Patent Publication Number: US-2003235227-A1

Title: Spot-size-converted laser for unisolated transmission

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates most generally to optical subassemblies. More particularly, the present invention relates to a TOSA (transmit optical subassembly) including a spot-size-converted laser passively aligned to an optical transmission medium for unisolated transmission.  
       BACKGROUND OF THE INVENTION  
       [0002] The trend in the optoelectronics industry is towards integrating more functionality into smaller packages. As the optoelectronics industry matures and expands from traditional telecommunications into newer areas like data communications, components are also evolving to meet more compact, integrated, and cost-sensitive requirements. One major step towards cost-effective optoelectronics is the development of lasers such as spot-size-converted (SSC) lasers which have high alignment tolerances and can be passively aligned to an optical transmission medium to reduce or eliminate alignment time and achieve a cost-savings. In a completed packaged transceiver or transponder, it is highly desirable to include the laser as a bare chip passively coupled to an optical transmission medium, rather than as a separately packaged pigtailed laser device. In addition to eliminating the time for active alignment, this eliminates additional packaging costs, and removes many of the high-speed limitations associated with the laser package. Consistent with the low cost, highly integrated approach, the lasers should desirably operate uncooled, requiring the laser to have good high temperature performance and excellent aging characteristics.  
       [0003] Such applications require a laser that can couple a large fraction of its emitted light into an optical transmission medium without optics. To achieve an optical coupling comparable to traditional packaged devices using passive alignment, it is essential that the device have a narrow far field pattern of the outgoing light. Typical buried heterostructure (BH) lasers with a 30×30 degree far fields can couple at most 10-15% of their light into a flat cleaved fiber. Spot size converted (SSC) lasers provide a reduced far field enabling passive alignment thereby reducing packaging costs. Understandably, the alignment tolerances, defined as the maximum excursions of the fiber that can still meet a minimum coupled power specification, are much greater for a narrow far field device.  
       [0004] To produce such an acceptably narrow far field, a relatively large spot size is required. A trade-off, however, is that large spot sizes are not consistent with good active laser performance.  
       [0005] Moreover, If optical isolators and associated lenses are required to couple the SSC laser to the optical transmission medium, the cost savings associated with passive alignment are lost due to the cost of these additional components and the need to align them. As such, unisolated transmission is desirable to reduce costs.  
       [0006] Another important aspect in the high speed optical communications industry is the need to produce lasers that provide optical signals having high data rates (bit rates of 1 Gbps and greater) to increase transmission capacity. As such the lasers used in the above-described integrated subassemblies, should desirably have a performance equivalent to standard BH 2.5 Gbps directly modulated lasers. Unisolated transmission is difficult, however at these high bit rates, because high reflection creates power penalties at such high bit rates.  
       [0007] It would therefore be advantageous to provide a high slope efficiency laser that can provide sufficient optical power into an optical transmission medium at a reduced coupling efficiency and without a lens or isolator. More particularly, it would be desirable to provide a spot size converted laser that produces an optical data signal along an optical transmission medium that satisfies industry standard specifications for acceptably low bit error rates, high speed, and sufficient resistance to external optical reflection, for suitably long transmission distances. Similarly, it would be advantageous to provide an optical subassembly including such a laser coupled to an optical transmission medium by passive alignment.  
       SUMMARY OF THE INVENTION  
       [0008] Accordingly, the present invention is directed to a laser and an optical subassembly (OSA) including the laser directly coupled to an optical transmission medium and producing an optical signal that provides high data rates, low bit error rates, and complies with United States and international industry standard specifications for data transmission. For purposes of the present invention, directly coupled means that there are no intervening components interposed between the laser and the optical transmission medium.  
       [0009] In one embodiment, the present invention provides a TOSA (transmit optical subassembly) that includes a spot-size-converted semiconductor laser directly coupled to an optical transmission medium without a lens or isolator, and provides an optical data signal having a bit error rate no greater than 10 −10  and at a data speed of 1-10 Gbps.  
