiode  612  is used for power monitoring of the feedback signal. 
     The first mirror  608  is also coupled, along a reflection path, to the steering lens  502 . The steering lens  502  is coupled to the isolator  402  such that the steering lens  502  is positioned between the first mirror  608  and the isolator  402 . The isolator  402  is coupled to the half wave plate  314 . The half wave plate is coupled to the second mirror  316 , which is coupled to the third mirror  318 . The third mirror is coupled to the third collimating lens  320 . In some other implementations, the isolator and the steering lens can be positioned in other locations on the feedback loop as described above. 
     The third collimating lens  320  is coupled to the optical amplifier  322 . The optical amplifier  322  can be an SOA. The optical amplifier  322  is configured to receive a focused optical signal from the third collimating lens  320  and output an amplified optical signal to the FP laser diode  302 . The FP laser diode  302  and the optical amplifier  322  can be positioned together on a submount  324 . 
     The TOSA  600  operates in a similar manner as described above. However, the beam splitter  602  causes a portion of the output optical signal from the FP laser diode  302  to be passed to the first photodiode  606  for power monitoring. Similarly, the first mirror  608  causes a portion of the feedback optical signal to be passed to the second photodiode  612  for power monitoring. 
       FIG. 7  shows an example of a compact WDM laser transmitter optical subassembly (TOSA)  700  with optical system placed and aligned on a carrier. In particular, the TOSA  700  is shown as including the components of TOSA  600  described above with respect to  FIG. 6  and operates in a similar manner. However, a number of the optical components within the package case  326  can be placed and aligned on a substrate  702 . The substrate  702  can be formed from a suitable material including silicon or ceramic. The substrate  702  can be configured to have precise dimension control and alignment indicators such as one or more alignment marks or fillisters. These can be used for passive alignment of optical components arranged on the substrate  702 . 
       FIG. 8  shows an example of a compact bidirectional WDM optical module  800 . The WDM optical module  800  can transmit and receive WDM optical signals through a single optical fiber. 
     The WDM optical module  800  includes a transmitter portion  801  and a receiver portion  802 . The transmitter portion  801  includes components similar to those of TOSA  600 . In particular, the transmitter portion of the WDM optical module  800  includes the FP laser diode  302 . An optical output end of the FP laser diode  302  is coupled to the first collimating lens  304 . The first collimating lens  304  is coupled to the beam splitter  602 . The beam splitter  602  can have a partial reflection coating that reflects a portion of an incident light along a first output path and passing a portion of the incident light along a second output path. The beam splitter  602  is coupled on the first output path to the first photodiode  606  having an integrated mirror lens  604 . The mirror lens  604  reflects and focuses light on the first photodiode  606 . The first photodiode  606  can include a bonding pad that is wire bonded to a lead pin along the package case  326  to provide for power monitoring electric signal access. 
     The beam splitter  602  is coupled on the second output path to the PBS  306  such that the beam splitter  602  is positioned between the PBS  306  and the first collimating lens  304 . The PBS  306  has a first output that is coupled to a C/L band filter  804 . The C/L band filter  804  is coupled to the second collimating lens  308 , which passes collimated light to the fiber  310 , e.g., through a receptacle assembly. This fiber  310  can be coupled to a WDM filter and FRM assembly, not shown, for example as described above with respect to  FIGS. 1 and 2 . 
     The C/L band filter  804  is also coupled to the receiver portion  802 . In particular, the C/L band filter  804  is coupled to a third photodiode  808  integrated with a mirror lens  806 . The third photodiode  808  and mirror lens  806  can be positioned on a receiver substrate delineating the receiver portion  802 . The mirror lens  806  is used to reflect and focus the collimating light reflected from the C/L band filter  804  onto the third photodiode  808 . The photodiode  808  is coupled to a transimpedance amplify (TIA) integrated circuit  810 . The TIA IC  810  can be used, for example, to provide wideband and low noise pre-amplification of signal current from the photodiode  808 . 
     Referring back to the transmitter portion  801 , the second optical output of the PBS  306  is coupled to the first mirror  608 . The first mirror  608  is a partial transmission mirror instead of a full reflection film. The first mirror  608  is optically coupled, along a transmission path, to the second photodiode  612  integrated with the mirror lens  610 . The mirror lens  610  reflects and focuses light on the second photodiode  612 . The second photodiode  612  is used for power monitoring of the feedback signal. 
     The first mirror  608  is also coupled, along a reflection path, to the steering lens  502 . The steering lens  502  is coupled to the isolator  402  such that the steering lens  502  is positioned between the first mirror  608  and the isolator  402 . The isolator  402  is coupled to the half wave plate  314 . The half wave plate is coupled to the second mirror  316 , which is coupled to a third mirror  318 . The third mirror is coupled to the third collimating lens  320 . 
     The third collimating lens  320  is coupled to an optical amplifier  322 . The optical amplifier  322  can be an SOA. The optical amplifier  322  is configured to receive a focused optical signal from the third collimating lens  320  and output an amplified optical signal to the FP laser diode  302 . The FP laser diode  302  and the optical amplifier  322  can be positioned together on submount  324 . 
     In operation, the transmitted and received wavelengths are arranged differently to separate the two signals, for example, C-band wavelengths can be used for transmitting and L-band wavelengths can be used for receiving, or vice versa. Thus, the transmitting and receiving signals allocated at different wavelength spans are spatially separated with C/L band filter  804 . 
     In some implementations, the wavelength of the received signal is in the L-band. As a result, it will follow the light path from the fiber  310 , but will be reflected by a C/L band filter  808  while the C-band wavelength signal transmitted out from the FP laser diode  302  and then returned by the fiber  310  as fed back from the self-seeding cavity will transmit through the C/L band filter  808  and into the PBS  306 . The angle of a C/L thin film filter of the C/L band filter  808  should be tilted sufficiently large to effectively separate the transmitting and receiving paths yet still small enough for the C/L band filter  808  to separate the C and L band signal effectively on the spectral domain. 
       FIG. 9  shows an example of a compact bidirectional WDM optical module  900  with a TEC cooler. The WDM optical module  900  is shown as including the components of WDM optical module  800  described above with respect to  FIG. 8  and operates in a similar manner. However, in the WDM optical module  900  the transmitter portion  801  is positioned on a TEC cooler  902  to control the temperature of the subassembly. This reduces the performance variation due to some of the optical devices&#39; characteristics being temperature sensitive, since there are many beam manipulating components on the subassembly including collimating, redirecting, steering, and focusing components. The controlled temperature will also reduce the movements of all the optical components due to temperature change and prevent performance degradation. 
     In some implementations, the transmitter subassembly and receiver subassembly described with respect to  FIGS. 8 and 9  are assembled into a package case  326  with lead pins implemented glass sealed feed throughs on both sides of the package case  326  near where either RF or CW signal connections are needed to come out of the package. With the compact transmitter and receiver subassembly designs, even though the relatively cost effective glass feed through type of lead pins occupy certain space and increase the size of the package, the whole package size can still be small enough for its application in the small form factor SFP type of transceiver module application. 
       FIG. 10  shows an exploded view of an example of SFP type transceiver  1000  built with proposed bidirectional WDM optical module. The transceiver  1000  includes an upper case  1002 , a bidirectional WDM optical module  1004 , a PCBA  1006 , a retainer  1008 , a TX FPC  1010 , a RX FPC  1012 , and a down case  1014 . 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system modules and components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.