Abstract:
An apparatus containing an integrated optical circuit that enhances phase stability of the injection-locking process of two slave lasers. The integrated optical circuit helps to reduce phase noise by keeping environmental or mechanical perturbations uniform everywhere on that circuit. Also, the integrated optical circuit provides connections for additional components to be coupled, which can monitor and control the performance characteristics of the integrated optical circuit and the injection-locking process.

Description:
CROSS REFERENCE TO RELATED APPLICATION.  
       [0001]    The present document claims the benefit of U.S. Provisional Application Ser. No. 60/373,742, filed Apr. 17, 2002, the contents of which are incorporated by reference herein.  
         [0002]    The present document is related to the copending and commonly assigned patent application document entitled “Low-Noise, Switchable RF-Lightwave Synthesizer,” Serial No. 60/373,739. The contents of this related application is hereby incorporated by reference herein. 
     
    
     
       FIELD  
         [0003]    The present invention relates to the generation of microwave signals. More specifically, this invention relates to an integrated optical circuit that enables the stable injection locking of two distributed feedback (DFB) diode lasers, whose outputs can be converted into microwave signals.  
         BACKGROUND  
         [0004]    Frequency synthesis is used to generate &#39;signals at one or more precise frequencies. These signals may then be used to perform frequency conversion in radio frequency (RF) sensor and communication systems. Frequency synthesis may be provided by several different methods. Of concern in frequency synthesis, are the phase, frequency and amplitude stability of the generated signal. Since the generated signal may be used as a local oscillator signal for frequency up-conversion or down-conversion, instability in the signal results in decreased signal-to-noise performance.  
           [0005]    One method of frequency synthesis involves the generation of a multiple tone lightwave signal that can be converted into a RF carrier or local oscillator signal. In this method, optical heterodyning is used to create a sum or difference beat frequency from two optical wavelength tones. The sum or difference beat frequency is detected by a photodetector or similar apparatus to generate an RF carrier or local oscillator signal. However, the stability of the beat frequency signal is limited by the relative stability of each of the optical wavelength tones.  
           [0006]    In, R. Logan, R. D. Li, Final Technical Report for DARPA Program, “Radio Frequency Photonic Synthesizer” an optical heterodyning circuit, shown in FIG. 1, is disclosed. This circuit contains a mode locked laser  100 , two DFB lasers  102 ,  104 , an optical splitter  122 , two optical circulators  114 ,  116 , an optical combiner  124 , two photodetectors  110 ,  112  connected to power-control feedback circuits  106 ,  108 , and two Mach-Zender modulators  118 ,  120 . The components of this optical circuit are interconnected using optical fiber. As the figure shows, the optical comb generated by a master laser (which in this case is a mode-locked laser) is split by a power splitter  122  and then is sent, via optical fibers, to injection-lock a pair of slave lasers, DFB-laser- 1   102  and DFB-laser- 2   104 . Each of these slave lasers would become injection-locked to a line of the optical comb if the spacing or detuning between the free-running laser&#39;s lasing wavelength and that line of the optical comb is less than the laser&#39;s lock-bandwidth, a wavelength range determined by the injected power Pi. In particular, temperature tuning (˜0.1 nm/° C.) was used to change the slave lasers lasing wavelengths, so that they were tuned to within the injection-locking bandwidth of two selected lines in the incident optical comb. In between the master and slave lasers, a pair of Mach-Zender modulators  118 ,  120  is used as variable attenuators to provide a way to adjust the injected optical power P i . The optical outputs from the Mach-Zender modulators  118 ,  120  are fed to port  1  of two optical circulators  114 ,  116 . The optical signals from the two slave lasers  102 ,  104  are used for heterodyning and are obtained through port  2  of those optical circulators  114 ,  116 . The slave laser outputs from port  2  of the two circulators  114 ,  116  are subsequently combined by combiner  124  and then sent to a photodetector (PD 3 ) for heterodyning.  
           [0007]    As shown in FIG. 1, the photonic components (optical isolators, Mach-Zender modulators for controlling injected optical power, optical circulators, and optical combiner) used to generate the optical outputs were all fiber-pigtailed/connectorized. Such use of optical fiber to interconnect multiple discrete components results in sensitivity to environmental disturbances such as mechanical or temperature perturbations, which ultimately cause reduced phase stability. In particular, the use of optical fiber links to interconnect the various components of FIG. 1 makes it difficult to keep the optical path lengths of arms I and II equal or balanced over the long term. Since the length of a section of optical fiber typically can be cleaved to an accuracy of only several millimeters, it is difficult to control and balance the overall path lengths of arms I and II to an accuracy of better than 1-2 cm. Also, because the fibers were not co-located physically, environmental perturbations (such as temperature or mechanical disturbances) could cause differential phase fluctuations between the optical inputs to the slave lasers. Theoretically, the phase noise (δφ 1  and δφ k , |i−k|=n) of the ith and kth lines in the optical comb generated by a mode-locked laser are given by:  
           δφ I =(δφ O ) I   +iδφ   R   (1)  
           δφ k =(δφ O ) k   =kδφ   R   (2)  
           [0008]    In equation 1 and 2 (δφ O ) I,k  and δφ R  are, respectively, the phase fluctuations in the mode-locked laser and the RF-source driving the mode-locked laser. For a high-quality RF source and for lines in the optical comb that correspond to higher order modes of the mode-locked laser, the magnitude of δφ O  is much larger than δφ R . The phase noise of the microwave signal (f s =nf R ) generated via optical heterodyning of diodes  1  and  2  is then given by;  
           [0009]    δφ 21 (f)=[δφ I (f)−δφ k (f)]+(residual due to the process of injection locking).  