       [0010] According to another exemplary embodiment, the present invention provides a TOSA including a spot-size-converted semiconductor laser coupled to an optical transmission medium without a lens or isolator and which provides an optical data signal having a maximum 1 dB optical path power penalty with a maximum −19 dB back reflection.  
       [0011] According to another exemplary embodiment, the present invention provides a TOSA including a spot-size-converted semiconductor laser coupled to an optical transmission medium without a lens or isolator and which provides an optical data signal that satisfies at least one of ITU-T SDH STM-16 and STM-48 standard specifications and SONET OC-48 and OC-192 standard specifications for data transmission.  
       [0012] According to another exemplary embodiment, the present invention provides a TOSA with a spot-size-converted semiconductor laser affixed to a submount. The submount includes a groove which receives and thereby passively aligns an optical transmission medium to the laser.  
       [0013] According to another exemplary embodiment, the present invention provides a method for transmitting an optical signal. The method includes providing a spot-size-converted semiconductor laser coupled to a submount that includes a groove for passively aligning an optical transmission medium to the laser, and also providing an optical fiber having an end capable of being received within the groove. The method further includes passively aligning the optical fiber to the laser without a lens or isolator by positioning the end of the optical fiber in the groove such that the laser provides an optical data signal along the optical fiber having a bit error rate less than 10 −10  and a data rate of 1-10 Gbps, and then causing the laser to emit light.  
       [0014] According to another exemplary embodiment, the present invention provides a method for providing a spot-size-converted semiconductor laser coupled to a submount that includes a groove for passively aligning an optical transmission medium to the laser and providing an optical fiber having an end capable of being received within the groove. The optical fiber is then passively aligned to the laser by positioning the optical transmission medium in the groove. The laser is then caused to emit light producing a coupling efficiency of at least 25%.  
       [0015] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory of the claimed invention and are not presented by way of limitation. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0016] The present invention is best understood from the following detailed description when read in conjunction with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Like numerals denote like features throughout the specification and drawing. Included in the drawing are the following figures:  
     [0017]FIG. 1 is a cross-sectional view of an exemplary spot-size-converted laser of the present invention;  
     [0018]FIG. 2 is a plan view of the exemplary spot-size-converted laser of the present invention;  
     [0019]FIG. 3 is a graph showing the far field of the spot-size-converted laser of the present invention;  
     [0020]FIG. 4 is a perspective view of the spot-size-converted laser of the present invention formed on a submount and passively aligned to an optical fiber;  
     [0021]FIG. 5 is a graph showing reflection tolerance versus coupling efficiency;  
     [0022]FIG. 6 is a graph showing bit error rate versus received power for various reflection levels for 5 meter transmission and including a 10% coupling efficiency; and  
     [0023]FIG. 7 is a graph showing bit error rate versus received power for various reflection levels for a 16.1 km transmission and a 10% coupling efficiency. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0024] To provide a high quality optical data signal transmission, a laser advantageously includes a narrow far field. The advantageous utilization of a narrow far field exiting from a laser implies a large spot-size such as four micron full width half maximum, inside the device cavity. Such a large spot size is not consistent with good active laser performance. Accordingly, the present invention provides a spot-size-converted semiconductor laser that includes an active region with a relatively small spot size and a mode expander region which enlarges the spot size. This combination of an active region with a relatively small spot size and an expander section with an enlarged spot size gives both good confinement to the active region and a narrow far field such as required for a high coupling efficiency and high quality data transmission.  
     [0025] The present invention provides such a spot-size-converted laser that includes a narrow far field, is capable of high coupling efficiency of the light directly into an optical fiber without an isolator or lens and has the ability to transmit without error floors or excessive power penalties. The laser of the present invention can therefore be packaged in an integrated transmitter or transceiver whereby the laser is mounted on a submount inside the transmitter, transceiver or transponder along with other electronics, uncooled, and directly coupled and passively aligned to an optical fiber or the like. The spot-size-converted laser so coupled to an optical fiber and passively aligned without a lens or isolator, provides sufficient resistance to external optical reflection to enable transmission up to 15 kilometers and greater, at reduced optical coupling efficiencies and is suitable for telecommunications and data communications.  
