Patent Publication Number: US-2005129077-A1

Title: Tunable electro-absorption modulator and tunable laser

Description:
FIELD OF THE INVENTION  
      The present invention is directed to a method an apparatus for modulating light, and more particularly to an integrated modulator for modulating laser light that is effective over a wide wavelength range.  
     BACKGROUND  
      Optical communications systems are typically based on light that is generated continuously from a laser, where the light is modulated in an external modulator. The external modulator may be, for example, a Mach Zehnder modulator formed on lithium niobate substrate, having an input waveguide coupled to the output form the laser. For ease of fabrication, however, it is preferred to integrate the modulator on the same chip as the laser, in which case the modulator is formed from a semiconductor material.  
      One form of readily integrable modulator is the electro-absorption modulator. An electro-absorption modulator may be formed in the waveguide that receives the output light from the laser, although it may also be formed as a separate component. An electro-absorption modulator uses a semiconductor material whose band gap, under normal conditions, is larger than the energy of the photons emitted by the laser. Therefore, the light output from the laser is transmitted through the modulator. When the modulator material is subjected to an electric field, however, the band gap reduces due to an electro-optic effect. When the modulator waveguide material includes bulk semiconductor materials, band gap reduces due to the Franz-Keldysh effect. When the modulator includes one or more semiconductor quantum wells, the band gap reduces as a result of the Stark effect.  
      If the band gap reduces by a sufficient amount, the band gap may become equal to or less than the photon energy of the light output from the laser, in which case the laser light is absorbed in the modulator. Therefore, application of a modulating voltage to the electro-absorption modulator results in a corresponding modulation in the light emitted from the modulator. The electro-absorption modulator is often fabricated in the form of a reverse-biased diode. The electro-absorption modulator has advantages over a Mach-Zehnder modulator formed in semiconducting material because the insertion loss of the electro-absorption modulator is lower and the electro-absorption modulator is typically shorter than a Mach-Zehnder modulator by about a factor of ten. Furthermore, the drive voltage for an electro-absorption modulator is typically around two volts, while a Mach Zehnder formed in semiconducting material may require ten volts.  
      In considering the design of an electro-absorption modulator, it is advantageous that signal absorption in the modulator is minimal when the modulator is in the transmissive state and that signal absorption is close to 100% when the modulator is in the absorptive state. Therefore, the band gap of the unexcited modulator material is preferably significantly higher than the energy of the laser photons so that transmission in the transmissive state is as high as possible.  
      On the other hand, the band gap of the unexcited modulator should not be too much greater than the photon energy. The band gap varies approximately linearly with electric field applied across the modulator. Consequently, in order to maintain constant extinction ratio, an increasingly large voltage has to be applied to the electro-absorption modulator when the band gap of the unexcited modulator material is significantly larger than the laser photon energy. The requirement of large modulation voltage reduces the bandwidth of the modulator. If the applied voltage is not increased, then the extinction ratio of the modulator may be reduced. Large modulation voltages are not desirable, since the drive electronics become more complex and consume more power, and the modulator itself suffers from increased heating.  
      The task of the designer, therefore, is to select a material for the electro-absorption modulator whose band gap is not so small as to produce significant transmission-state losses, nor so large as to require a large drive voltage. It has been found that a satisfactory compromise in band gap energy is that the unexcited semiconductor material has a band gap that differs in energy from the laser output by approximately 0.033 eV. This difference between modulator band gap and the laser photon energy is referred to as detuning. For optical communications lasers operating at about 1550 nm, the detuning corresponds to a wavelength difference of approximately 65 nm.  
     SUMMARY OF THE INVENTION  
      There are difficulties, however, when the output wavelength of the laser is tunable. For example, distributed Bragg reflector (DBR) lasers are typically tunable over a range of about 10 nm, and grating coupled, sampled reflector (GCSR) lasers are tunable over several tens of nm. Tuning such lasers moves the laser out of the optimum regime for operating the electro-absorption modulator. If tuned sufficiently far, the laser may tune into a region where the modulator absorbs the output even without an applied electric field, thus reducing the extinction ratio. Furthermore, the laser may tune into a region where the applied voltage required for a particular level of attenuation is significantly higher than at other wavelengths. In general, it is more difficult to produce higher drive voltages at high modulation frequencies, and so tuning the laser may result in reduced modulation bandwidth for the new wavelength. Alternatively, if the voltage is kept constant, the absorption of the light in the modulator under applied voltage may be incomplete, thus reducing extinction ratio.  