           [0010]    If the optical path lengths of arms I and II are identical, then δφ O , i.e. (δφ O ) I =iδφ O ) k , is common to the injection locking of diodes  1  and  2 , and δφ I (f)−δφ k (f)=nδφ R (f), where |i−k|=n. We thus obtain the minimum value of [δφ I (f)−δφ k (f)], and the best phase-noise for the microwave signal f s . From the above discussion, it is obvious that one needs to keep the optical path lengths of  1  and  2  equal and stable to attain the lowest phase noise for the microwave signal f s  that we generate via optical heterodyning. Likewise, optical phase stability in the output paths of DFB lasers  1  and  2  (e.g. from the output of the DFB laser through the associated optical circulator and to the optical combiner) translates into amplitude stability for the optically synthesized microwave signal. In the prior art described above, it was difficult to maintain differential phase stability between the fiber links of arms I and II. This weakness, in turn, deters the field deployment of the photonic synthesizer described in ref.  1 . Therefore, there is a need in the art for a photonic synthesizer which can increase phase stability in different environments.  
         SUMMARY  
         [0011]    Optical heterodyning of the optical outputs of two separate lasers to produce an electronic signal is a technique well known in the art. However, one of the major problems with optical heterodyning, which this invention solves, is the phase drift associated with the optical signals used in and produced for the heterodyning process. This phase drift results in a degradation of the phase and amplitude noise associated with the heterodyned signal.  
           [0012]    The present invention provides an integrated optical circuit that enables the stable injection locking of a first and second slave laser to lines in an optical comb generated from a master laser, whose lasing lines are locked in phase. An optical comb is comprised of a series of optical lines, where each of the lines is at a different frequency. By injection-locking the first and second slave lasers to two different lines in the optical comb, and then heterodyning the slave lasers&#39; outputs, one can synthesize microwave signals (f s ) over a wide frequency range.  
           [0013]    For example, if the master laser is a mode-locked fiber laser, one can synthesize frequencies that range from a minimum frequency equal to the mode-locking frequency of f s =f m , to a maximum frequency of f s ˜100 GHz. Specifically, the upper frequency limit of f s  is determined by the spectral width of the mode-locked optical pulses. For this example, the theoretically predicted phase noise (at f s ) that one can accomplish using the present invention is the sum of:  
           [0014]    (i) the phase noise due to the RF-oscillator (at f m ), scaled by the n 2 −law, where n=f s /f m , and  
           [0015]    (ii) a phase noise incurred by the injection-locking process.  
           [0016]    One advantage of the present invention over the prior art lies in the exploitation of integrated optics for the interconnection of the photonic components. Specifically, the splitters, directional couplers, and waveguides of the present invention are all integrated on a common substrate and located close to each other. As mentioned earlier, the theoretically predicted phase noise is the sum of the phase noise due to the RF-oscillator and the phase noise incurred by the injection locking process. As a result, by integrating the components, environmental disturbances such as mechanical vibrations and temperature changes which cause degradations to the amplitude stability and phase noise of the synthesized RF-signal are common to all components, which helps reduce the phase and amplitude instability. Also, because the waveguides to/from the slave lasers are formed on a common substrate and in proximity to each other, the relative phase-drifts between the above components is reduced. Therefore, we can expect long term differential phase stability in the optical injection inputs supplied to the first and second slave lasers. In addition, the two optical signals (the slave laser outputs) to be heterodyned are combined in a common waveguide inside the integrated optical chip. The prior art combines the optical signals using discrete components interconnected by optical fiber, which decreases phase stability.  
           [0017]    It is therefore an object of the present invention to provide an integrated optical circuit. The integrated optical circuit is formed on a single substrate, receives an optical comb preferably generated from a mode-locked master laser, and transfers the optical comb to a first and a second slave laser, using a first and a second waveguide path, and a plurality of optical couplers. The first and second slave lasers produce first and second laser outputs which are coupled to an optical coupler using the first and second waveguide paths. The optical coupler combines the first laser output and the second laser output creating at least one combined optical output. The combined optical output is then preferably sent to a primary or secondary output port where the combined optical output can be used for heterodyning.  
           [0018]    It is also an object of this invention to use optical couplers instead of Mach-Zender modulators and optical circulators. The optical couplers are preferably 2×2 directional couplers. The use of directional couplers, which have 4 inputs/outputs, provides the integrated optical circuit of this invention with monitoring and control output ports located on the substrate that can be used to monitor chosen characteristics and to implement external control circuits which enhance the performance of the integrated optical circuit.  
           [0019]    The directional couplers of this invention as aforementioned, are preferably 2×2 directional couplers, but other optical couplers such as X-junction couplers, or multimode interference couplers could be used as well. The 2×2 directional couplers have two ports on each side of the directional coupler, which can be used as either an input or an output for light. Furthermore each port is not bound to remain as an input or an output permanently. Each port has the ability to function as both an input and an output, depending on whether light is entering or exiting the port. If light enters the port, it is acting as an input, and when light exits the same port, the port is acting as an output. In this way, a 2×2 directional coupler has 4 inputs and 4 outputs, or any combination thereof. A 2×2 directional coupler is well known in the art. However, bi-directional coupler use of a 2×2 directional coupler in which a given port is used as both an input and an output is not a matter of normal routine.  
