Abstract:
Electroabsorption-modulated Fabry-Perot lasers and methods of making the same are described. In one aspect, a light source includes a Fabry-Perot (FP) laser that is operable to generate multimode laser light, an electroabsorption modulator (EAM) that is configured to selectively absorb and transmit laser light traveling therethrough, and an optical isolator. The optical isolator is on an optical path between the FP laser and the EAM. The optical isolator is configured to transmit laser light traveling along the optical path from the FP laser to the EAM.

Description:
BACKGROUND  
       [0001]     Two general approaches typically are used to modulate the intensity of light: direct modulation and external modulation.  
         [0002]     In a direct modulation approach, a laser (e.g., a laser diode) is directly modulated by an information signal to generate a modulated laser output. The laser output power often is modulated directly by modulating the input drive current to the laser. The laser begins lasing when the drive current exceeds a threshold current level. Typically, the modulation range of input drive current that is applied to a directly modulated laser extends above and near the threshold current level.  
         [0003]     In an external modulation approach, a modulator modulates the intensity of light generated by a continuous wave laser in accordance with an information signal. The modulator and laser may be disposed on separate, discrete substrates or they may be fabricated together on a single substrate. External modulators fall into two main families: electro-optic type modulators, such as Mach-Zehnder type electro-optic modulators, which modulate light through destructive interference; and electro-absorption modulators, which modulate light by absorption (e.g., through the quantum-confined Stark effect).  
         [0004]     Under direct modulation linear and nonlinear effects within the laser create chirp. Chirp is a variation in optical signal wavelength over the duration of a laser light pulse during modulation. For positive transient chirp, the leading edge of the laser light pulse comprises shorter wavelengths than the trailing edge. In positive dispersion fibers, shorter wavelengths travel faster than longer wavelengths. The pulse therefore broadens as it propagates. Regenerators often are required in order to compensate for this positive chirp, raising the cost of communications networks considerably. Chirp effects are manageable at direct laser modulation bit rates up to a few GHz. Direct modulation of lasers typically is not used at bit rates above a few GHz, especially when the laser is driven to create sharp laser pulses with abrupt rising and falling edges.  
         [0005]     External modulation is favored for applications that are sensitive to chirp because external modulation introduces very little chirp into the output signal. For this reason, external modulation is used almost exclusively in long-distance digital optical communications, where excessive spectral broadening in a directly modulated laser due to chirp leads to a greater pulse distortion during propagation and a reduction in overall performance.  
         [0006]     Distributed feedback (DFB) lasers are typically used for long-distance optical communication applications. A DFB laser produces an output that is characterized by a narrow spectral linewidth, which allows a DFB laser to transmit signals over long distances. This feature also allows a DFB laser to be used in narrow-linewidth applications, such as wavelength-division multiplexing (WDM) where it is desirable to carry as many multiplexed signals as possible without interference in the same optical fiber. DFB lasers, however, are extremely sensitive to back-reflections, which broaden the spectral linewidth and increase noise. For this reason, DFB lasers typically are assembled in one package with an optical isolator that blocks back-reflections.  
         [0007]     The narrow linewidth features of DFB lasers and the low chirp characteristics of external modulators are leveraged in long-haul optical data transmission systems. The output wavelength temperature coefficient of a DFB laser and the absorption edge wavelength coefficient of an electroabsorption modulator, however, typically are significantly different, which degrades operation over wide temperature ranges. For this and other reasons, systems that include DFB lasers and electroabsorption modulators also typically include direct active temperature-regulating devices, such as thermoelectric coolers. In one such approach, a DFB laser and an electroabsorption modulator are mounted on an optical platform that is mounted on a submount, which is attached to a thermoelectric cooler. A thermistor mounted on the submount provides thermal feedback that allows the thermoelectric cooler to maintain the temperature of the DFB lasers and the electroabsorption modulators within a prescribed narrow temperature range.  
         [0008]     For the reasons explained above, DFB laser designs tend to be bulky, expensive, and high in power consumption.  
       SUMMARY  
       [0009]     In one aspect, the invention features a light source that includes a Fabry-Perot (FP) laser that is operable to generate multimode laser light, an electroabsorption modulator (EAM) that is configured to selectively absorb and transmit laser light traveling therethrough, and an optical isolator. The optical isolator is on an optical path between the FP laser and the EAM. The optical isolator is configured to transmit laser light traveling along the optical path from the FP laser to the EAM.  