     [0026] Transmitter optical subassemblies, transceiver optical subassemblies and transponders are hereinafter referred to collectively as TOSAs. Transceiver optical subassemblies include receive components in addition to transmit components and transponders include transceiver capabilities along with electrical mutiplexing and demultiplexing components in the same package. In one embodiment, the optical data signal provided by the laser in the TOSA of the present invention can withstand −14 dB reflection and remain within the ITU-T specified 1 dB optical path power penalty at a bit error rate of 10 −10 , thereby meeting and surpassing ITU-T SDH G.957 STM-16 S-16.1 standards for 15 km transmission which allows a maximum reflectance of −19 dB. The ITU-T specified optical path power penalty is a power penalty due to dispersion, reflection, and other sources. Bit error rate (BER) may also be referred to as the bit error ratio and the terms may therefore be used interchangeably, hereinafter. The TOSA includes a data signal having a data rate as high as 10 Gbps, and generally within the range of 1-2.5 Gbps.  
     [0027] The TOSA including the passively aligned, spot size converted laser of the present invention, provides an optical output in the form of a data signal that complies with ITU-T Synchronous Digital Hierarchy (SDH), Synchronous Transport Module (STM) specifications STM-16 and STM-48 for 2.5 Gbps and 10 Gbps transmissions, respectively, in various embodiments. In other exemplary embodiments, the optical output data signal of the SSC laser/TOSA of the present invention also complies with SONET (Synchronous Optical Network) standard specifications OC-48 and OC-192 for 2.5 Gbps and 10 Gbps transmission, respectively. Moreover, in other exemplary embodiments, the optical output/data signal of the present invention may comply with Fiber Channel (ANSI X3T11), Gigabit Ethernet (IEEE 802.3z 1000BASE-LX) and 10 Gigabit Ethernet standards for 1.3 μm lasers (IEEE 802.ae 10GBASE-L) and for 1.55 μm lasers (IEEE 802.ae 10GBASE-E).  
     [0028]FIG. 1 is a cross-sectional view showing an exemplary spot-size-converted (SSC) semiconductor laser of the present invention. SSC laser  10  is formed on substrate  1 . SSC laser  10  includes opposed ends or facets  41  and  45 . SSC laser  10  includes active region  3  and expander region  5 . In one exemplary embodiment, active region  3  includes a strained multi-quantum well (MQW) InGaAsP conventional buried-heterostructure (BH) laser operating at 1.3 μm. SSC laser  10  may be a Fabry-Perot laser or it may be a distributed feedback (DFB) laser including grating structure  27  formed within substrate  1 . Substrate  1  may be formed of Si, GaAs, InP or other suitable materials. In the Fabry-Perot embodiment, grating structure  27  is not present. Grating structure  27  may be formed using holographic or other methods and is a repeating sequence of a material formed at regular intervals within substrate  1  to tune the laser to a desired output wavelength. In one exemplary embodiment, grating structure  27  may be a periodic loss or gain structure. MQW  7  includes a sequence of alternating layers  9  and  11  and, in an exemplary embodiment, forms a mesa structure which includes a beveled edge  13 . Alternating layers  9  and  11  are chosen to have different refractive indices and in one embodiment may be a repeating sequence of InGaAsP with a 70:30 arsenic to phosphorus ratio, and InGaAsP with a 60:40 arsenic to phosphorus ratio. According to other embodiments, other compositions may be used. According to still other exemplary embodiments, other semiconductor heterostructure families such as InGaAlAs may be used. Layers  9  and  11  of MQW  7 , as well as waveguide  15  and cladding layer  33 , may each be formed using MOCVD (metalorganic chemical vapor deposition) or other suitable techniques.  
