Abstract:
A tunable laser device produces a laser energy over a range of frequencies. A property of an ultrasonic transducer or, more specifically, the frequency of an ultrasonic wave created by the ultrasonic transducer is altered to change the laser frequency of the tunable laser device. The ultrasonic transducer may couple its ultrasonic wave directly into the laser source or may form a tunable acousto-optic modulator external to the laser source. In both cases, the ultrasonic wave creates an index of refraction perturbation in an optical substrate, through which the laser energy of the laser source passes. Changes in the ultrasonic wave cause changes in the perturbation through which the laser energy passes, which in turn, changes the frequency of the laser energy, thereby enabling the frequency of the laser to be tuned.

Description:
FIELD OF THE DISCLOSURE 
     This patent generally relates to lasers and more specifically to ultrasonic tunable lasers. 
     BACKGROUND OF THE PRIOR ART 
     Communication networks are known to employ laser sources in many different forms, from transponders to gain regions in amplifiers. Generally, these laser sources are characterized by an output bandwidth and output (or laser) frequency. In smaller-scale communication networks, like an enterprise network for example, a single frequency laser (850 nm) may be used to transmit data. For larger bandwidth applications, for example, links between metro-area networks (MANs), a dense wavelength division multiplexing (DWDM) system employs lasers over a range of frequencies. A DWDM system may include 100 channels, each channel emitting from a laser producing a different output frequency within a range of 1525-1565 nm. Output frequency and output bandwidth are the key design criteria for the network transponders housing these lasers. In the DWDM system in particular, control over frequency and bandwidth parameters is important to ensure proper channel spacing. Numerous industry standards, for example, pertain to output frequency and bandwidth spacing control. 
     Using tunable lasers in network devices is known. With tunable lasers, a manufacturer can choose from numerous, different output frequencies/wavelengths. Tunable lasers reduce manufacturing costs and eliminate the need for producing separate lasers of different output frequencies. Tunable lasers are also desirable because they are less susceptible to fabrication errors. If the desired output frequency is not exact, tuning the laser will take an otherwise useless output and tune it to a useful one. Some non-tunable/single frequency lasers may be tuned by temperature tuning, but the tuning is over a very small frequency range, one too small for tuning between different channels. 
     There are two primary ways of configuring a tunable laser. The first is via multi-segment Bragg reflectors (DBR). The second is via an external laser cavity. With respect to the former, each DBR serves as a highly reflective mirror cavity that produces a narrow-bandwidth output signal at a characteristic frequency. In some devices, selectively activating the DBR corresponding to the desired output frequency provides tuning. Each DBR is connected to a lead and separately activated. In other devices, like sampled-grating DBRs, electrical current is controlled to align reflection peaks between gratings, thereby tuning the device. Control over the electrical current is very difficult to achieve, however. 
     Multi-segment DBRs, while functional, have numerous drawbacks. These devices require individual control of each grating, which translates into increased control complexity. These devices have limited scalability. Additionally, because multi-segment DBRs must be formed to exacting layer width tolerances, more expensive fabrication techniques are required to create large numbers of DBRs and electrodes. 
     Another problem common to the multi-segment DBR tunable lasers is mode hopping. Here, the frequency of the output signal inadvertently hops from one output value to another output value. In the DWDM context, for example, such mode hopping would result in a multi-segment DBR tunable laser emitting at a frequency corresponding to one data channel and then inadvertently hopping to another frequency corresponding to another data channel. As a result of this mode hopping, a single data stream would be transmitted on different data channels, which is undesirable. 
     In an external cavity laser, a gain medium is placed between two mirrors, and one or both of the mirrors are moved to change the lasing cavity length. Additionally, a tuning element may be inserted into the cavity to select which of the modes will lase. Unfortunately, these external cavity lasers have moving parts that are expensive to fabricate. For example, some have proposed costly microelectronic mechanical systems (MEMS) devices to adjust the position of the moving components. 