         [0010]     In another aspect, the invention features a method of making the above-described light source.  
         [0011]     Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0012]      FIG. 1  is a block diagram of an embodiment of a light source that includes a Fabry-Perot laser, an optical isolator, and an electroabsorption modulator.  
         [0013]      FIG. 2  is a diagrammatic view of an implementation of the Fabry-Perot laser of  FIG. 1 .  
         [0014]      FIG. 3A  is a graph of a set of exemplary optical longitudinal modes and an exemplary gain profile plotted as a function of wavelength for the Fabry-Perot laser implementation of  FIG. 2 .  
         [0015]      FIG. 3B  is a graph of the optical power spectrum plotted as a function of wavelength for the exemplary Fabry-Perot laser implementation of  FIG. 3A .  
         [0016]      FIG. 4  is a diagrammatic view of an implementation of the optical isolator of  FIG. 1 .  
         [0017]      FIG. 5  is a diagrammatic side view of one possible implementation of embodiment of the electroabsorption modulator of  FIG. 1 .  
         [0018]      FIG. 6A  is a top view of an implementation of the electroabsorption modulator embodiment of  FIG. 5  that has a signal electrode formed from a continuous strip of electrically conductive material.  
         [0019]      FIG. 6B  is a top view of an implementation of the electroabsorption modulator embodiment of  FIG. 5  that has a signal electrode formed from multiple spaced-apart electrode segments of electrically conductive material that are connected in series by inter-stage microstrip lines.  
         [0020]      FIG. 7  is an illustrative graph of the absorption coefficient of the electroabsorption modulator implementation of  FIG. 5  plotted as a function of wavelength for different bias conditions.  
         [0021]      FIG. 8A  is a block diagram of an implementation of the light source of FIG.  
         [0022]      FIG. 8B  is a diagrammatic side view of the light source implementation of  FIG. 8A .  
         [0023]      FIG. 9A  is a block diagram of an implementation of the light source of  FIG. 1 .  
         [0024]      FIG. 9B  is a diagrammatic side view of the light source implementation of  FIG. 9A .  
         [0025]      FIG. 10  is a flow diagram of a method of making the light source embodiment of  FIG. 1 .  
         [0026]      FIG. 11  is a block diagram of an embodiment of a drive circuit for driving the light source embodiment of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0027]     In the following description, like reference numbers are used to identify like elements. Furthermore, the drawings are intended to illustrate major features of exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale.  
         [0028]     An FP laser produces an output that is characterized by a multiple longitudinal mode (or multimode) spectrum over a relatively wide spectral linewidth. The wide spectral linewidths of FP lasers preclude their use in long-haul data transmission applications and in narrow linewidth applications such as WDM. For example, the dispersion for each wavelength within the FP laser spectrum is different, giving rise to material dispersion that spreads the optical signal. In addition, FP lasers are characterized by mode hopping behavior, which gives rise to mode partition noise that causes phase jitter in the optical signal. The spectral linewidths of FP lasers also are too wide to be used in WDM applications without interference between the various longitudinal modes. For these reasons, the use of FP lasers hitherto has been limited to relatively short-distance applications that are not sensitive to chirp and where direct modulation techniques provide sufficient modulation rates.  
         [0029]     In the light source embodiments described in detail below, the structural similarities between FP lasers and electroabsorption modulators are leveraged to enable high-speed light source designs that exhibit reliable and substantially temperature-independent data transmission capabilities over a wide temperature range. These designs therefore can omit direct active temperature regulating devices, such as thermoelectric coolers, which increase fabrication costs and operating costs. In this way, these embodiments enable practical light source designs that are characterized by high data rate, temperature-independent operation over short and medium distances.  
         [0030]      FIG. 1  shows an embodiment of a light source  10  that includes a Fabry-Perot (FP) laser  14 , an optical isolator  16 , and an electroabsorption modulator  18 . The light source  10  generates digital impulse output signals  22  that are encoded in accordance with any one of a wide variety of known optical communication protocols (e.g., amplitude shift keying (ASK) modulation, frequency shift keying (FSK) modulation, phase shift keying (PSK) modulation, and the like). In some embodiments, light source  10  generates digital impulse output signals  22  at bit rates ranging from approximately 1 gigabit per second (Gb/sec) up to approximately 50 Gb/sec over a wide range of operating temperatures.  
