CROSS REFERENCE TO RELATED APPLICATION 
   The present application is related to and claims priority to U.S. Provisional Patent Application, entitled “Bidirectional Photonic Integrated Circuit-Based Subsystem,” which was filed on Apr. 7, 2005, and assigned Ser. No. 60/669,798. The U.S. Provisional Patent Application 60/669,798 is hereby incorporated by reference in its entirety. 


   BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The present invention relates to photonics. In particular, the present invention relates to a bidirectional photonic integrated circuit (PIC) based subsystem. 
   SUMMARY 
   According to one embodiment of the present invention, an intelligent subscriber subsystem may include a bidirectional photonic integrated circuit. The bidirectional photonic integrated circuit may include a distributed reflector (DR) laser or a distributed feedback (DFB) laser that emits light of a first wavelength, and a longitudinal waveguide portion that transmits light of a second wavelength, while attenuating backwardly propagated light of the first wavelength. The bidirectional photonic integrated circuit may be coupled to a single mode optical fiber to provide a two-way optical communication between a headend and a subscriber. 
   According to one embodiment of the present invention, the bidirectional photonic integrated circuit where wavelength λ 1  nm (for example 1310 nm) carry interactive commands from a subscriber to a headend and signals of wavelength λ 2  nm (for example 1550 nm) carry content from a headend to a subscriber. In one embodiment, wavelength λ 1  nm is shorter than wavelength λ 2  nm. The photonic integrated circuit integrates various optical functions on a common wafer for simpler construction, smaller size and lower cost. 
   The present invention is better understood upon consideration of the detailed description below and the accompanying drawings. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows, schematically, a vertical cross-section through a structure for use in a photonic integrated circuit (PIC)  380 , in accordance with one embodiment of the present invention. 
       FIG. 2  is a perspective view of the structure shown in  FIG. 1 . 
       FIG. 3  shows, schematically, a vertical cross-section of the structure of  FIG. 1  including gratings  240  and  260  provided in laser MQW  200  (which may be used to form a DR laser  300  with a phase control region  320 , respectively, such as shown as shown in  FIG. 4 ). 
       FIG. 4  shows, in a schematical perspective view of PIC  380 , a directly modulated DR laser  300  with a phase control region  320 , a reversed biased monitor photodiode  340  and a transmission region  360 , in accordance with one embodiment of the present invention. 
       FIG. 5  shows a schematic perspective view of an apparatus in one application of PIC  380 , in accordance with one embodiment of the present invention. 
       FIG. 6  shows the apparatus of  FIG. 5  encapsulated in a PIC optical module  500 . 
       FIG. 7  is a block diagram of a subscriber communication subsystem in which PIC optical module  500  may be used. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows, schematically, a vertical cross-section through a structure for use in a photonic integrated circuit (PIC), in accordance with one embodiment of the present invention. As shown in  FIG. 1 , the structure includes 400±10 μm of laser multiple quantum well (MQW) material  200 , and 1200±10 μm of bulk or MQW waveguide material  220  (hereinafter, “bulk waveguide material  220 ” to simplify reference). In this embodiment, the bandgap wavelength of laser MQW material  200  is selected to be λ 1  nm and the bandgap wavelength of bulk waveguide material  220  is selected to be about (λ 1  nm+λ 2  nm)/2. 
   Laser MQW material  200  and bulk waveguide material  220  may be formed as repeated structures separated from each other by angular etched windows  180  (as shown in  FIG. 1 ) on a wafer. Laser MQW material  200  and bulk waveguide material  220  may be provided on a buffer layer  160 , followed absorption layer  140  on a by an substrate  120 . 
   A portion of the unprotected area of the substrate is etched away, and laser MQW  200  and waveguide  220  are selectively grown sequentially using, for example, metalorganic chemical vapor depositions (MOCVD), or any other suitable method. Windows provide oxidation-free growth of laser MQW  200  and non-absorbing interfaces for reliable laser operation. Angled windows provide localized reflection immunity. Laser MQW material  200  typically includes InAlGaAs or InGaAsP material. Bulk waveguide material typically includes InGaAsP. Electrical contacts may be made onto these structures by providing a contact layer, typically formed using a p+ InGaAs contact material. Substrate  120 , absorption layer  140 , buffer layer  160  and cladding layers are typically InP material. The structure is fabricated at an approximate 7-degree angle with respect to the major cleavage crystal planes of the semiconductor material to reduce localized reflection. Absorption layer  140  absorbs substrate-induced stray light of wavelength λ 1  nm. 
     FIG. 2  is a perspective view of the structure shown in  FIG. 1 . In one embodiment, laser MQW material  200  is fabricated into a distributed reflector (DR) laser and bulk waveguide material  200  forms a waveguide that preferentially transmits light of wavelength λ 2  nm. Therefore,  FIG. 3  shows, schematically, a vertical cross-section of the structure of  FIG. 1  including gratings  240  and  260  provided in laser MQW material  200  (which may be used to form DR laser  300  and a phase control region  320 , respectively, such as shown in  FIG. 4 ). 