           [0020]    It is also an object of the present invention to provide optical waveguides having substantially shortened lengths. One problem associated with optical heterodyning, which this invention solves, is that as the length of the optical path increases, the phase stability decreases. The prior art as mentioned earlier, uses components that are pigtailed and connected with lengths of optical fiber. It is very likely that when implementing the prior art circuit, it will have lengths of optical-fiber paths that are on the order of meters to 10 s of meters long. This is a problem because as mentioned earlier, when the optical path length increases the phase stability decreases. By integrating the components on an integrated optical circuit, it is possible to reduce the optical path length to a length in the range of 6-20 centimeters. This difference in optical path lengths using discrete components and integrating the components is roughly a factor of 100. In addition, it is difficult to integrate all of the components of the prior art circuit on a common substrate. The optical circulators of the prior art are generally bulky items and are especially difficult to integrate.  
           [0021]    It is also an object of this invention to provide optical waveguides having substantially equal lengths. This is one advantage of using the present invention over the prior art. In the present invention the integrated optical circuit is created using photolithography techniques, which achieve sub-micron accuracy, to ensure that the two optical paths which carry the two optical signals to be heterodyned are of substantially equal lengths. Also, the components of the integrated optical circuit are completely symmetrical about an axis on that circuit. By creating symmetrical waveguide paths with substantially equal lengths on a common substrate, the phase instability is greatly reduced.  
           [0022]    It is also a further object of this invention to provide additional output ports for monitoring and adjusting performance characteristics of the integrated optical circuit. As aforementioned, one advantage to using 2×2 directional couplers is that the directional couplers provide additional output ports. External devices may be connected to these additional output ports, which can monitor and control different characteristics of the integrated optical circuit. One device that may be connected is the a Fabry-Perot etalon, and another is a fixed Fabry-Perot etalon. The Fabry-Perot etalon can be used to monitor the slave lasers&#39; lasing wavelengths and compare them to the desired wavelengths in the optical comb. One or more Fabry-Perot etalons can be connected to a processor that can adjust the slave lasers so that they lase at the desired wavelengths, approximately aligned with the selected wavelengths in the optical comb.  
           [0023]    It is also a further object of this invention to provide an integrated optoelectronic module for generation of optical signals for heterodyning to synthesize microwave signals. This integrated optoelectronic module comprises the integrated optical circuit hybrid integrated with one or more injection-power control feedback circuits. The feedback circuit is coupled to one or more of the output ports of the integrated optical circuit. The feedback circuit contains one or more photodetectors that measure the intensity of the optical comb and/or the slave laser output. By monitoring the photocurrents, the feedback circuit can adjust the power level of the optical-comb light that is injected into a slave laser as well as the relative optical-power levels of the two slave-laser outputs as they are combined for heterodyning.  
           [0024]    It is also a further object of this invention to provide heterodyne and homodyne phase lock loops for monitoring and adjusting the integrated optical circuit. The heterodyne and homodyne phase lock loops are coupled to the output ports of the integrated optical circuit. The heterodyne phase lock loop is used to ensure that the beat frequency of the slave lasers is kept constant. If the beat frequency changes, the heterodyne phase lock loop provides a feedback system that can adjust one of the slave lasers and regain the desired beat frequency. A homodyne phase lock loop is used to adjust the lasing wavelength of the other slave laser to match a selected line in the optical comb. If the lasing wavelength of that slave laser changes, the homodyne phase lock loop provides feedback that readjusts that slave laser to lase at the desired wavelength in the optical comb. Furthermore, by using homodyne and heterodyne phase lock loops with an integrated optical circuit, loop bandwidths on the order of 10s of KiloHertz can be achieved. In contrast, homodyne and heterodyne phase lock loops comprising discrete components connected by optical fiber only achieve loop bandwidths on the order of 100s of Hertz.  
           [0025]    In summary, the integrated optical circuit of this invention, which has optical waveguides, a splitter and directional couplers formed on the same chip, allows us to:  
           [0026]    (i) have a much more compact physical size, for reduced sensitivity to environmental perturbations;  
           [0027]    (ii) form a feedback circuit that will modify the power injected to each DFB laser for optimal injection locking;  
           [0028]    (iii) ensure that the power of each of the combined laser output signals going to the output ports for heterodyning is equal, for increased efficiency of frequency synthesis; and  
           [0029]    (iv) have shorter optical and electrical delays for improved phase locking using phase-lock loops.  
           [0030]    As a result, synthesized signals will have increased phase and amplitude stability.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 shows a photonic synthesizer according to the prior art;  
         [0032]    [0032]FIG. 2 a  shows an integrated optical circuit according to the present invention;  
         [0033]    [0033]FIG. 2 b  shows a photonic microwave synthesizer based on the integrated optical module according to the present invention;  
         [0034]    [0034]FIG. 3 shows a block diagram showing the path of the optical comb and the first and second laser output according to the present invention;  
         [0035]    [0035]FIG. 4 shows the integrated optical circuit according to an alternate embodiment of the present invention;  
         [0036]    [0036]FIG. 5 shows a block diagram showing the path of the optical comb and the first and second laser output according to an alternate embodiment of the present invention;  
         [0037]    [0037]FIG. 6 a  shows an integrated optoelelectronic module for photonic microwave synthesis according to an alternate embodiment of the integrated optical circuit;  
         [0038]    [0038]FIG. 6 b  shows an alternate embodiment of the integrated optoelelectronic module for photonic microwave synthesis according to an alternate embodiment of the integrated optical circuit;  
         [0039]    [0039]FIG. 7 shows a hybrid integration approach for constructing the integrated optoelectronic module of the present invention; and  
         [0040]    [0040]FIG. 8 shows a larger version of the 2×2 directional couplers used. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0041]    The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.  