         [0031]      FIG. 2  shows an implementation of FP laser  14  that includes first and second reflectors  28 ,  30  that define an optical resonant cavity  32 . The optical resonant cavity  32  contains an active region  34  interposed between first and second cladding regions  36 ,  38 . In the illustrated implementation, the first cladding region  36  is a semiconductor layer of material doped p-type, the second cladding region  38  is a semiconductor layer of material doped n-type, and the active region  34  is an undoped layer of semiconductor material. When a drive current (I Drive ) is applied to the FP laser  14 , electron-hole pairs in the active region  34  combine to generate light  40 . In some implementations, the active region  34  includes one or more quantum wells, which tailor the characteristics of the light  40  generated in the active region  34 . The first and second reflectors  28 ,  30  are cleaved facets of semiconductor material. In the illustrated embodiment, the reflector  28  is 100% reflective of light  40  and the reflector  30  is partially reflective of light  40  so that polarized output light  42  exits the edge of the FP laser  14  corresponding to the partially reflective reflector  30 .  
         [0032]     The optical resonant cavity  32  limits light oscillation to a discrete set of evenly-spaced longitudinal optical modes  44  shown in  FIG. 3A . The wavelength mode spacing (Δλ) of the output light  42  is given by Δλ=C(2nL) −1 , where C is the velocity of light, L is the length of cavity  32  and n is an effective refractive index of the medium for light propagation and n is a number greater than 1. The active region  34  is characterized by a gain versus wavelength function  46 , which results in the amplification of only a limited number of optical modes (e.g., 3-30 modes) within a relatively narrow wavelength band. The output power spectrum  48  of the output light  42  generated by FP laser  14  is shown in  FIG. 3B . The wavelength spectrum  48  is characterized by a root-mean-squared (RMS) linewidth that corresponds to the weighted mean of the square root of the sum of the squared magnitudes of the spectral components of the output light  42 . Maintaining a relatively narrow spectral linewidth (though still not a single-mode spectrum) enables the light source  10  to reliably transmit output data signals  22  over longer distances. In some implementations, the RMS spectral linewidth is at most 3 nanometers (nm). In some of these implementations, the RMS spectral linewidth is at most 1 nm.  
         [0033]      FIG. 4  shows an embodiment of optical isolator  16  that includes a pair of polarizers  50 ,  52  and a Faraday rotator  54  interposed between the polarizers  50 ,  52 . In some implementations, the polarizers  50 ,  52  are birefringent prisms or polarizing beam splitters, and the Faraday rotator  54  is a magnetic garnet crystal. An annular permanent magnet  56  surrounds and applies a magnetic field to the Faraday rotator  54 . In the illustrated embodiment, the polarizer  50  has a polarization axis that is oriented parallel to the polarization of the light  42  received from FP laser  14 , the Faraday rotator  54  rotates the polarization of the light 45°, and the polarizer  52  has polarization axis that is oriented parallel to the rotated light received from the Faraday rotator  54 . In this way, the polarized output light  42  received from FP laser  14  passes through the optical isolator without substantial amplitude reduction. Back-reflected light, on the other hand, passes through the Faraday rotator  54  twice and therefore has an orthogonal polarization relative to the polarization axis of the polarizer  50 . For this reason, such back-reflected light is substantially blocked by the optical isolator  16 . By preventing backreflections from reaching the FP laser  14 , the optical isolator  16  prevents spectral broadening of the output data signals  22  that otherwise would occur and allows the light source  10  to produce output light signals  22  without substantial spectral broadening of the output light  42  generated by FP laser  14 . In the illustrated embodiment, the polarization axis of polarizer  52  is oriented at an angle of 45° relative to the polarization axis of polarizer  50 . In the illustrated embodiment, the electroabsorption modulator  18  is polarization-independent. In another embodiment, the optical isolator  16  is a polarization-maintaining isolator, in which the input and output polarization states are along the same axis. In this other embodiment, the electroabsorption modulator  18  may be polarization-independent or polarization-dependent (e.g., TE mode only).  
         [0034]      FIG. 5  shows an embodiment of an electroabsorption modulator  18  that includes first and second electrodes  62 ,  64 , first and second cladding regions  66 ,  68 , and an active region  70 .  
