Abstract:
A semiconductor optical amplifier system comprises a hermetic package. In the typical implementation, this hermetic package is a standard 0.75 inch×0.5 inch package, such as a butterfly package. An optical bench is sealed within this package. A first fiber pigtail enters this package via a feed-through to connect to the bench and terminate above the bench. A second optical fiber pigtail enters the package via a second fiber feed-through to connect to the bench and similarly, terminate above the bench. A semiconductor amplifier chip is connected to the bench to provide amplification. Isolators are further incorporated along with a monitoring diode to yield a fully integrated system.

Description:
BACKGROUND OF THE INVENTION 
     Today, the most common modality for optical signal amplification is the rare-earth doped fiber amplifier. These devices have good amplification characteristics and a well-understood long-term behavior. Moreover, they can be inserted into a fiber link via fiber splicing, which is a low loss coupling technique. 
     An alternative amplification modality is the semiconductor optical amplifier (SOA). SOA systems have a number of advantages relative to the common erbium-doped amplifier scheme. SOA&#39;s are typically electrically, rather than optically, pumped. As a result, they are more efficient and avoid the need for ancillary, expensive laser pumps. Moreover, they are usually physically smaller than fiber amplifiers, which require a relatively long length of doped fiber. This quality is especially relevant when amplification is required in larger systems offering higher levels of functionality, such as optical add-drop multiplexers and other types of switching devices. The semiconductor optical amplifier can be as small as the semiconductor chip. 
     Nonetheless, the principal barrier to the commercial deployment of semiconductor optical amplifiers is the difficulty associated with coupling optical signals in and out of the semiconductor amplifier chip. The coupling issues are analogous to coupling light from a laser transmitter/laser pump into an optical fiber with the additional problems associated with back-reflection suppression, which can convert the amplifier into a laser, resulting in unintended operation. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a semiconductor optical amplifier system and specifically, the implementation of a semiconductor optical amplifier system on a single substrate or optical bench. As such, the present invention is applicable to the inclusion of a physically-compact amplification system into larger optical systems. 
     When constructing semiconductor optical amplifiers, it is typically desirable to have some type of feedback mechanism to control the amplification level of the semiconductor optical amplifier. Proposed techniques for diverting a portion of the optical signal from the amplifier chip, however, can be susceptible to polarization shifts in the optical signal. As a result, they can introduce some noise into the feedback scheme. 
     In general, according to one aspect, the present invention features a semiconductor optical amplifier system. It comprises a hermetic package. In the typical implementation, this hermetic package is a standard 0.75 inch×0.5 inch package, such as a butterfly package. An optical bench is sealed within this package. A first fiber pigtail enters this package via a feed-through to connect to and terminate above the bench. A second optical fiber pigtail enters the package via a second fiber feed-through to connect to and similarly terminate above the bench. A semiconductor amplifier chip is connected to or installed on the bench. 
     In a preferred embodiment, at least one isolator is included in the hermetic package and specifically on the optical bench for suppressing back-reflections into the fiber pigtail and/or the semiconductor optical amplifier chip. Specifically, in the preferred embodiment, a first isolator suppresses back-reflections into the input or first fiber pigtail and a second isolator suppresses back-reflections into the semiconductor optical amplifier chip. 
     In the preferred embodiment, additional optical components are provided to facilitate the transmission of optical signals through the system. Specifically, a first collimation lens is installed on the bench between the first isolator and the termination of the first fiber pigtail to improve the collimation of light emitted from the first fiber pigtail. A focusing lens is installed on the bench between the first isolator and the semiconductor optical amplifier chip to couple light from the first isolator into the semiconductor optical amplifier chip. Further, a second collimation lens is installed between the second isolator and the chip to couple light from the chip into the second isolator. Finally, a second focusing lens is installed on the bench for coupling light from the second isolator into the second pigtail. 
     Although, in the preferred embodiment, discrete optics are used to couple light into and out of the fiber pigtails, in alternative embodiments, fiber lenses may be formed on the fiber endfaces to reduce or eliminate the need for discrete coupling optics between the fiber endfaces and the other components of the system. 