   A DR laser may be implemented using shallow etched gratings  240  and deeply etched gratings  260 . Gratings  260  function both as a phase control region and a reflector region for wavelength λ 1  nm as to obtain a higher front output power, a higher slope efficiency and a narrower linewidth. Gratings  240  and  260  are either uniform or phase-shifted. Gratings  240  and  260  may have different pitches, different waveguide widths, tapered profiles, or curvatures, or mixture of tapered profiles and curvatures so as to have refractive index differences. Gratings may have near-perfect pitches to allow precise control of the emission wavelength (λ 1  nm) by utilizing an additional thermo electrical cooler (TEC). Care should be taken to prepare the grating surfaces, prior to carrying out any material regrowth by MOCVD or any other suitable method. 
   The difference Δ nm between the bandgap wavelengths of laser MQW material  200  and bulk waveguide material  220  may be selected based on the relative change of the emission wavelength (λ 1  nm) per ° C., the gain curve shift of laser MQW material  200  per ° C. and the gain curve shift of bulk waveguide material  200  per ° C. with respect to entire operating temperature range of the PIC  380 . 
   Typically, a DR laser or a distributed feedback (DFB) laser includes either a buried hetrostructure (BH) or a ridge waveguide (RW) as a longitudinal device structure. In a BH longitudinal structure, a suitable active width may be about 1.4±0.10 μm, with a height or depth of about 2.75±0.05 μm. In a ridge waveguide longitudinal structure, a suitable ridge width may be about 2±0.10 μm, with a height or depth of 1.75±0.05 μm. 
   As mentioned above, the structure of  FIG. 3  can be used to build a PIC  380 .  FIG. 4  shows, in a schematical perspective view of PIC  380 , directly modulated DR laser  300  with a phase control region  320 , reversed biased monitor photodiode  340  and transmission region  360 . A transmission region  360  also simultaneously absorbs light of wavelength λ 1  nm (under reversed bias) and transmits light of wavelength λ 2  nm at a lower loss. DR laser  300  and its phase control region  320  corresponds to gratings regions  240  and  260  in laser MQW material  200 . In this embodiment, a reversed bias monitor photodiode  340  is formed in an electrically isolated 250-μm region of bulk waveguide material  220 . A reversed bias monitor photodiode  340 , which measures a photo current in the microampere range, is provided to allow a conventional laser driver integrated circuit with controlled feedback circuitry to be used with PIC  380 . A reversed biased monitor photodiode  340  is sufficiently electrically isolated to monitor back output power, which is proportional to front output of DR laser  300 . 
   Each of the cleaved end facets of PIC  380  is provided with a single layer or multiple layers of anti-reflective (AR) coating of about 0.1% in a suitable wavelength range. Thus, as shown in  FIG. 4 , PIC  380  is about 1600±10 μm in length, 400±10 μm in width, and 120±10 μm in thickness to facilitate cleaving, handling and mounting. 
   As shown in  FIG. 4 , PIC  380  provides four control electrodes: an electrode for direct modulation to a laser  300  (about 350 microns in length), an electrode for phase control region  320  (about 50 microns in length—required only for a DR laser), an electrode for monitor photodiode  340  (about 250 microns in length), and an electrode for section  360  (950 micron in length). 
   Additional electrodes may be provided to achieve further electrical isolation between monitor photodiode  340  and laser  300 , and for switching, varying or tuning wavelength around λ 1  nm. 
   Direct modulation bandwidth and chirp of laser  300  may be improved by incorporating quantum dots in laser MQW material  200 . The material gain profile and operating temperature range can be increased by incorporating quantum dots in both laser MQW material  200  and bulk waveguide material  220 . 
   In addition, PIC  380  may also include a thin-film resistor (on top) which is isolated by a thermally conducting, but electrically non-conducting insulator material from the top electrodes. The thin-film resistor allows fast heating (e.g., about 50 milliseconds) upon an initial cold start. 
   In one embodiment of the present invention, a gain-coupled directly modulated DFB laser may be formed by carefully etching through laser MQW material  200 . Extreme care should be taken to prepare the laser MQW material  200  grating surfaces, prior to carrying out any material regrowth by MOCVD or any other suitable method. 
   An exemplary structure for forming a λ 1 =1310 nm emission wavelength index-coupled directly modulated DR laser (or index- or gain-coupled directly modulated DFB laser) on n+ InP substrate  120  is given below in Table 1. Table 1 shows, for each layer from the top surface, the layer&#39;s purpose, composition, thickness, dopant type and concentration, and layer strain. As set forth in Table 1, the structure includes 6 quantum wells and 7 quantum barriers. The figures provided in Table 1 are provided merely for illustrative purposes. In any implementation, the actual number of quantum wells and barriers, or any other layers, thicknesses, dopant concentrations, and the amounts and polarities of strains are tuned to achieve the specification of a desired laser. In general, any structure that includes multiple quantum wells and quantum barriers in the active layers may be used to practice this invention. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Layer 
               Composition 
               Thickness (nm) 
               Doping (10 18 /cm 3 ) 
               Strain (%) 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               Contact 
               In 0.53 Ga 0.47 As (1657 nm) 
               200 
               Zn: &gt;30 
               0.0 
             