         [0042]    An apparatus for generating a microwave signal by optical heterodyning that has improved phase and amplitude stability according to a first embodiment of the present invention is described with reference to FIG. 2 a , FIG. 2 b , and FIG. 3, blocks  2 - 28 . This apparatus comprises an integrated optical circuit, illustrated in FIG. 2 a . Use of the integrated optical circuit to form a photonic synthesizer of microwave signals is illustrated in FIG. 2 b . In this apparatus, an integrated optical circuit  201  couples optical energy from a master laser  203  to a first and second slave laser  216 ,  218 . The integrated optical circuit  201  contains two ports  219 ,  221  at which the first and second slave lasers  216 ,  218  are butt coupled to the integrated optical circuit  201 . The first and the second slave laser  216 ,  218  each receive the optical energy.  
         [0043]    The first slave laser  216  produces a first laser output based on the physical characteristics of the first slave laser  216  and the received optical energy. The second slave laser  218  produces a second laser output based on the physical characteristics of the second slave laser  218  and the received optical energy. The first and second laser outputs are then combined in a directional coupler  222  creating a primary and a secondary combined laser output. The primary combined laser output is sent to at least one primary output port  220  where the primary combined laser output can be used for heterodyning. The secondary combined laser output is preferably also sent to one or more secondary output ports  230  on the integrated optical circuit  201 , where the secondary combined laser output can be used for monitoring or feedback control.  
         [0044]    The components of the integrated optical circuit  201  consist of an optical splitter  202 , preferably a 1:2 optical splitter, a first directional coupler  212 , a second directional coupler  214 , a combining directional coupler  222 , and lengths of optical waveguide that interconnect those components with each other and with input/output ports of the integrated optical circuit  201 . First and second directional couplers  212 ,  214  and the combining directional coupler  222  each have two sides with two ports on each side. The directional couplers can conduct light from the master laser  203  to the first and second slave laser  216 ,  218 , hereinafter referred to as the forward direction, as well as from the first and second slave laser  216 ,  218  to the primary output port  220  and the secondary output port  230 , hereinafter referred to as the reverse direction. A directional coupler is a well-known device in the art. It can be adjusted electrically to split the light from either one of its inputs into its outputs according to a selected ratio. It is preferred to have the powers of the first and second laser outputs be equal as they are combined and delivered to the primary output port  220  for more efficient heterodyning. This can be accomplished by adjusting the first and second directional coupler  212 ,  214  and the combining directional coupler  222 . First and second directional coupler  212 ,  214  also can be adjusted to achieve the desired optical powers P i  of the optical comb injected into first and second slave lasers  216 ,  218 , respectively. The integrated optical circuit  201  is preferably constructed from a GaAs, LiNbO 3 , InP, or Si substrate. For this invention, we prefer to use 2×2 directional couplers for the first, second, and combining directional couplers  212 ,  214 ,  222 .  
         [0045]    The components of the apparatus are connected by a first waveguide path  215 , and a second waveguide path  217 . The first waveguide path  215  is formed between one of the two outputs of optical splitter  202 , the first directional coupler  212 , and the first slave laser  216 . The first waveguide path  215  also includes the lengths of optical waveguide that interconnect the combining directional coupler  222 , and the first slave laser  216 . A second waveguide path  217  is formed between the other of the two outputs of optical splitter  202 , the second directional coupler  214 , and the second slave laser  218 . The second waveguide path  217  also includes the lengths of optical waveguide that interconnect the combining directional coupler  222 , and the second slave laser  218 .  
         [0046]    The master laser  203  of the apparatus  200  is preferably a mode-locked laser. The optical energy generated by the master laser  203  is preferably an optical comb, which is a series of optical lines at different frequencies. The spacing between the lines is determined by a RF oscillator  260  (shown in FIG. 6 b ), which drives the master laser  203 . The output of the master laser  203  is sent through an optical isolator  205  to prevent reflected power from destabilizing the optical comb.  
         [0047]    The first and second slave laser  216 ,  218  of the apparatus are preferably distributed feedback (DFB) lasers. The first and second slave laser  216 ,  218  are preferably each tuned to a different optical line of the optical comb. By tuning the first and second slave laser  216 ,  218  to different optical lines, they produce a first and second laser output, which when combined, can then be used to generate a microwave signal by optical heterodyning.  
         [0048]    For the embodiments of this invention, described in FIGS.  2 - 7 , the optical splitters, waveguides and directional couplers formed on integrated optical circuit  201  are preferably manufactured using photolithography, which has sub-micron accuracy. This ensures that the first and second waveguide paths  215 ,  217  are of substantially equal lengths. Also, the components of the integrated optical circuit  201  are preferably laid out in a symmetrical fashion, with the axis of symmetry shown by the dashed line in FIGS.  2   a ,  2   b ,  4 ,  6   a  and  6   b . This is another technique to help ensure that the first and second waveguide paths  215 ,  217  are of equal length.  