      There is, therefore, a need to reduce or avoid the deleterious effects of tuning the laser away from optimum detuning with the electro-absorption modulator. Furthermore, it is desirable that the modulation voltage applied to the electro-absorption modulator to achieve a selected extinction ratio be constant, regardless of wavelength.  
      Generally, the present invention relates to an electro-absorption modulator that is tunable along with the laser. Tuning the electro-absorption modulator permits optimum detuning to be maintained, even though the laser is tuned over several tens of nm. One approach to tuning the electro-absorption modulator is to heat the electro-absorption modulator.  
      One particular embodiment of the invention is directed to a semiconductor laser device that has a substrate and a semiconductor laser positioned on the substrate. The semiconductor laser produces output light that is tunable over a tuning range between a first wavelength and a second wavelength. An electro-absorption modulator is disposed to modulate the light produced by the semiconductor laser. The operating temperature of the electro-absorption modulator is tunable so as to maintain constant detuning over at least a portion of the tuning range.  
      Another embodiment of the invention is directed to an optical communications system that includes an optical transmitter, and optical receiver and an optical communication link coupled between the optical transmitter and the optical receiver. The optical transmitter has at least one laser operable at a plurality of wavelengths. The laser includes a substrate and a semiconductor laser positioned on the substrate and producing output light tunable over a tuning range between a first wavelength and a second wavelength. The laser also includes an electro-absorption modulator disposed to modulate the output light produced by the semiconductor laser, an operating temperature of the electro-absorption modulator being tunable to maintain constant detuning over at least a portion of the tuning range.  
      Another embodiment of the invention is directed to a method of operating a semiconductor laser modulated by an electro-absorption modulator. The method includes tuning output light from the semiconductor laser to a desired wavelength between first and second wavelengths and directing the output light through an electro-absorption modulator. The method also includes adjusting an operating temperature of the electro-absorption modulator so as to achieve a preselected extinction level in the electro-absorption modulator.  
      Another embodiment of the invention is directed to a laser device that includes means for tuning output light from the semiconductor laser to a desired wavelength between first and second wavelengths and means for directing the output light through an electro-absorption modulator. The laser device also includes means for adjusting an operating temperature of the electro-absorption modulator so as to achieve a preselected extinction level in the electro-absorption modulator.  
      Another embodiment of the invention is directed to a modulator that has a waveguide positioned on a substrate, the waveguide passing through an electro-absorption modulator disposed on the substrate. A heater is disposed on the substrate proximate the electro-absorption modulator.  
      The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
       FIG. 1  schematically illustrates a semiconductor laser integrated with an electro-absorption modulator according to the present invention;  
       FIG. 2  presents a graph showing various characteristics of the laser module according to the present invention;  
       FIG. 3  illustrates the cross-sectional view AA′ of the device in  FIG. 1  mounted on a heatsink according to the present invention.;  
       FIG. 4  illustrates another embodiment of mounting a laser module of the present invention on a heatsink; and  
       FIG. 5  schematically illustrates an optical communications system that incorporates a tunable electro-absorption modulator according to the present invention. 
    
    
      While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
     DETAILED DESCRIPTION  
      The present invention is applicable to electro-absorption modulators that are required to modulate light at different wavelengths, and is believed to be particularly suited to modulating the output of a tunable laser.  
      One particular embodiment of the invention is shown in schematic form in  FIG. 1 . A laser  102  is positioned on a chip substrate  100 . The laser typically includes at least one bonding pad  104  for the drive current and may include other bonding pads  106  that connect to different wavelength control regions of the laser  102 . The laser  102  may be any type of tunable diode laser, for example a distributed Bragg reflector (DBR) laser, a grating coupled, sampled Bragg reflector (GCSR) laser. The laser may also be a dual sampled DBR laser with vernier gratings, for example as is taught in U.S. Pat. No. 4,896,325. Typically, a tunable diode laser includes an active section, where a drive current produces optical gain, and a tuning section where one or more tuning currents control the operating wavelength of the device.  
      The light output from the laser  102  propagates along a waveguide  108  through an electro-absorption modulator  110 . The waveguide may be integrated with the laser  102 . The electro-absorption modulator  110  is typically provided with at least one bond pad  112  for connecting to a modulation voltage source.  