     According to another embodiment, in a single physical port embodiment, the optical signal is received into the hermetic package by a fiber, focused onto the amplifier chip, reflected to pass back through the chip, and then refocused onto the fiber so that the amplified optical signal exits from the system. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
     FIG. 1A is a perspective view of a semiconductor optical amplifier system according to the present invention; 
     FIG. 1B is a close-up view of the inventive semiconductor optical amplifier system; 
     FIG. 2 is a block/schematic view of a second embodiment of the semiconductor optical amplifier system of the present invention; and 
     FIG. 3 is a perspective view showing a composite mounting structure used to hold lenses  134 ,  136  to provide for z-axis alignment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A shows a semiconductor optical amplifier system (SOA), which has been constructed according to the principles of the present invention. 
     Generally, the SOA  100  comprises a hermetic module or package  104 . In the illustrated example, the package  104  is a butterfly package with leads  114 . In the view of FIG. 1A, the top of the hermetic module  104  has been removed. 
     The hermetic package  104  has an input fiber feed-through to which an input fiber ferrule  112  is attached. An input optical fiber pigtail  118  enters into the hermetic package  104  via the input fiber feedthrough. 
     The hermetic package  104  also has an output fiber feedthrough in which an output fiber ferrule  110  is installed. The output fiber pigtail  120  passes through the hermetic package  104  exiting from the module. 
     Within the hermetic module  104 , an optical substrate or bench  116  is installed. In the typical implementation, the optical bench  116  is installed on a thermoelectric cooler  106 . A thermocouple  108  is typically attached to the top of the bench  116  to detect the temperature within the module  104  to enable the temperature stabilization. 
     FIG. 1B is a close-up view, better illustrating the configuration of components on the optical bench  116 . 
     Specifically, the input optical fiber  118  passes through the ferrule  112  and is secured to the bench  116  via a fiber mounting structure  150 . This structure secures the fiber endface or optical signal source  122  such that it is terminated above the optical bench  116 . 
     The diverging input beam that is emitted from the fiber endface  122  is collimated or has its collimation improved via a first collimating lens  132 , which is supported above the substrate on an optical component mounting structure  152 . This first collimating lens  132  generates a generally cylindrical, but diffracting input signal beam, which enters a first isolator  128 . The first isolator prevents back reflections into the optical signal input port/fiber endface  122 . 
     The optical signal after exiting the first isolator is focused by a first focusing lens  134 , which is supported above the substrate on a second optical component mounting structure  154 . Specifically, the optical signal is focused and thus coupled into a semiconductor optical amplifier chip  102 . 
     The optical signal is amplified in the semiconductor optical amplifier chip  102 . Typically, these amplifier chips are constructed from AlGaAs substrates with ridge waveguide structures. The invention, however, is of course applicable to chips made with other material systems/chip configurations. 
     The amplified optical signal is emitted from the chip  102  in a typically diverging beam. A second collimating lens  136 , which is supported above the bench  116  on a third optical component mounting structure  156 , generates a collimated diffracting beam. 
     In the preferred embodiment, the mounting structures  154  and  156  are preferably composite structures that allow for alignments in the x- and y-axes, but also the z-axis as illustrated in FIG.  3 . Specifically, two z-axis flexure pieces  1102 A,  1102 B are used to control rotation around the x-axis or in the direction of angle Θ x , thereby determining the resistance to force components along the z-axis. Preferably, the z-axis flexure pieces  1102  are separately fabricated and bonded to base surface of portion  1101 . Base surfaces of the pieces  1102  are then bonded to the bench  116  with the lens bonded to optical element interface  1112 . As a result, the z-axis position of the focal point of the lens can be controlled relative to the SOA chip facets. 
     Returning to FIG. 1B, the amplified optical signal beam then passes through a second isolator  130  for preventing back-reflections into the chip  102 . The beam of the amplified optical signal, which exits from the second isolator  130 , is focused by a second focusing lens  138 , which is supported above the substrate on a fourth optical component mounting structure  160 , and coupled into an output port or the endface  124  of the output fiber pigtail  120 . The termination of the output fiber is supported above the substrate  116 , via a second fiber mounting structure  162 . 
     In this way, the SOA system is integrated on a common substrate with isolation. This implementation allows for the addition of amplification capabilities in a very compact form-factor, which is applicable not only to general amplification applications but also as a subsystem in larger optical systems providing higher levels of functionality. 