         [0049]    The transverse spacing between any two sets of directional couplers, for example the first and second directional coupler  212 ,  214  of the integrated optical circuit  201 , that are placed in parallel can be as small as approximately 0.05-0.10 mm and still ensure that those directional couplers do not interact with each other, except through their inputs and outputs. Thus, the maximum separation between the first and second waveguide paths  215 ,  217  of the embodiment shown in FIGS. 2 a  and  2   b  can be as small as 0.2 mm. Similarly, the maximum transverse separation between the first and second waveguide paths  215 ,  217  of the embodiments shown in FIGS. 4, 6 a  and  6   b  can be as small as 0.4 mm, but is generally less than 10 mm. This close proximity of the first and second waveguide paths  215 ,  217  ensures that they see the same environment, which results in improved phase stability.  
         [0050]    Referring now to FIG. 3, blocks  1 - 28 , the path of the optical comb and the first and second optical output can be followed throughout the apparatus shown in FIG. 2 a . The optical comb generated by the master laser  203  (See block  2 ) first goes through an optical isolator  205  (See block  4 ) to prevent power from being reflected. The optical comb enters the integrated optical circuit  201  at an input port  207  to a center waveguide  213  (See block  6 ). After entering the integrated optical circuit  201  through center waveguide  213  (See block  6 ), the optical comb enters the optical splitter  202  where the optical comb is divided between two outputs of the optical splitter  202  (See block  8 ) into first and second waveguide paths  215 ,  217 . The first and second waveguide paths  215 ,  217  carry the optical comb to the first and second slave laser  216 ,  218 , respectively, traveling through first and second directional coupler  212 ,  214 , respectively. The first and second slave laser  216 ,  218  generate a first and second laser output. The first and second waveguide paths  215 ,  217  carry the first and second laser output from first and second slave laser  216 ,  218  to the combining directional coupler  222 , again traveling through first and second directional coupler  212 ,  214 , respectively.  
         [0051]    The divided optical comb from optical splitter  202  enters the first and second directional coupler  212 ,  214  (See blocks  10 , 12 ). Both the first and second directional coupler  212 ,  214  each split the optical comb into two outputs. The first and second directional coupler  212 ,  214  each have an output that is connected to the first and second slave laser  216 ,  218 , respectively. The other output of first and second directional couplers  212 ,  214  are connected to output ports  226 ,  228  (See blocks  14 ,  16 ), respectively. The output ports  226  or  228  could, for example, be used to couple the optical comb to Fabry-Perot etalons, which monitor the alignment of the lasing wavelength of the first and second slave laser  216 ,  218  to the desired lines in the optical comb. The optical comb that leaves the first directional coupler  212  enters the first slave laser  216 , and the optical comb that leaves the second directional coupler  214  enters the second slave laser  218  (See blocks  18 ,  20 ). The first slave laser  216  is adjusted so that its emission is at a desired line in the optical comb. Likewise, the second slave laser  218  is adjusted so that its emission is at, a usually different, desired line in the optical comb, thereby producing a first and second laser output, respectively. The first laser output reenters the first directional coupler  212  (See block  22 ), and the second optical output reenters the second directional coupler  214  (See block  24 ). The first directional coupler  212  splits the first laser output into two portions; one portion is supplied to a combining directional coupler  222  (See block  26 ) through an output of the first directional coupler  212 . The second directional coupler  214  likewise splits the second laser output into portions; one portion being supplied to the combining directional coupler  222  (See block  26 ) through an output of second directional coupler  214 . The first and second laser outputs that enter the combining directional coupler  222  (See block  26 ) are combined to form a primary and a secondary combined laser output. The primary combined output is then sent to a primary output port  220  on the integrated optical circuit  201  (See block  28 ), where the signal can be used for heterodyning to generate the microwave signal. It is preferred that in the primary combined output, the power of the first laser output be equal to the power of the second laser output. This can be achieved by adjusting the combining directional coupler  222 , as well as the first and second directional coupler  212 ,  214 . The secondary combined output can be sent to a secondary output port  230  on the integrated optical circuit  201 .  
         [0052]    In an alternate embodiment, as shown in FIG. 2 b , the relative splitting of light between the two outputs each of first and second directional coupler  212 ,  214  and combining directional coupler  222  can be controlled electrically. This control permits the first, second, and combining directional coupler  212 ,  214 ,  222  to be set to obtain the desired optical powers P i  of the optical comb injected into the first and second slave lasers  216 ,  218 . This control permits the first, second, and combing directional coupler  212 ,  214 ,  222  to also be set to obtain equal powers of the first and second laser outputs in the primary combined laser output of the integrated optical circuit  201 . This embodiment makes use of the output ports  226 ,  228  and the secondary output port  230  of the integrated optical circuit. The output ports  226 ,  228 , and the secondary output port  230  are coupled to optical fibers and supplied to an optical spectrum analyzer (OSA)  300  that may contain at least one scanning Fabry Perot etalon. A portion of the primary combined laser output also may be coupled, by means of an additional optical fiber splitter  240 , into the OSA  300 . The OSA  300  monitors the amplitudes and frequencies of individual lines of the optical comb and also of the first and second slave laser outputs. This information is then supplied to a processor  310  and to a directional coupler controller  246  for controlling the first and second directional coupler  212 ,  214  and the combining directional coupler  222 .  