     [0029] Waveguide  15  extends under both active region  3  and expander region  5  of SSC laser  10 . In an exemplary embodiment, waveguide  15  may be a quaternary InGaAsP material surrounded by InP cladding material  33  in expander region  5 . In one particularly advantageous embodiment, waveguide  15  may be an InGaAsP layer with a characteristic luminescence of 1.17 microns and zero strain, but other compositions may be used in other exemplary embodiments. The mode transfer of light generated in MQW  7  of active region  3  to underlying waveguide  15  is accomplished through a lateral taper etch removal of the active MQW layers. This lateral taper etch provides beveled end  13  as will be shown more clearly in FIG. 2. Underlying waveguide  15  may be grown using selective area growth (SAG) in order to produce a relatively thick waveguide portion  17  at the mode transition region of active region  3  while maintaining a relatively thin waveguide portion  19  in expander region  5  to achieve a large spot size and narrow far field. It can be seen that thickness  23  of relatively thick waveguide portion  17  in the mode transition region, is greater than corresponding thickness  25  of relatively thin waveguide portion  19  in the expander region. Waveguide  15  also includes taper  21  between relatively thick waveguide portion  17  and relatively thin waveguide portion  19 . Cladding layer  33  may be formed of InP in an exemplary embodiment.  
     [0030] SSC laser  10  includes active region length  29 , which may vary from 200 to 400 microns in various exemplary embodiments, and may be about 300 microns in a particular embodiment. Expander region length  31  may vary from 150 to 300 microns in various exemplary embodiments. Such dimensions are intended to be exemplary only, and the described materials of formation are exemplary and not limiting of the various structures used for the present invention. Multiple lasers of the present invention may be formed simultaneously on a substrate, then cleaved into individual SSC lasers. Photolithographic and conventional wet and dry etching techniques may additionally or alternatively be used to size the individual SSC lasers. After the laser structure is formed, conductive contact layers are formed to provide electrical contact. In an exemplary embodiment, N-contact metal  39  may be formed on the bottom surface of substrate  1  and P-contact metal  37  may be formed over SSC laser  10 . Conventional metalization and dry and/or wet etching techniques may be used. Conventional electronic circuitry may be coupled to N-contact metal  39  and P-contact metal  37  using conventional means and conventional means may be used to provide electrical power to SSC laser  10  and cause it to lase and emit light.  
     [0031] The materials that form the various layers and the thicknesses may be chosen to desirably produce a laser that emits light having various wavelengths such as 1.3 μm and 1.55 μm. In one exemplary embodiment, light having a median wavelength of about 1.3 μm (1290-1330 nm) may be used, where chromatic dispersion in a standard single mode optical fiber is minimal, but other embodiments may include light of various other wavelengths such as about 1.55 μm (1530-1565 nm), where optical fiber loss is small and erbium doped fiber amplifiers are used. SSC laser  10  includes opposed facets  45  and  41  and light is emitted from facet  41  along direction  43  in the illustrated embodiment. In an exemplary embodiment, facet  45  may be coated with a reflective material to enhance reflectivity within MQW  7  and facet  41  may be coated with an antireflective material to enhance transmission and reduce reflection back into the lasing chamber. In combination, such coatings provide a high slope efficiency laser capable of launching sufficient power into an optical transmission medium at a reduced coupling efficiency.  
     [0032] The spot size within active region  3  is relatively small to insure good active laser performance, and the spot size within expander region  5  is relatively large to provide a narrow far field. In an exemplary embodiment, spot size within active area  3  may be 1 μm and expanded to a spot size of 4 μm in expander region  5 , but other absolute and relative spot sizes may be used in other exemplary embodiments depending on device application, materials and thicknesses of materials used to form SSC laser  10 , optical coupling considerations and data transmission requirements. Various thicknesses may be used and various numbers of alternating layers  9  and  11  may be used to form MQW  7 , as would be appreciated by one of ordinary skill in the art.  