         [0035]     The first and second electrodes  62 ,  64  include one or more metal layers. In one exemplary embodiment, each of the first and second electrodes  62 ,  64  includes an underlying layer of titanium, which promotes adhesion and forms an ohmic contact interface between the electrodes  62 ,  64  and the supporting semiconductor material, and an overlying layer of gold that forms electrical contacts for the electroabsorption modulator  18 . In the illustrated embodiment, the first electrode  62  is a traveling-wave signal electrode formed from a continuous strip of electrically conductive material, as shown in  FIG. 6A , or from multiple spaced-apart electrode segments of electrically conductive material that are connected in series, with each pair of signal electrode segments connected by a respective inter-stage microstrip line  73 , as shown in  FIG. 6B .  
         [0036]     In some implementations, the first and second electrodes  62 ,  64  are connected to input and output bonding pads by respective microstrip lines. The input bonding pad is connected to a drive circuit by a first bonding wire and the output bonding pad is connected to an external termination load through a second bonding wire. The electro-absorption modulator  18 , the input and output bonding pads, and the input and output microstrip lines are fabricated on the same substrate  74  (e.g., a wafer of semiconductor material, such as InP or GaAs). The external termination load is any suitable termination load, such as a resistor. The termination load and the drive circuit typically are impedance-matched to reduce reflections and maximize the electrical voltage that can be delivered across the active region  70  of the electro-absorption modulator  18 .  
         [0037]     In the illustrated embodiment, the substrate  74  is electrically insulating and the electroabsorption modulator  18  and first and second metal film transmission lines  76 ,  78  are formed on an electrically conducting semiconductor layer  80  (e.g., n++InGaAs or n++InGaAsP), which is formed on the substrate  74 . In other embodiments, the substrate  74  is electrically conducting, and the electroabsorption modulator  18  and the transmission lines  76 ,  78  are formed directly on the substrate  74 .  
         [0038]     Each of the first and second cladding regions  66 ,  68  and the active region  70  includes one or more semiconductor layers. In the illustrated embodiment, the first and second cladding regions  66 ,  68  are doped n-type and the active region is undoped and, therefore, contains relatively small amounts of impurities (e.g., less than about 5×10 15  cm −3 ). The first and second cladding regions  66 ,  68  are formed of material compositions that have lower refractive indices than the material composition of the active region  70 . In this way, the cladding regions  66 ,  68  and the active region  70  operate as a waveguide for light traveling through the electroabsorption modulator  18 . The active region  70  includes a light absorption region  82  that includes at least one quantum well with a conduction band alignment and a valence band alignment that create bound electron and hole states that are involved in the electro-absorption process.  
         [0039]     The implementation of electroabsorption modulator  18  shown in  FIG. 5  corresponds to a ridge-type waveguide structure. In other embodiments, the electroabsorption modulator  10  may by implemented by different types of waveguide structures. For example, in some embodiments, the electroabsorption modulator  10  includes a buried heterostructure.  
         [0040]      FIG. 7  shows a graph of the absorption spectrum of the electroabsorption modulator  18  under different bias conditions and the output spectrum  48  of the FP laser  14 . As shown in  FIG. 7 , the absorption edge (corresponding to the “knee” in the absorption spectrum curves) of the electroabsorption modulator  18  moves to longer wavelengths with increasing applied reverse bias (V Reverse ). In some implementations, the zero-bias (V Reverse =0) absorption edge wavelength of the electroabsorption modulator  18  is designed to be shorter than a specified target wavelength (e.g., the dominant peak) in the output spectrum  48  to be modulated. For example, in one exemplary implementation, the zero-bias (V Reverse =0) absorption edge wavelength of the electroabsorption modulator  18  is approximately 50-70 nm shorter than a target lasing wavelength of approximately 1555 nm. As the reverse bias applied to the electroabsorption modulator  18  increases, the absorption edge wavelength shifts to longer wavelengths and the optical signal  22  emitted from the electroabsorption modulator  18  is reduced. The ratio of the “on” state to the “off” state is referred to as the extinction ratio of the electroabsorption modulator  18 .  