         [0053]    Once a directional coupler is set to achieve a particular splitting of light from one of its inputs into its two outputs, the light from the other input also is set. In addition, that setting applies for light traveling in either direction, both forward and reverse. In general, the settings of the first and second directional couplers  212 ,  214  will be determined by the desired optical powers Pi of the optical comb injected into first and second slave lasers  216 ,  218 . The setting of the combining directional coupler  222  is then determined by the desire to obtain equal powers of the first and second laser outputs in the primary combined laser output which is supplied to the primary output port  220  of the integrated optical circuit. This means that the secondary combined laser output supplied to the secondary output port  230  of the integrated optical circuit may not have equal powers of the first and second laser outputs.  
         [0054]    Another alternate embodiment of the present invention is shown in FIG. 4. In this embodiment, the components of the integrated optical circuit  201  comprise the optical splitter  202 , the first directional coupler  212 , the second directional coupler  214 , the combining directional coupler  222 , a third directional coupler  204 , and a fourth directional coupler  206 . The first and second waveguide paths  215 ,  217  interconnect these components to each other output ports  215 ,  217  of the integrated optical circuit  201 .  
         [0055]    This embodiment is similar to the one shown in FIG. 2 a  but has two additional directional couplers, a third directional coupler  204  and a fourth directional coupler  206 . These additional directional couplers  204 ,  206 , when used in combination with the first and second directional couplers  212 ,  214 , permit the control of optical powers P i  of the optical comb injected into first and second slave lasers  216 ,  218  as well as the attainment of equal powers of the first and second laser outputs in the primary combined laser output of the integrated optical circuit  201 . This control can be achieved without needing to use an OSA  300  for controlling the first and second directional coupler  212 ,  214 . Instead, an integrated optoelectronic module  250  (shown in FIG. 6 a  and  6   b ) can be constructed from the integrated optical circuit  201  of this embodiment for generating a microwave signal by heterodyning. An OSA  300 , however, can still be used for monitoring the optical comb and the first and second laser outputs. An OSA  300  also can still be used for controlling the emission wavelengths of the first and second slave lasers  216 ,  218  so that they coincide with the selected lines of the optical comb. Again, in this embodiment, we prefer to use 2×2 third and fourth directional couplers  204 ,  206 .  
         [0056]    The third and fourth directional couplers  204 ,  206  each have two ports on each side of the directional coupler, which may act as either inputs or outputs. The third directional coupler  204  is connected between one output of splitter  202  and an input of first directional coupler  212  that was associated with that output of splitter  202  in the prior embodiment. The fourth directional coupler  206  is connected between the other output of splitter  202  and the input of second directional coupler  214  that was associated with that output of splitter  202  in the prior embodiment. One input of third and fourth directional coupler  204 ,  206  for light traveling in the forward direction is connected to splitter  202 . One output of third and fourth directional coupler  204 ,  206  for light traveling in the forward direction is connected to first and second slave laser  216 ,  218 . Note that the output of third and fourth directional coupler  204 ,  206  for the optical comb traveling in the forward direction acts as an input for the first and second laser output, which travel in the reverse direction through the third and fourth directional coupler  204 ,  206 . The third and fourth directional coupler  204 ,  206  also each have another output for coupling the optical comb, traveling in the forward direction through the coupler, to output ports  244 ,  246 , respectively. The third and fourth directional couplers  204 ,  206  are also each coupled to another output port  242 ,  248  for providing a portion of the first and second laser output, which travels in the reverse direction through the third and fourth directional coupler  204 ,  206 .  
         [0057]    [0057]FIG. 5 blocks  2 - 44  show the path of the optical comb and the first and second laser output in this alternate embodiment. FIGS. 6 a  and  6   b  show additional components that may be coupled to the various output ports of this alternate embodiment. Some of these additional components are photodetectors that are part of control circuits and phase-lock loop circuits. Scanning Fabry-Perot etalons can be connected to the monitoring and control output ports for monitoring the optical comb and the first and second laser output.  
         [0058]    As shown in FIG. 5, the optical comb generated by the master laser  203  (See block  2 ), first goes through the optical isolator  205  (See block  4 ) to prevent power from being reflected. The optical comb enters the integrated optical circuit  201  at input port  207  to a center waveguide  213  (See block  6 ). After entering the integrated optical circuit  201  through center waveguide  213  (See block  6 ), the optical comb enters the optical splitter  202 , where the optical comb is divided between two outputs of the optical splitter (See block  8 ) into a first and second waveguide path  215 ,  217 . The divided optical comb then enters inputs of third and fourth directional coupler  204 ,  206  (See blocks  10 , 12 ). A portion of the optical comb leaving the third directional coupler  204  is directed to the output port  244  (See block  14 ). A portion of the optical comb leaving the fourth directional coupler  206  is directed to another output port  246  (See block  16 ). The output ports  244 ,  246  may be used to connect photodetectors and feedback circuits. The photodetectors and feedback circuits may be used for adjusting the power of the optical comb injected into the first and second slave lasers  216 ,  218 . The optical powers P i  of the optical comb injected into first and second slave lasers  216 ,  218  and the detuning of those slave lasers from the selected lines of the optical comb determine the phase noise associated with the optical injection locking process, as is known in the art.  