             
               Contact 
               In 0.60 Ga 0.40 As 0.87 P 0.13  (1520 nm) 
               25 
               Zn: 5.0 
               0.0 
             
             
               Contact 
               In 0.72 Ga 0.28 As 0.61 P 0.39  (1300 nm) 
               25 
               Zn: 3.0 
               0.0 
             
             
               Cladding 
               InP (918.6 nm) 
               1500 
               Zn: 0.7-2.0 
               0.0 
             
             
               Etching-Stop 
               In 0.89 Ga 0.11 As 0.24 P 0.76  (1050 nm) 
               10.5 
               Zn: 0.5 
               0.0 
             
             
               Spacer 
               InP (918.6 nm) 
               140 
               Zn: 0.5 
               0.0 
             
             
               *Protection 
               In 0.53 Ga 0.47 As (1657 nm) 
               20 
               Zn: 0.3 
               0.0 
             
             
               *Protection 
               InP (918.6 nm) 
               10 
               Zn: 0.3 
               0.0 
             
             
               ***Waveguide 
               In 0.75 Ga 0.25 As 0.54 P 0.46  (1250 nm) 
               40 
               Zn: 0.3 
               0.0 
             
             
               Spacer 
               InP (918.6 nm) 
               40 
               Zn: 0.3 
               0.0 
             
             
               Blocking 
               In 0.52 Al 0.48 As (834 nm) 
               80 
               Zn: 0.3 
               0.0 
             
             
               GRIN-SCH 
               In 1−x−y Al x Ga y As 
               50 
               Undoped 
               0.0 
             
             
                 
               x/y = 0.35/0.13 − 0.38/0.09 
             
             
                 
               (1000 nm − 950 nm) 
             
             
               Barrier 
               In 0.52 Al 0.35 Ga 0.13 As (1000 nm) 
               8.5 
               Undoped 
               0.0 
             
             
               *Well-6 
               In 0.74 Al 0.18 Ga 0.08 As (1390 nm) 
               5 
               Undoped 
               Compressive 
             
             
                 
                 
                 
                 
               strain (CS) 
             
             
                 
                 
                 
                 
               1.5 
             
             
               Barrier 
               In 0.52 Al 0.35 Ga 0.13 As (1000 nm) 
               8.5 
               Undoped 
               0.0 
             