     [0033]FIG. 2 is a plan, top view of SSC laser  10  shown in FIG. 1. FIG. 2 shows P-contact metal  37  formed over the top of SSC laser  10 . The mesa which forms MQW  7  includes a maximum width  49  which may range from 0.5 to 2 μm according to various exemplary embodiments and may advantageously be 1 μm in one exemplary embodiment. MQW  7  includes beveled end  13  which produces angled face  47 . Together, the taper of beveled end  13 , and the decreasing thickness of waveguide  15  towards emitting facet  41 , produces a low loss mode transfer and narrow far field. SSC laser  10  of the present invention is therefore suitable for use within a transmit optical subassembly or a transceiver or other optical subassembly of reduced size and increased functionality because an isolator or lens is not needed and SSC laser  10  does not require cooling.  
     [0034]FIG. 3 is a graph showing the far fields achieved by exemplary SSC (spot-size converted) laser  10  compared to standard buried heterostructure (BH) lasers. It can be seen that the far field is desirably reduced considerably for the spot-size converted laser of the present invention, compared to conventional lasers. Far fields of 10×10 to 15×15 are achievable according to the SSC laser of the present invention, compared to BH laser far fields of approximately 30×30. The advantageously reduced far field allows for optical coupling by passive alignment to an optical transmission medium such as an optical fiber, high optical coupling efficiency, and high quality optical data signal transmission that can withstand considerable reflection, even when coupled without a lens or isolator.  
     [0035] Spot-size-converted laser  10  of the present invention provides the advantage that it can be coupled to an optical transmission medium such as an optical fiber using passive alignment to provide sufficiently high optical coupling. Another advantage of SSC laser  10  of the present invention is that it can be used with a low optical coupling efficiency advantageously chosen to minimize reflection from the optical fiber to which it is coupled, such that the laser can provide an optical signal to the optical fiber or other optical transmission medium to enable transmission up to 15 kilometers within industry standard specifications, including low optical path power penalties, without the use of an isolator or lens to couple the laser to the optical transmission medium.  
     [0036]FIG. 4 is a perspective view showing SSC laser  10  formed on submount substrate  51  of TOSA  100 . SSC laser  10  may be affixed to submount substrate  51  using various suitable techniques. TOSA  100  will include additional components among which may be receiver components in various exemplary embodiments. SSC laser  10  operates over a −40° C. to 85° C. temperature range and does not require cooling. This results in increased miniaturization of TOSA  100  as a cooling medium is not required. In one embodiment, the device operates at 65° C. or within the range of 65° C. to 85° C. SSC laser  10  is capable of providing an optical signal having a data rate as high as 10 Gbps along an optical transmission medium such as an optical fiber and in one exemplary embodiment may provide a data rate of 2.5 Gbps. SSC laser  10  may be produced to provide light having various wavelengths. In one embodiment, SSC laser  10  may be a DFB laser tuned to provide light having a wavelength of about 1.3 or 1.55 microns. Submount substrate  51  may be formed of silicon or other suitable materials and includes surface  53  which includes metalization  55 . Metalization  55  may provide contact to the subjacent one of P-contact metal  37  and N-contact metal  39  shown in FIG. 1. In the exemplary embodiment illustrated in FIG. 4, SSC laser  10  is oriented such that P-contact metal  37  is oriented upward and N-contact metal  39  is underneath and in contact with metalization  55 , but the relative positioning may be reversed according to other exemplary embodiments. Submount substrate  51  includes groove  59  formed therein and including surfaces  61 . Groove  59  may be formed using various suitable conventional means. In an exemplary embodiment, groove  59  is V-shaped, but other shapes may be used in other exemplary embodiments. Groove  59  is formed with precision such that, once SSC laser  10  is mounted on submount substrate  51 , an optical transmission medium such as an optical fiber may be passively aligned to SSC laser  10 , which is coupled to submount substrate  51 , such that an acceptable optical coupling efficiency is achieved between SSC laser  10  and the optical transmission medium. Groove  59  may be formed and SSC laser  10  may be joined to submount substrate  51  such that an alignment tolerance of about 1 micron is achieved between the SSC laser  10  and the optical transmission medium.  
