Abstract:
To provide economical high-speed optical communications through an optical module a silicon substrate with a ball lens and semiconductor luminescence element mounted thereon, and a laser driver IC are provided. A silicon subassembly includes a silicon substrate with a pyramidal cavity etched into the substrate near its edge. A ball lens is precision-mounted in the pyramidal cavity by bonding it at multiple points. This provides a high-strength connection and a small, inexpensive silicon substrate. Also provided on the silicon substrate is a semiconductor luminescence element and a slit on the opposite side of the ball lens. The slit enables collimated light emitted from the ball lens to be optically coupled without being blocked or reflected by the substrate. The semiconductor luminescence element and its laser driver IC are placed in close proximity to each other, and ribbon wires are kept short (approximately 300 μm) to reduce signal line impedance mismatch for high-speed transmission.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optical module and an optical transceiver, and in particular, to a semiconductor optical module appropriate for use in optical fiber communication, including a silicon substrate, a semiconductor optical element, and a ball lens. 
     BACKGROUND OF THE INVENTION 
     Recent growth in the demand for economical, high-traffic-volume optical fiber communication systems has encouraged the development of small, high-speed, low-cost optical modules. In particular, the use of passive alignment with semiconductor substrates has attracted significant attention because it uses low-cost materials and simplifies assembly. Passive alignment is a method in which a semiconductor substrate is etched to precise dimensional tolerances in order to eliminate complex alignments that would otherwise be required at the time of assembly. Another improvement entailed includes the laser IC driver for the semiconductor luminescence element (hereinafter, laser diode) inside the optical module package. This allows smaller optical modules that are capable of functioning with smaller input signals. 
     An example of a laser diode using a semiconductor substrate as described above may be found in Japanese pre-grant patent publication No. H9-222540. Here, a trapezoidal trench formed in a silicon substrate is configured to preclude the obstruction of light emitted from a luminescence element and collimated by a ball lens. An example of placing both the laser diode and its laser driver IC inside an optical module package may be found in Japanese pre-grant publication No. 2000-91695. Here, the laser diode and the laser driver IC are mounted on separate members to provide thermal isolation. 
     The above described prior art technologies, however, have problems. In the optical coupling device with a trapezoidal trench as described in JP pre-grant publication H9-222540, bonds are made at three points. This means that the pressure applied during bonding must be applied, not perpendicular to the plane of the substrate, but perpendicular to a virtual plane that includes the three bonding points. The direction in which the force is applied is at an angle to the plane of the silicon substrate results in the unevenly applied force, which leads to improper positioning and degraded bond strength. 
     In the optical module described in JP pre-grant publication No. 2000-91695, because the laser diode and the laser driver IC are mounted on separate members, it is difficult to place them close together, and they also complicate signal line (bonding wire) impedance matching. Also, because the input signal to the laser driver IC for the laser diode is a single phase (single-ended) signal, parasitic impedance introduced by the bond wires, etc. can degrade the input signal waveform. These factors tend to degrade high-speed transmission characteristics. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a laser diode and a ball lens are mounted on a semiconductor substrate, and the ball lens is mounted/bonded in a pyramidal cavity that is etched into the substrate. This configuration allows the pressure during bonding to be applied perpendicular to the surface of the substrate so as to achieve high-precision mounting and a high-strength bond. In addition, to prevent the blocking or reflection of collimated light being emitted from the ball lens, the light-emitting side of the pyramidal cavity where the ball lens is mounted is removed. 
     According to another aspect of the present invention, the signal input to the laser driver IC that drives the laser diode is a differential input. The input lines to the module are alternately arranged between ground pins and signal pins so as to facilitate signal path impedance-matching. Furthermore, rather than placing the laser diode and the ball lens in the middle of the silicon substrate, they are placed at the edge on a side that is perpendicular to the edge from which the light is emitted (a side parallel to the direction of emission). The above placement allows the laser diode and its driver IC to be placed closer to each other. This close placement allows the length of the wire ribbons between the laser driver IC and the lands of the laser diode to be a maximum of 300 μm, for minimizing impedance mismatch due to the wire ribbons in the signal path, and improving the high speed propagation characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a first preferred embodiment of a subassembly of the optical module according to the present invention. 
     FIG. 2 is an isometric view of a second preferred embodiment of the subassembly of the optical module according to the present invention. 
     FIGS. 3A and 3B are detailed diagrams illustrating the ball lens and semiconductor luminescence element of the optical module of the first preferred embodiment according to the present invention. 