     In the preferred embodiment, a photodetector is additionally integrated within the SOA system  100 . Specifically, a photodetector  126  is installed in the bench  116  to detect the power of the amplified optical signal. Preferably, this signal is used as a feedback control signal to regulate the level of electrical-drive being provided to the semiconductor optical amplifier chip  102 . 
     According to the preferred embodiment, a polarization independent scheme is used to detect the strength of the amplified optical signal. Specifically, a reflecting component is inserted into the beam path of the amplified optical signal to reflect a portion of this optical signal to a photodetector  126 . In the preferred embodiment, a small portion of the cross-section of the amplified optical signal beam is scattered. This has advantages relative to half mirrors, for example, that are installed across the entire beam path since the reflectivity of such devices is typically very polarization dependent. 
     In the preferred embodiment, a mounting structure  158  is inserted to nick an outer cross-sectional portion of beam of the amplified optical signal to scatter a portion of the amplified optical signal to be detected by the photodetector  126 . In alternative embodiments, the portion of the amplified optical signal can be specularly reflected to the photodetector  126 . 
     According to one manufacturing technique, the optical signal link or path through the system  100  is activated and the mounting structure is placed or deformed into the beam path such that it interrupts less than 5% of the beam&#39;s power, and specifically less than 1% in the preferred embodiment. 
     FIG. 2 illustrates a second embodiment of an SOA system  100 , which has been constructed according to the principles of the present invention. 
     Specifically, a wavelength division multiplex (WDM) signal source  10  generates the input optical signal to be amplified. This signal is received by a circulator  30 , in one embodiment, which circulator passes the optical signal to the SOA system  100 . Alternatively other coupling systems can be used. 
     As described previously, the optical fiber passes into the hermetic package  112  via a fiber feedthrough and is terminated above the optical bench  116 . Specifically, the endface is held above the optical bench  116  via a fiber mounting structure  150 . 
     This embodiment is a single physical-fiber port design. Specifically, only a single fiber passes into the module  112 . As a result, fiber  118  functions both the input fiber and output fiber. Additionally, the fiber endface functions both as the optical signal input port  122  and the output port  124  for the amplified optical signal. 
     Specifically, the diverging beam from the fiber endface or input port  122  is collimated by a collimating lens  132 . As described previously relative to FIG. 1B, the lens is held on an optical component mounting structure  152  on the bench  116 . 
     The optical signal beam is then focused by a focusing lens  134  (held on an optical component mounting structure  154 ) onto the semiconductor optical amplifier chip  102 . 
     The optical signal is amplified in the chip. The partially amplified optical signal having made one pass through the chip is then reflected to pass through the chip  102  a second time. This double pass arrangement can be accomplished by reflectively coating the back facet B of the chip  102 . In an alternative embodiment, a discrete reflector  144  is located behind the back facet B of chip  102 . This reflects the light to re-enter the chip  102 . In one implementation of this discrete reflector configuration, the reflector  144  has a concave shape to refocus the beam onto the back facet B of the chip  102 . In alternative embodiments, additional focusing optics can be installed in the beam path between the back facet B and the reflecting structure  144 . 
     The fully amplified optical signal is emitted from the front facet F of chip  102  on the second pass. It is emitted as a diverging beam and is collimated by the focusing lens  134 . The amplified optical signal passes from the focusing lens  134  to the collimation lens  132 , which now functions as a focusing lens to couple the amplified optical signal into the fiber  118  via focusing it onto the endface  122 / 124 . The amplified optical signal now passes through the fiber  118  now functioning as the output fiber  120  to circulator  130  to be directed to the WDM photodetector  20 . 
     The embodiment of FIG. 2 has provisions for detecting the amplitude of both of the input optical signal and the amplified optical signal. Specifically, an input photodetector  142  detects the level of the input optical signal. Output photodetector  126  detects the level of the amplified optical signal. 
     Specifically, reflective structures  158 ,  164  are inserted into the beam paths of both the input optical signal and the amplified optical signal. Specifically, structure  164  specularly reflects or scatters the input optical signal to be detected by photodetector  142 . Structure  158  specularly reflects or scatters light to be detected by the output signal detector  126 . As a result, the second embodiment is capable of modulating the level by which the chip  102  is energized based upon and in response to both the level of the input optical signal and the level of the amplified optical signal. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.