             
               *Well-5 
               In 0.74 Al 0.18 Ga 0.08 As (1390 nm) 
               5 
               Undoped 
               CS1.5 
             
             
               Barrier 
               In 0.52 Al 0.35 Ga 0.13 As (1000 nm) 
               8.5 
               Undoped 
               0.0 
             
             
               *Well-4 
               In 0.74 Al 0.18 Ga 0.08 As (1390 nm) 
               5 
               Undoped 
               CS1.5 
             
             
               Barrier 
               In 0.52 Al 0.35 Ga 0.13 As (1000 nm) 
               8.5 
               Undoped 
               0.0 
             
             
               *Well-3 
               In 0.74 Al 0.18 Ga 0.08 As (1390 nm) 
               5 
               Undoped 
               CS1.5 
             
             
               Barrier 
               In 0.52 Al 0.35 Ga 0.13 As (1000 nm) 
               8.5 
               Undoped 
               0.0 
             
             
               *Well-2 
               In 0.74 Al 0.18 Ga 0.08 As (1390 nm) 
               5 
               Undoped 
               CS1.5 
             
             
               Barrier 
               In 0.52 Al 0.35 Ga 0.13 As (1000 nm) 
               8.5 
               Undoped 
               0.0 
             
             
               *Well-1 
               In 0.74 Al 0.18 Ga 0.08 As (1390 nm) 
               5 
               Undoped 
               CS1.5 
             
             
               Barrier 
               In 0.52 Al 0.35 Ga 0.13 As (1000 nm) 
               8.5 
               Undoped 
               0.0 
             
             
               GRIN-SCH 
               In 1−x−y Al x Ga y As x/y = 0.38/0.09 − 0.35/ 
               100 
               Undoped 
               0.0 
             
             
                 
               0.13 (950 nm − 1000 nm) 
             
             
               Blocking 
               In 0.52 Al 0.48 As (834 nm) 
               50 
               Si: 2.0-0.8 
               0.0 
             
             
               Buffer 
               InP 
               800 
               Si: 1.0 
               0.0 
             
             
               Substrate 
               InP (100) 
               — 
               Si: 3.0 
               0.0 
             
             
                 
             
             
               *Photoluminescence (PL) peak position at room temperature: (1300 ± 5) nm, across the wafer ±2 nm 
             
           
        
       
     