     [0037] In the illustrated embodiment, the optical transmission medium is optical fiber  63 , including fiber core  65 . Optical fiber  63  also includes outer surface  67  and end facet  69 , which may be a cleaved surface in an exemplary embodiment. In one exemplary embodiment, optical fiber  63  may be a standard single mode fiber. Cleaved end facet  69  faces the emitting end, facet  41  of SSC laser  10 . When SSC laser  10  is powered by electrical connection (not shown), light is emitted substantially at portion  57  of end facet  41  and coupled into fiber core  65 . The passive alignment between optical fiber  63  and the light emitted at portion  57  of end facet  41  of SSC laser  10  is achieved by positioning optical fiber  63  within groove  59 . Otherwise stated, groove  59  receives optical fiber  63  such that portions of outer surface  67  form a conterminous boundary with both surfaces  61  of groove  59 . End facet  69  of optical fiber  63  may be formed by cleaving or other means and may be substantially parallel to end facet  41  or end facet  69  may be a substantially planar surface that is angled with respect to end facet  41 .  
     [0038] According to another exemplary embodiment, the optical fiber or other optical transmission medium may be terminally encased within a ferrule or other member. Groove  59  formed in submount substrate  51  may be correspondingly sized to receive the ferrule or other member containing the optical transmission medium such that the optical transmission medium will be passively aligned to SSC laser  10  to include a suitable coupling efficiency when the ferrule or other member is received within groove  59 . According to either of the exemplary embodiments, various conventional means may be used to secure the optical transmission medium within groove  61 . According to either of the aforementioned exemplary embodiments, a time and cost savings is achieved because an active alignment procedure is not required.  
     [0039] It is an aspect of the present invention that the mounted SSC laser  10  and passively aligned optical fiber  63  achieve a suitably high coupling efficiency without a lens to focus the laser, or an isolator. In various exemplary embodiments, coupling efficiencies greater than 25% can be achieved. A device far field of approximately 15×10, achievable by SSC laser  10 , provides a coupling efficiency of greater than 25%. In another exemplary embodiment, a coupling efficiency within the range of 30-50% is achieved.  
     [0040] It is another aspect of the present invention that SSC laser  10 , notably without a lens or isolator, provides sufficient resistance to optical reflection to enable high speed data transmission up to 15 kilometers at a reduced coupling efficiency, with optical path power penalties that satisfy and exceed the various aforementioned industry-standard specifications. Such high quality data transmission is achievable with the reduced coupling efficiencies advantageously used to minimize reflection from the optical transmission medium. Reflection is defined as the power ratio directed back into the TOSA along the optical transmission medium such as an optical fiber. The optical transmission medium is also coupled to a receiver (not shown). Either or both of the receiver and the optical transmission medium itself, may contribute to the reflection directed along the optical transmission medium and back into the TOSA.  
     [0041]FIG. 5 is a graph showing reflection tolerance in decibels (dB) versus coupling efficiency in percentage. FIG. 5 is an exemplary embodiment that covers 5 meter transmission. According to the exemplary embodiment in which the criteria is to maintain less than a 1 dB optical path power penalty and less than a 10 −12  error floor, FIG. 5 shows that, as the reflection tolerance increases (approaches 0, which represents 100% reflection), coupling efficiency decreases. This demonstrates that a lower coupling efficiency is advantageously used to provide an increased reflection tolerance. Stated alternatively, a lower coupling efficiency between the laser and optical transmission medium reduces sensitivity to reflection. It is an advantage of the present invention that, with a coupling efficiency of about 10% to provide an acceptably high reflection tolerance, the TOSA of the present invention delivers an acceptably high quality data signal. While an even lower coupling efficiency may further reduce sensitivity to reflection, a minimum coupling efficiency is required for acceptable data transmission.  
     [0042] SSC laser  10  of the present invention is capable of producing an output power between −5 dBm and 0 dBm. For typical 15 km transmission lengths, desired power is approximately −3 dBm which typically utilizes a coupling efficiency of approximately 10%. Such is exemplary only, and other power levels using different coupling efficiencies may be used in other exemplary embodiments.  