         [0041]     The optical isolation provided by the optical isolator  16  renders the light source  10  substantially immune to any reflections originating beyond the optical isolator  16 . The spectrum and the amplitude of the output optical signal  22  are determined primarily by the ambient temperature, the drive current to the laser  14  and the voltage applied to the electroabsorption modulator  18 . The temperature-dependence of the output signal  22  is substantially eliminated by designing the FP laser  14  and the electroabsorption modulator  18  so that they have output wavelength and absorption edge temperature coefficient parity and they share a mutual thermal environment.  
         [0042]     In some implementations, the FP laser  14  and the electroabsorption modulator  18  are designed so that the FP laser  14  has an output wavelength temperature coefficient that is substantially equal to the absorption edge wavelength temperature coefficient of the electroabsorption modulator  18 . For example, in some implementations, the output wavelength temperature coefficient of the FP laser  14  and the absorption edge wavelength temperature coefficient of the electroabsorption modulator  18  are substantially equal (i.e., they differ by at most ±25%). In some implementations, this temperature coefficient parity is achieved by forming the FP laser  14  and the electroabsorption modulator  18  of materials selected from the same semiconductor material family. As used herein, the term “semiconductor material family” refers to a group of semiconductor materials that are composed of, for example, two or more members of a discrete set of suitable elemental atoms (e.g., Group III and Group V elemental atoms) suitable for forming an epitaxial thin film a compatible substrate. Exemplary semiconductor material families include: In x Ga 1-x As y P 1-y  on an InP substrate, where 0≦x≦1 and 0≦y≦1; Al x Ga y In 1-x-y As on an InP substrate, where 0≦x≦1 and 0≦y≦1; In x Ga 1-x As on a GaAs substrate, where 0≦x≦1; and Al x Ga 1-x As on a GaAs substrate, where 0≦x≦1.  
         [0043]     In addition to output wavelength and absorption edge temperature coefficient parity, the FP laser  14  and the electroabsorption modulator  18  share a mutual thermal environment such that the FP laser  14  and the electroabsorption modulator  18  are at substantially the same temperature throughout the range of operating conditions specified for the light source  10 . For example, in some implementations, the FP laser  14  and the electroabsorption modulator  18  differ in temperature by at most 15 degrees Celsius (° C.) over an operating temperature range of 20° C. to 90° C.  
         [0044]     As explained in detail below, the components of light source  10  may be packaged separately but contained within a shared thermal environment, or the components of light source  10  may be packaged in a single package that defines a shared thermal environment.  
         [0045]      FIGS. 8A and 8B  show an implementation of the light source  10  in which the Fabry-Perot laser  14  and the optical isolator  16  are contained within an optoelectronic package  90  and the electroabsorption modulator  18  is contained within a separate optoelectronic package  92 , where both optoelectronic packages  90 ,  92  are contained within a shared thermal environment  93 . A lens element  94  (e.g., an optical lens or a diffractive lens) focused the output light  42  from the FP laser  14  through the optical isolator  16  and onto the end of an optical fiber  96 , which is held by a ferrule  98 . The FP laser  14 , the lens element  94 , and the optical isolator  16  are mounted on a substrate  100  within the first optoelectronic package  90 . Optical bench alignment techniques are used to align these components before they are secured to substrate  100 . The FP laser  14  electrically connects to the drive circuit  12  through an electrical interface  102 . In the implementations illustrated  FIG. 8A  and  FIG. 8B , a high degree of flexibility is achieved by using a polarization-independent electroabsorption modulator  18 .  
         [0046]     A ferrule  104  holds the end of optical fiber  96  in optoelectronic package  92 . In another embodiment, ferrule  104  holds the end of a separate fiber that can be connected to optical fiber  96 . A lens element  106  (e.g., an optical lens or a diffractive lens) within the second optoelectronic package  92  focuses light received from the optical fiber  96  onto the input of the electroabsorption modulator  18 . An optical fiber  108 , which is held by a ferrule  109 , carries the output optical signal  22  from the output of the electroabsorption modulator  18 . The electroabsorption modulator  18  and the lens element  106  are mounted on a substrate  111  within the second optoelectronic package  92 . Optical bench alignment techniques are used to align these components before they are secured to substrate  111 . The electroabsorption modulator  18  electrically connects to the drive circuit  12  through an electrical interface  110 .  