         [0059]    The third directional coupler  204  has an output for sending a portion of the optical comb to an input of the first directional coupler  212  (See block  18 ), and the fourth directional coupler  206  has an output for sending the optical comb to an input of the second directional coupler  214  (See block  20 ). Both the first and second directional coupler  212 ,  214  have an output connected to an output port  226 ,  228  for monitoring the optical comb power injected into slave laser  216 ,  218  (See blocks  22 ,  24 ). The output port  226 ,  228  could, for example, be used to connect to Fabry-Perot etalons. The output port  226 ,  228  also could be connected to a photodetector, which is a part of feedback circuit. The optical comb that leaves the first and second directional coupler  212 ,  214 , also enters the first slave laser  216  and the second slave laser  218  (See blocks  26 ,  28 ). The first and second slave laser  216 ,  218  are adjusted so that they lase at wavelengths coincident with the desired lines in the optical comb. The first and second slave laser  216 ,  218  produce a first and second laser output, respectively. The first laser output reenters an input of the first directional coupler  212  (See block  30 ), and the second laser output reenters an input of the second directional coupler  214  (See block  32 ), in the reverse direction. After being split by the first directional coupler  212 , a portion of the first laser output leaves an output of the first directional coupler  212 , and reenters an input of the third directional coupler  204  (See block  34 ), in the reverse direction, and the remaining portion of the first laser output enters the combining directional coupler  222  (See block  42 ). After being split by the second directional coupler  214 , a portion of the second laser output leaves an output of the second directional coupler  214 , and enters the combining directional coupler  222  (See block  42 ), and the remaining portion of the second laser output reenters an input of the fourth directional coupler  206  (See block  36 ). A portion of the first and second laser outputs that reentered the third and fourth directional coupler  204 ,  206  leaves an output of the third and fourth directional coupler  204 ,  206  and is then coupled to output ports  242 ,  248  of the integrated optical circuit  201  (See blocks  38 ,  40 ). These output ports  242 ,  248  can be coupled to photodetectors that are a part of a feedback circuit. The portion of the first and second laser outputs that enter the combining directional coupler  222  (See block  42 ) are combined to form a primary combined laser output and a secondary combined laser output. The primary combined laser output signal is then sent to the primary output port on the integrated optical circuit (See block  44 ), where the signal can be used for heterodyning to generate the microwave signal. The secondary combined laser output is sent to the secondary output port  230  (See block  44 ), where the secondary combined laser output may be used for heterodyning.  
         [0060]    According to this alternate embodiment, a first waveguide path  215  is formed between one of the two outputs of optical splitter  202 , the third directional coupler  204 , the first directional coupler  212 , and the combining directional coupler  222 . The first waveguide path  215  interconnects those components with each other and with first slave laser  216 . A second waveguide path  217  is formed between the other of the two outputs of optical splitter  202 , the fourth directional coupler  206 , the second directional coupler  212 , and the combining directional coupler  222 . The second waveguide path  217  interconnects those components with each other and with second slave laser  218 .  
         [0061]    An integrated optoelectronic module  250  can be constructed from the embodiment, described above, of the integrated optical circuit  201  illustrated in FIG. 4. A example of the integrated optoeletronic module is shown in FIG. 6 a . This example contains feedback circuits for controlling the directional couplers. The integrated optoelectronic module  250  comprises the integrated optical circuit  201  and a first and second feedback circuit  210 ,  211 . First feedback circuit  210  controls first directional coupler  212  and third directional coupler  204 . Second feedback circuit  211  controls second directional coupler  214  and fourth directional coupler  206 . First feedback circuit  210  also contains first photodetector  302 , for detecting photocurrent intensity of the optical comb, third photodetector  304 , for detecting the photocurrent intensity of the first laser output, and fifth photodetector  306 . The first, third and fifth photodetectors  302 ,  304 ,  306  are coupled to output ports  242 ,  244  and  226  of the integrated optical circuit. Second feedback circuit  211  contains second photodetector  308 , for detecting the photocurrent intensity of the optical comb, fourth photodetector  310 , for detecting the photocurrent intensity of the second laser output, and fifth photodetector  306 . The second, fourth and fifth photodetectors  308 ,  310 ,  306  are coupled to output ports  246 ,  248 , and  228  of the integrated optical circuit  201 . The first and second feedback circuit  210 ,  211  monitor the optical comb power coupled into the first and second slave laser  216 ,  218  and the first and second laser output from first and second slave laser  216 ,  218  that is delivered to the combining directional coupler  222 . First feedback circuit  210  then electrically adjusts the first and third directional coupler  212  and  204 , and second feedback circuit  211  electrically adjusts the second and fourth directional coupler  214  and  206 , to obtain the desired levels of optical power P i  of the optical comb injected into first and second slave lasers  216 ,  218 , respectively. The first, second, third, and fourth directional couplers  212 ,  204 ,  214 ,  206  also are electrically adjusted to obtain equal powers for the first and second slave laser outputs that are delivered to the combining directional coupler  222 . The combining directional coupler in this embodiment is nominally set to be a 3-dB splitter, which divides the power evenly between its two outputs.  
         [0062]    In yet another alternate embodiment of this invention, as shown in FIG. 6 b , heterodyne and homodyne phase lock loops are connected to the integrated optical circuit  201  to construct an integrated optoelectronic module  250 . The function and benefits of heterodyne and homodyne phase lock loops for optical-heterodyne microwave synthesis using injection-locked slave lasers is discussed in a related patent application document entitled “Low-Noise, Switchable RF-Lightwave Synthesizer,” Ser. No. 60/373,739 which is filed on even date herewith. The process of generating a frequency-converted microwave signal by optical heterodyning also is discussed in this related patent application. Although an integrated optoelectronic module  250  with heterodyne and homodyne phase lock loops and with feedback circuits for controlling the directional couplers are described separately herein. That separation is done only for purposes of clarity. An integrated optoelectronic module  250  of this invention could contain heterodyne and homodyne phase lock loops as well as feedback circuits for controlling the directional couplers.  