     [0043] The SDH/SONET standards (STM-16/OC-48) for 15 kilometer transmission, require an optical path (reflection/dispersion) power penalty of less than 1 dB for reflection up to −19 dB. This back reflection from the optical fiber may be due to the optical fiber itself (up to −24 dB) and/or the receiver coupled to the optical fiber (up to −27 dB). The optical path power penalty generally is the additional power required to overcome system reflectance and dispersion, etc., and maintain a given bit error ratio achievable by the same system without such influences. In the present example, the optical path power penalty is the penalty produced due to system reflection, and not considering any dispersion effects. The TOSA of the present invention exceeds the requirements of the SDH/SONET standards because it maintains a maximum optical path power penalty of 1 dB at a bit error rate of 10 −10  with system reflection up to 14 dB, higher than the allowable −19 dB maximum system reflection of ITU-T SDH standard G.957 STM-16 S-16.1, for example (2.48832 Mb/s, short reach application using 1.3 μm directly modulated lasers at up to 15 km). In one embodiment, the 1 dB optical path power penalty is maintained with system reflection of −8.5 dB. In another embodiment, the passively aligned TOSA and distributed feedback SSC laser of the present invention provide a data signal along an optical transmission with the above characteristics using a coupling efficiency of about 10% for transmission up to 15 km and at a data rate in the range of 1-2.5 Gbps and a wavelength of 1.3 microns, without a lens or isolator. In summary, the TOSA arrangement of the present invention is robust with respect to high system reflections and meets and exceeds 2.5 Gbps SDH/SONET standards (STM-16/OC-48) for 15 km transmission and at a bit error ratio of 10 −10 , and SDH/SONET standard specifications (STM-48/OC-192) at 10 Gbps. Furthermore, in various embodiments, the TOSA arrangement of the present invention meets and exceeds the aforementioned Gigabit Ethernet, 10 Gigabit Ethernet and Fiber Channel specifications.  
     [0044]FIG. 6 is a graph showing bit error rate versus received power in dBm for an exemplary TOSA of the present invention including the passively aligned SSC laser coupled to an optical transmission medium without a lens or isolator. FIG. 6 covers an exemplary embodiment of 5 meter transmission with a 10% coupling efficiency. It can be seen that for various exemplary reflection levels, a bit error rate of 10 −10  is achievable with the specified optical path power penalty of 1 dB or less. FIG. 6 illustrates that to maintain a bit error rate of 10 −10  with a reflection of −8 dB, a 0.8 dB optical path power penalty results, i.e. the difference between the received power of approximately −24.6 dBm to maintain a 10 −10  bit error rate with −8 dB reflection and the received power of approximately −25.4 dBm for the bit error rate of 10 −10  without reflection. Various output powers in the −5 dBm to 0 dBm range may be provided by the SSC laser in various exemplary embodiments, to provide the indicated received powers.  
     [0045]FIG. 7 is a graph showing bit error rate versus received power in dBm for an exemplary TOSA of the present invention including the passively aligned SSC laser coupled to an optical transmission medium without a lens or isolator. The graph in FIG. 7 covers the exemplary embodiment for 16.1 km transmission and using a 10% coupling efficiency. FIG. 7 shows that, for a −16.5 dB reflection, a bit error rate of 10 −10  is achievable within the maximum 1 dB optical path power penalty.  
     [0046] The preceding graphs of FIGS. 6 and 7 are intended to be exemplary and explanatory and are used to illustrate the fundamental concept of the present invention that, according to various exemplary embodiments, the passively-aligned TOSA of the present invention, including the spot-size-converted semiconductor laser of the present invention coupled to an optical fiber or the like without a lens or isolator, can provide a data signal with a bit error ratio of 10 −10  or less and enable 1-10 Gbps transmission within the optical path power penalty specified by various SDH/SONET standards, for various system reflection values.  
     [0047] The preceding merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its scope and spirit. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes and to aid in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and the functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. The scope of the present invention, therefore, is not intended to be limited to the exemplary embodiments shown and described herein. Rather, the scope and spirit of the present invention is embodied by the appended claims.