         [0047]     In some implementations, the first and second optoelectronic packages  90 ,  92  are mounted on the same printed circuit board (e.g., a motherboard or a daughterboard), which is contained in an enclosure of an optical data transmission system that defines the shared thermal environment  93 . In other implementations, the first and second optoelectronic packages  90 ,  92  are mounted on different printed circuit boards that are contained in an enclosure of an optical data transmission system that defines the shared thermal environment  93 . In one exemplary implementation of this type, one of the first and second optoelectronic packages  90 ,  92  is mounted on a motherboard and the other optoelectronic package is mounted on a daughterboard connected to the motherboard in the optical data transmission system enclosure. In another exemplary implementation of this type, the first and second optoelectronic packages  90 ,  92  are mounted on different respective daughterboards that are connected to the same motherboard in the optical data transmission system enclosure. In these implementations, the first and second optoelectronic packages are decoupled from any direct active temperature-regulating devices.  
         [0048]      FIGS. 9A and 9B  show an implementation of the light source  10  in which the Fabry-Perot laser  14 , the optical isolator  16 , and the electroabsorption modulator  18  are contained within the same optoelectronic package  112 , which defines a shared thermal environment for the light source components. A lens element  114  (e.g., an optical lens or a diffractive lens) collimates the output light  42  from the FP laser  14 . The collimated light passes through the optical isolator  16 . A lens element  116  (e.g., an optical lens or a diffractive lens) focuses the light output from the optical isolator  16  onto the input of the electroabsorption modulator  18 . The FP laser  14 , the lens elements  114 ,  116 , the optical isolator  16 , and the electroabsorption modulator  18  are mounted on a substrate  118  within the optoelectronic package  112 . Optical bench alignment techniques are used to align theses components before they are secured to substrate  118 . An optical fiber  120 , which is held by a ferrule  122 , carries the output optical signal  22  from the output of the electroabsorption modulator  18 . The FP laser  14  and electroabsorption modulator  18  electrically connect to the drive circuit  12  through respective electrical interfaces  124 ,  126 .  
         [0049]      FIG. 10  shows an embodiment of a method of making the light source  10 . In accordance with this method, the FP laser  14  is provided (block  130 ). The electroabsorption modulator  18  is provided (block  132 ). The optical isolator  16  is provided (block  134 ). The FP laser  14 , the optical isolator  16 , and the electroabsorption modulator  18  are mounted in at least one optoelectronic package (block  136 ).  
         [0050]     As a result of the above-described output wavelength and absorption edge temperature coefficient parity and the shared thermal environment, the relative wavelength offset between the center of the FP laser output spectrum  48  and the zero-bias absorption edge wavelength of the electroabsorption modulator  18  is substantially constant over a relatively wide temperature range (e.g., 20° C. to 90° C.). As a result, the insertion loss of the electroabsorption modulator  18  does not shift substantially with temperature since the respective band edges track and the optical output amplitude and the extinction ratio for a given electroabsorption modulator bias and signal amplitude are substantially temperature-independent.  
         [0051]     The multimode spectrum of the optical signals  22  produced by light source  10  limits the distance over which the output optical signals  22  can propagate. The relatively broad spectral nature of the modulated output optical signals  22  still may be used over a range of useful distances for a number of practical optical data transmission applications. For example, output optical signals  22  with data rates up to approximately 40 Gb/sec are able to propagate up to approximately twenty meters on multimode optical fibers using, for example, a 2 nm root-mean-squared linewidth, 0 dBm of launched optical power, and a suitable receiver sensitivity. Among the applications for such short-distance data transmissions are high-speed signal transmission between computer chips, between printed circuit boards within a data transmission system, between back-planes, and between racks of separate data transmission systems.  
         [0052]      FIG. 11  shows an embodiment of a drive circuit  12  for driving the light source  10 . An external digital signal source, such as a non-return to zero (NRZ) driver, transmits digital input drive signals  20  to drive circuit  12 . Drive circuit  12  includes respective sets  24 ,  26  of RF components (e.g., attenuators, filters, and couplers) that modify the digital input drive signals  20  with respective transfer functions T laser , T mod , and synchronously apply the drive signals  140 ,  142  to the FP laser  14  and the electroabsorption modulator  18 . Drive circuit  12  also may include circuit elements for establishing appropriate direct current (DC) bias conditions for operating the FP laser  14  and the electroabsorption modulator  18 .  
         [0053]     Other embodiments are within the scope of the claims.