     FIGS. 4A and 4B are diagrams illustrating a subassembly of the optical module of the first preferred embodiment according to the present invention. 
     FIG. 5 is a plan view of the optical module in a preferred embodiment according to the present invention. 
     FIG. 6 is a block diagram of the optical transmission system in a preferred embodiment according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments according to the present invention are described below, with reference to the figures. In general, the same reference numerals are used to the same components across the figures. 
     FIG. 1 is an isometric view of a diagram for illustrating an optical coupling in a first preferred embodiment of the optical module according to the present invention. As shown in FIG. 1, a silicon subassembly  100  comprises a silicon substrate  101 , a semiconductor luminescence element (laser diode)  102 , a ball lens  103 , and a thermistor  105 . The ball lens  103  is shown to be removed from the substrate to simplify the drawing. Formed in the silicon substrate  101  is a pyramidal cavity  104 . A slit  106  is formed in a portion of the pyramidal cavity  104  by a dicing process. The slit  106  lowered one of the pyramidal cavity walls along an entire edge that is perpendicular to the optical axis at the emitting side. The laser diode  102  and the ball lens  103  are placed at the front of the silicon substrate  101  on the emitting side and on a side that is perpendicular to the emitting side. 
     Using an ordinary hot etch, the depth of the pyramidal cavity formed in the silicon substrate  101  is controlled to an accuracy of ±5 μm. With metallization provided on portions of the pyramidal cavity  104  and the ball lens  103  (metallization not shown in the drawing), the ball lens can be bonded to the cavity by application of pressure in a heated environment. Light emitted from the laser diode  102  is collimated or converted to substantially parallel rays by the passively-aligned ball lens  103 . The thermistor  105  on the silicon substrate  101  measures the temperature of the laser diode  102  for controlling the amplitude of the laser diode drive current. Based on a reference value determined by measurements performed in advance, the thermistor  105  maintains a constant output. 
     The pyramidal cavity  104  as shown in FIG. 1 is a four-sided pyramidal cavity. It is known, however, that a three-sided pyramidal cavity can be formed by proper selection of the growth direction of the silicon substrate. That is, in the first preferred embodiment and other embodiments as well, a three-sided pyramidal cavity may also be used. In addition, a bonding adhesive could be used to make the bond between the pyramidal cavity  104  and the ball lens  103 . This, of course, would eliminate the need for metallization. 
     FIG. 2 is an isometric view of a diagram illustrating an optical coupling in a second preferred embodiment of the optical module according to the present invention. The elements  101  through  106  are substantially identical to those of FIG.  1 . Only the differences between the first and second preferred embodiments will be described. In the second preferred embodiment of a silicon assembly  110 , a surface mounting-type semiconductor photodiode  108  for monitoring the optical output of a laser diode  102 ′ is placed on the opposite side from the ball lens  103  across the laser diode  102 ′. 
     In the second preferred embodiment, the laser diode  102 ′ is a “junction-down” device. Both the laser diode  102 ′ and photodiode  108  have large tolerances for wafer thickness. If a “junction-up” device were used, there could be cases in which the photodiode  108  would not receive the light emitted from the back of the laser diode  102 ′. The second preferred embodiment function substantially the same as the first preferred embodiment except that the second preferred embodiment additionally monitors the operation of the laser diode  102 ′. 
     FIGS. 3A and 3B are detailed diagrams illustrating the semiconductor luminescence element (laser diode)  102  and the ball lens  103  of FIGS. 1 and 2. FIG. 3A is a plan view, and FIG. 3B shows a cross-sectional review at the section I-I′ of FIG.  3 A. The ball lens  103  is bonded to the pyramidal cavity  104  of the silicon substrate  101  at four bonding locations  109 . The bonding is performed using adhesive, solder, or aluminum oxide. Light emitted from the laser diode  102  is converged by the ball lens  103  to form a collimated light beam  107  over the slit  106 . 
     Referring to FIG. 3B, because a slit  106  is provided in the path of this collimated light beam  107  through the ball lens  103  from the laser diode  102  on the silicon substrate  101 , it can be converged into a fiber by the next optical component without being blocked or reflected by the inner surface of the pyramidal cavity  104 . 
     In the configuration of the second preferred embodiment, the pressure during bonding is applied perpendicular to the surface of the substrate so as to achieve highly precise mounting and a high-strength bond. In addition, the removal of the exit side of the pyramidal cavity where the ball lens is mounted prevents blocking or reflection of the collimated light that is emitted from the ball lens. 