         [0063]    The integrated optical circuit  201  shown in FIG. 6 b  is similar to the integrated optical circuit  201  shown in FIGS. 4 and 6 a  with the following exceptions. The integrated optical circuit  201  of FIG. 6 b  has two additional optical splitters  252  and  254 , two additional output ports  256  and  258 , and additional waveguides that connect the additional optical splitters  252 ,  254  to the additional output ports  256 ,  258 . Splitter  252  receives the secondary combined laser output from combining directional coupler  222  and splits the secondary combined laser output into two signals that are provided to the secondary output port  230  and output port  256  of the integrated optical circuit  201 . Splitter  254  receives a portion of the second laser output from fourth directional coupler  206  and splits that second laser output signal into two signals that are provided to output ports  248  and  258  of the integrated optical circuit  201 .  
         [0064]    Photodetector  257  is a part of a heterodyne phase lock loop  262  that is coupled to output port  256 . Heterodyne phase lock loop  262  is described in more detail in the related patent application document entitled “Low-Noise, Switchable RF-Lightwave Synthesizer,” Ser. No. 60/373,739. The heterodyne phase lock loop  262  is electrically connected to the first slave laser  216  to provide fine control of the current driving first slave laser  216 . An external RF reference oscillator  260  is connected to both the heterodyne phase lock loop  262  and the mode-locked master laser  203 . The purpose of this external reference oscillator  260  also is discussed in the referenced related patent application document.  
         [0065]    Photodetector  259  is a part of a homodyne phase lock loop  264  that is coupled to output port  258 . Homodyne phase lock loop  264  is also coupled to output port  226  to receive the optical comb. Homodyne phase lock loop  264  also is described in more detail in the above referenced related patent application document. The homodyne phase lock loop  264  is electrically connected to the second slave laser  218  to provide fine control of the current driving second slave laser  218 .  
         [0066]    A hybrid integration approach for constructing the integrated optoelectronic module  250  of FIGS. 6 a  and  6   b  is illustrated in FIG. 7. The integrated optical circuit  201 , and photodetectors for first and second feedback circuits (not shown) as well as for the heterodyne phase lock loop (not shown) and the homodyne phase lock loop (not shown) are mounted on a common substrate platform  270 . FIG. 7 shows a portion of the integrated optoelectronic module  250  and integrated optical circuit  201 . FIG. 7 also shows, as an example, first slave laser  216  coupled to port  219 , a V-shaped groove  272  for aligning an optical fiber (not shown) to output port  226  of the integrated optical circuit  201  and a photodetector  306  coupled to another output port  228  This substrate platform  270  could be fabricated from a variety of materials, such as silicon, glass, III-V semiconductors (e.g. InP or GaAs) or metals (e.g. copper). V-shaped grooves  272  can be machined into the substrate platform  270  using known techniques and aligned with the integrated optical circuit  201  using other known techniques. These V-shaped grooves  272  serve to hold and align optical fibers that are coupled to input port  207 , (shown in FIG. 4) and primary output port  220 , secondary output port  230  and output port  226  of the integrated optical circuit (as shown in FIG. 4). First and/or second slave lasers  216 ,  218  also can be mounted on substrate platform  270  and aligned to first and second waveguide paths  215 ,  217 , (as shown in FIG. 4) of integrated optical circuit  201 . Photodetectors  302 ,  304 ,  306 ,  308 ,  310 ,  257 , and  259  also can be mounted on substrate platform  270  and aligned to their respective output ports of the integrated optical circuit  201 .  
         [0067]    A micro-lens or tapered-waveguide mode-expander can be used to accomplish mode-matching between the first and second slave laser  216 ,  218  and the first and second waveguide path  215 ,  217  (not shown) on integrated optical circuit  201 . The lens and mode-expander also can be formed on the integrated optical circuit  201  chip or on the first and second slave laser  216 ,  218  chip. The lens also can be formed as a physically separate unit that is mounted on the substrate platform  270  between first and second slave laser  216 ,  218  and integrated optical circuit  201 . Alignment features (not shown) that are known in the art can be machined into substrate platform  270  to assist in the alignment of first and second slave laser  216 ,  218  to first and second waveguide path  215 ,  217  of integrated optical circuit  201 .  
         [0068]    [0068]FIG. 8 shows a larger view of the first, second, third, fourth, and combining 2×2 directional coupler shown in FIGS.  2 - 6 . As shown in FIG. 8, the 2×2 directional coupler contains ports  402 ,  404 ,  406 , and  408 . Each of the ports acts as either an input or an output for light. For example, suppose light exits port  402 . After the light has exited, light could then enter port  402 . In this way the 2×2 directional coupler has in effect 4 inputs and 4 outputs, as aforementioned. The ports  402 ,  404 ,  406 , and  408  give the first, second, third, fourth, and combing directional coupler, the ability to send/receive the optical comb, send/receive the first laser output, send/receive the second laser output, and send the primary or secondary combined laser output.  
         [0069]    Let it be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the spirit of the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications, and variances which fall within the scope of the appended claims.