      A heater  114  may be disposed on the substrate  100  close to the electro-absorption modulator  110 . The heater  114  may be of any suitable type. In the particular embodiment illustrated, the heater  114  is a thin film resistor. One advantage of providing the heater  114  as a thin film resistor is that little extra wafer processing is required, since a suitable resistor material, namely the electrically conductive material used to form the bond pads, is already part of the wafer fabrication process.  
      The heater  114  may be connected to one or more bond pads  116  and  118  for connecting to a source for the heating current. The heating current may be a dc current or an ac current. The heater may previously have been calibrated so that the user knows the operating temperature of the electro-absorption modulator  110  for a given heating current and substrate temperature. A temperature sensor  120  may optionally be placed close to the electro-absorption modulator  110  to measure the temperature of the material close to the modulator  110 , and thus derive an estimate of the modulator temperature. The temperature sensor  120  may be connected to a bond pad  121  for coupling to external electronics.  
      The operating temperature of the electro-absorption modulator  110  may be used to control the band gap of the material that forms the electro-absorption modulator  110 . The heater current may be changed at the same time that the laser  102  is tuned from one wavelength to another, so as to change the modulator band gap along with the laser&#39;s operating wavelength.  
      The graph illustrated in  FIG. 2  shows several characteristics of the laser/electro-absorption modulator combination. All curves are shown plotted against channel number. The left hand axis shows wavelength in nm. The right hand axis shows two parameters, namely current in mA and modulator temperature in ° C. The first curve, curve  200 , shows the laser wavelength as a function of channel number. There is a one-to-one mapping of channel number to wavelength. The channel numbers may represent optical channels having frequencies set according to the International Telecommunications Union (ITU) standards.  
      The second curve, curve  202 , illustrates the wavelength that corresponds to the modulator band gap under no heating. As can be seen, the difference between this curve  202  and the laser wavelength, curve  200 , increases with increasing channel number. In other words, the detuning varies as a function of laser operating wavelength.  
      The third curve  204  shows the heater current as a function of channel number. The heater current is assumed for a 100 Ω heater at 80 W/K. The fourth curve  206  shows the modulator temperature that results from heating. The fifth curve  208  shows the wavelength that corresponds to the modulator band gap with the heater operating. For InP-based semiconductors, the wavelength change with temperature is approximately 0.5-0.6 nm/K. For the case illustrated, the modulator band gap wavelength increases with channel number at approximately the same rate as the laser wavelength illustrated in curve  200 . Therefore, the variation in the detuning, in other words the difference between curves  200  and  208 , over a large number of channels is reduced relative to the variation in detuning when the heater is not used to heat the electro-absorption modulator, i.e. between curves  200  and  202 . With judicious control of the heater current, the detuning may be kept approximately constant across a large number of channels, if not the entire tuning range.  
      Heating the electro-absorption modulator should advantageously not affect the operation of the laser  102 . In particular, the heat provided to the electro-absorption modulator  110  by the heater  114  may affect, for example, the operating temperature of the laser  102 . It is generally preferred that the operating wavelength of the laser  102  be tunable independent of the operating temperature of the electro-absorption modulator  110 .  
      One approach to reducing heat flow from the heater  114  to the laser  102  is to insert thermal resistance to the heat conduction path between the heater  114  and the laser  102 , is described with reference to  FIGS. 1 and 3 . In the particular embodiment illustrated, one or more slots  122  and  124  are cut into the substrate  100 . The side faces of the slots  122  and  124  are referred to as faces and  128 . The slot faces  126  and  128  may be seen more clearly in  FIG. 3  which shows section AA′. The slots  122  and  124  increase the length of the path that heat has to travel between the heater  114  and the laser  102 , thus reducing adverse heating effects on the laser  102  that arise due to the heater  114 .  
      Where the band gap wavelength tunes at a rate of 0.5-0.6 nm/K, the band gap is tuned over a range of about 20-24 nm by raising the temperature of the modulator by 40 K. Without taking any precautions, some heat may be transferred from the temperature-tuned modulator to the laser, which might result in affecting the operating wavelength of the laser, for example by changing the effective optical length of the laser cavity, thus tuning the Fabry/Perot mode or modes. This need not be a significant problem if the period over which the wavelength of the laser and electro-absorption modulator are tuned is in the microsecond range or longer. Furthermore, the laser  102  may include a phase control section that may be adjusted to compensate for any increase in temperature of the laser  102  that occurs due to the modulator heater  114 .  