   
     FIG. 5  shows a schematic perspective view of an apparatus in one application of PIC  380 , in accordance with one embodiment of the present invention. As shown in  FIG. 5 , the apparatus includes a PIC  380 , a single-mode optical fiber  400  (hereinafter, “optical fiber  400 ” to simplify reference), lens  440  and back-illuminated lensed avalanche photo-diode (APD) with a thin film cut off or band bass filter  460 , and trans-impedance amplifier (TIA)  480 . As shown in  FIG. 5 , lens  440  and optical fiber  400  are affixed in a V-groove  420 . Optical fiber  400  may be a nominal single-mode optical fiber or a large numerical aperture thermally expanded core single-mode optical fiber  400 . The apparatus of  FIG. 5  may be encapsulated in a PIC optical module  500  in a ceramic-metal package, for example, as shown in  FIG. 6 . For example, the components of  FIG. 5  may be packaged into 12-pin hermetic ceramic-metal, surface-mount, butterfly or TO can-style PIC optical module  500 . 
   In one application, optical fiber  400  runs from a head end (i.e., equipment on the service provider side) and connects to PIC optical module  500 , located at a subscriber. In that application, modulated light signals (from subscriber) of wavelength λ 1  nm are transmitted through anti-reflection coated ball lens  440  (or any other suitable lens) into optical fiber  400 . Lens  440  is affixed into V-groove  420 , which is etched out of a silicon or ceramic material. A suitable material for forming V-groove  420  supports mechanically, electrically and thermally the components residing on V-groove and can be precisely etched or shaped. At the same time, modulated signals (from the head end) of wavelength λ 2  nm are transmitted in a single-mode optical fiber  400 , and are received into PIC module  500  through a lens  440  and PIC  380 , and detected by a back-illuminated APD or a PIN photodiode  460 . A back-illuminated APD or PIN photodiode  460  may be integrated with an etched or shaped focusing microlens and a thin-film cut-off or bandpass filter. Such an APD or a PIN photodiode  460  may be closely mounted and connected to a TIA integrated circuit  480 . As shown in  FIG. 5 , APD or PIN photodiode  460  and a TIA integrated circuit  480  may be provided in a common assembly substrate or platform with an edge wrap metallization, which may be provided on another substrate or as a part of PIC optical module  500 . 
   In the application of  FIG. 5 , unwanted backward propagated light of wavelength λ 1  nm from PIC  380  may cause erroneous signals at an APD or a PIN photodiode  460 . Therefore, under reversed-biased condition, waveguide section  360  absorbs backward propagated light of wavelength λ 1  nm and prevents transmission of the wavelength λ 1  nm light from reaching back-end facet  280 , which faces APD or PIN photodiode  460 . The amount of absorbed λ 1  nm light may be increased by increasing the length of waveguide section  360 . 
   An APD or PIN photodiode  460  and a TIA integrated circuit  480  may be provided in a common assembly substrate or platform with an edge wrap metallization. 
   When assembling the components of  FIG. 5 , side steps may be etched onto a PIC  380 , such that they match accurately with the side steps of V-groove  420 . PIC  380  may be mounted in either a “substrate-down” configuration, or a “substrate-up” configuration, as long as the signal path vertical distance is “actively in-situ positioned” for a maximum TIA sensitivity. 
   For a “substrate up” configuration, all vertical epitaxy layer thicknesses may be precisely controlled using a MOCVD method, or any other suitable growth method. Consequently, very high precision control of the emission distance or emission point can be achieved, thus facilitating positioning of the other components. 
   To minimize reflection back into laser  300  of PIC  380 , single-mode fiber  400 &#39;s tip is fabricated at an approximately 12 degree angle, and with an anti-reflection coating of 0.1% in the suitable wavelength range. Optical fiber  400  may be fixed in place by either a rigid pigtailed or a flexible detachable receptacle. 
   PIC  380  can be coupled to an input arm of a 1×2 optical switch. The output arms of a 1×2 optical switch may be coupled to two separate single-mode optical fibers. In such a configuration, signals may be routed through a standby single-mode optical fiber when a cut occurs in the other optical fiber. 
     FIG. 7  is a block diagram of a communication subsystem in which PIC optical module  500  may be used. As shown in  FIG. 7 , PIC optical module  500  is shown driven by laser driver  520  to provide data output modulated at wavelength λ 1  nm. Limiting amplifier  540  receives the data input that PIC optical module  500  receives from signals modulated at λ 2  nm on optical fiber. Clock and data recovery circuit  560  then recovers data and clock signals from the amplified output signal of limiting amplifier  540 . The clock and data signals are then fed into FPGA with a processor  580 , which may be implemented, for example by a system-on-a chip implementation. 
   FPGA with a processor  580  interacts with various hardware or software modules to perform numerous communication functions. For example, security functions may be carried out by authentication module  600 , in-situ real-time diagnostic module  620 , Internet firewall device  640 , and Internet spyware firewall device  660 . Communication functions may be carried out by plain old telephony service (POTS)  680 , voice over IP service  700 , data over IP  720 , communication over wireless (including a millimeter wave wireless) service  740 , communication over coaxial cable  760 , and communication over Cat 5/6 service  780 . 
   In addition, multimedia services, such as video over IP to regular TV converter  800 , set top box  820 , video recorder  840  and T-1  860  or other smart home connections can also be provided. Most of the above circuit functions can be integrated into one or two application specific integrated circuits. 
   The detailed description above is provided to illustrate the specific embodiments of the present invention and is not intended to be limiting. Many modifications and variations within the scope of the present invention are possible. The present invention is set forth in the following claims.
A reconfigurable intelligent subsystem may include a bidirectional photonic integrated circuit. The bidirectional photonic integrated circuit may include a distributed reflector (DR) laser or a distributed feedback laser (DFB) that emits light of a first wavelength, and a longitudinal waveguide portion that transmits light of a second wavelength, while attenuating light of the first wavelength. The bidirectional photonic integrated circuit may be coupled to a single mode optical fiber to provide two-way optical communication between a service provider ("headend") and a subscriber.