     FIGS. 4A and 4B illustrate the construction of a stem subassembly of a preferred embodiment of the optical module according to the present invention. FIG. 4A is a plan view, while FIG. 4B illustrates a cross-sectional view at the section II-II′ of FIG.  4 A. The stem subassembly  200  includes a silicon substrate subassembly  100  and an optical isolator  201  on a stem  202  for preventing returned light, a laser driver IC  203  for driving a semiconductor luminescence element or laser diode  102 , a wiring-board-within-stem  206 , a semiconductor photodiode  208  for monitoring light output, and a block  207  for mounting the semiconductor photodiode  208  thereon. Electrical connections between the silicon substrate subassembly  100  and the laser driver IC  203  are made by ribbon bonding wires  204 , which are approximately 300 μm long. The distance between the laser diode  102  and its driver IC (including leads) is approximately 600 μm. The diameter of the ball lens  103  is approximately 600 μm, and the distance between its optical axis and a side of the substrate perpendicular to the light emitting side parallel to the direction of emission is approximately 650 μm. A noise suppression element  205  and a power supply bypass capacitor  209  are also located on the wiring-board-within-stem  206 . Copper-tungsten (CuW) is used as the material for the stem  202  because it has low thermal resistance and also has a coefficient of thermal expansion that is almost the same as that of the silicon substrate. 
     In the stem subassembly configuration, the distance between an optical axis  103 A of the lense  103  and a side  100 A that is perpendicular to the exit side  102 A of the laser diode  102  and that is parallel to the optical axis  103 A is made 1 mm or less and preferably less than 700 μm. This distance puts the laser driver IC  203  physically close to the laser diode  102 , and reduces impedance mismatching that is caused by the signal line ribbon bonding wires. The configuration thus improves the high-speed transmission characteristics. The semiconductor photodiode  208  makes it possible to monitor the operation of the semiconductor luminescence element or laser diode  102 . 
     FIG. 5 is a plan view or partial cross-section of a preferred embodiment of the optical module according to the present invention. A package  301  includes a module wiring board  302 , lead pins  303 , and a stem subassembly  200 . Electrical connections between the module wiring board  302  and the stem subassembly  200  are made by wire bonding. The reason for implementing the subassembly on a stem is that it would be difficult to install the optical components with the required precision if they were installed directly on the bottom of the module package  301 . The stem subassembly  200  is constructed with a dimensional variance as large as ±50 μm which is not considered high precision when working optical components. Once the stem subassembly  200  has been constructed as described above, it is acceptable for installation in the module package  301 . Variances of this magnitude can be adjusted for on the fiber holder end. 
     The signal input to the stem subassembly  200  is configured as a differential input. The input signal lines are arranged in the following sequence of ‘ground pin  304 -signal pin  305 -ground pin  304 -signal pin  305 -ground pin  304 -’. This arrangement provides impedance matching for the input signal from the module wiring board  302  to the laser diode driver IC. Light emitted from the stem subassembly  200  is converged by a holder-with-lens  308  to be optically coupled to a fiber ferrule  306  that is secured in a fiber holder  307 . In the preferred embodiment, impedance matching and good high-speed transmission characteristics are achieved by arranging the signal and ground lines such that each of the two signal lines of the differential input has a ground line on either side of it. 
     FIG. 6 is a block diagram for illustrating an optical transceiver and optical transmission system in another preferred embodiment according to the present invention. An optical transceiver  404  includes an optical module  408 , a photodiode module  402 , a limiting amplifier  403 , a multiplexer  407 , a demultiplexer  405 , and an electrical array connector  406 . An electrical signal from the optical communication system is input via the electrical array connector  406  to the optical transceiver  404 , where it is multiplexed by a multiplexer  407 . The multiplexed electrical signal is converted to light in the optical module  408 , and transmitted at high speed over an optical fiber  401 . This transmitted light is converted to an electrical signal by the photodiode module  402  of another optical transceiver. The resulting electrical signal is passed through the limiting amplifier  403  and demultiplexed by a demultiplexer  405 . This demultiplexed signal is output through the electrical array connector  406  to the optical communication system. 
     By providing for precision-mounting of a semiconductor luminescence element (laser diode) and a laser driver IC in an optical module, the present invention provides an optical module with improved high-speed transmission characteristics.