      The substrate  100  may be mounted on a heatsink  300  which removes heat from the substrate  100 . The heatsink  300  may be formed from any suitable material, including ceramics such as aluminum nitride, beryllia, or alumina, metals such as copper or titanium/tungsten, or semiconductor materials such as silicon.  
      The effect of temperature tuning the electro-absorption modulator on the laser may be reduced by increasing the thermal resistance between the modulator and the laser and by making the modulator easy to heat. For example, the modulator may be made to be have as low a thermal mass as possible, and should be placed very close to the modulator.  
      Another approach to preventing the heater from affecting the operation of the laser is illustrated in  FIG. 4 . In this case, a cross-section through the substrate  400  is given, showing the electro-absorption modulator  408  and the heater  414  on the bottom surface of the substrate. This configuration is commonly referred to as “p-side down”, since the p-doped layer of the laser is mounted on the heatsink  420 . The substrate  420  may be provided with a groove  422 . The groove  422  may be sufficiently large so as to provide clearance for contacts to the individual components on the ship substrate  400 .  
      One of the advantages of the p-side down configuration is that the heat generated at the laser and at the heater does not have to pass through the substrate to be extracted. Another advantage is that the thermal path length between the heater  414  and the heatsink  420  may be made to be very short, thus significantly reducing the amount of heat that is conducted over the longer path length to the laser. An advantage provided by using the groove  422  is that a fiber may conveniently be positioned in the groove  422  for coupling to the waveguide  408  to receive the modulated output light from the chip  100 .  
      One particular embodiment of a communications system that utilizes a laser having an integrated electro-absorption modulator is illustrated in  FIG. 5 . The communications system  500  includes a transmitter  502  and a receiver  504  coupled through a fiber communications link.  506 . The transmitter  502  includes a laser module  508 . The laser module  508  includes a laser and an electro-absorption modulator. A controller  510  is connected to the laser module  508  to control operation of the laser. The controller  510  may include a current control unit  512  to control the drive current to the laser, and a tuning control unit  514  to control the wavelength emitted by the laser. The controller  510  may also include a heater control unit  516  that controls the temperature of the heater, thus controlling the band gap of the electro-absorption modulator. The heater control  516  may typically be operated in tandem with the tuning control unit  514 , so that the band gap of the electro-absorption modulator is adjusted when the operating wavelength of the laser is tuned, so as to maintain optimum, or near optimum, detuning. A modulator controller  517  may be coupled to provide a modulation signal to the electro-absorption modulator.  
      Where the transmitter  502  includes more than one light source, the output from the laser module  508  is combined with the output from the other sources (not shown) in a combiner  518 . The combiner  518  may be a wavelength division multiplexer (WDM), a polarization division multiplexer, or any other suitable type of combiner.  
      The output from the laser module  508 , combined with the output from any other light source in the transmitter  502 , propagates along the fiber link  506  to the receiver  504 . The fiber link  506  may include various switching elements, such as one or more add/drop filters  520 , and one or more amplifiers  522 .  
      The signal is detected in the receiver by at least one detector  526 . Where more than one detector is used in the receiver  504 , the signal may pass into a demultiplexer (DMUX)  528  where it is split into respective wavelength components and directed to different detectors  526 ,  530  corresponding to the different components.  
      As noted above, the present invention is applicable to electro-absorption modulators and is believed to be particularly useful for electro-absorption modulators used in conjunction with tunable lasers. The present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.  
      For example, the tunable electro-absorption modulator need not be integrated with a laser, but may be integrated with other components, or may be an independent component. It will be appreciated that the electro-absorption modulator is formed from semiconductor material that is useful for the desired wavelength range. For example, where the electro-absorption modulator is used for modulating an optical communications signal in the range 1500 nm -1650 nm, the electro-absorption modulator may be formed from an InP-based semiconductor. A tunable electro-absorption modulator may be formed from other suitable types of semiconductor material for operating at different wavelength ranges. For example, for operation in the range 800-900 nm, the electro-absorption modulator may be formed from a GaAs-based semiconductor material. Furthermore, the electro-absorption modulator may be based on bulk semiconductor material or may be based on the use of one or more quantum wells.