     Other drawbacks exist. Moving parts must be reliably adjustable over extremely short distances, leaving them susceptible to performance degradation over time. Additionally, external cavity lasers are large in size and can result in undesirable coupling loss as energy is coupled into and out of the resonant cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of an example metro-area network having a plurality of routers and transponders. 
         FIG. 2  is a block diagram of a transponder used in the metro-area network of FIG.  1 . 
         FIGS. 3A and 3B  illustrate of an example tunable laser wherein a tunable acoustic wave has been formed within a gain region. 
         FIG. 4  is a side illustration of another example ultrasonic tunable laser. 
         FIG. 5  is an illustration of another example tunable laser with a laser source and tunable acousto-optic modulator. 
         FIG. 6  is an illustration of another example tunable laser with a laser source and two tunable acousto-optic modulators. 
         FIG. 7  is an illustration of an example tunable laser having a gain region coupled to a tunable acousto-optic modulator via a fiber coupling. 
         FIG. 8  is a side illustration of an example tunable laser in which only portions of a gain region receive an acoustic wave. 
         FIG. 9  is a block diagram of an example feedback control circuit for any of the tunable lasers depicted in FIGS.  3 - 8 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example metro-area network (MAN)  100 , which may be a DWDM network. The MAN  100  includes a plurality of routers  102  that may be interconnected in a ring configuration, for example. The routers  102  form a backbone of the MAN  100 , through which data from a wide-area network (WAN)  104  or a second MAN  106  may be received and locally routed. In the illustrated example, the MAN  100  also has routers  102  connected to a first local-area network (LAN)  108 , a second LAN  110 , and an access network  113 , such as a remote access network or server access network. The MAN  100  may be connected to an enterprise network or other network. At the head of each router  102  is a transponder  112  for sending and receiving data. The transponder  112  may be a multi-channel transponder, such as DWDM device communicating between MANs, or the transponder  112  may transmit and receive at a single frequency, as might be sufficient for communicating with smaller networks, like LANs. While identical transponders  112  are shown, it will be appreciated by persons of ordinary skill in the art that any of the transponders  112  may be different from one another. Furthermore, the illustration of  FIG. 1  is by way of example and may include additional networking components not shown. 
       FIG. 2  shows an example high-level block diagram of the transponder  112 . The transponder  112  includes a transceiver  114  for transmitting and receiving data streams along fibers  116  and  118 , respectively. The receiver line  119  includes a photodiode  120  which performs optical-to-electrical signal conversion, a trans-impedance amplifier  122 , and a separate boosting amplifier  124 . The transmitter line  125  includes a laser  126 , a modulator  128 , and an amplifier  130 . While a single transceiver  114  is shown, it will be understood by persons of ordinary skill in the art that the transponder  112  may have multiple transceivers or that each depicted block may represent a bank of blocks; for example, blocks  120  and  126  may be a plurality of photodiodes or lasers, respectively. 
     The transceiver  114  is connected to a controller  132 , which may represent a single application specific integrated circuit (ASIC), multiple integrated circuits, or a microprocessor, for example. The controller  132  may be formed from a microcontroller like the 8051 microcontroller available from Intel Corporation. A microprocessor may also be used, such as any one of the Intel family of microprocessors, including Pentium®, Xeon™, and Itanium™-based microprocessors. Alternatively, a chipset like the LXT16768, LX16769, or LXT16759 products also made by Intel Corporation may be used. For the receiver line  119 , the controller  132  includes a deserializer  134  coupled to the amplifier  124  and a decoder  136  coupled to the deserializer  134 . For the transmitter line  125 , the controller  132  includes an encoder  138  and a serializer  140 . 
     In operation, a multi-channel or single channel data stream is received via the fiber  118 . The multi-channel data-stream is coupled into the photodiode  120  for optical-to-electrical signal conversion. Data from the photodiode  120  is coupled to the trans-impedance amplifier  122  and on to the amplifier  124  prior to appearing at the deserializer  134 . The deserializer  134  provides a 10 bit signal to decoder  136  that decodes the input signal. The 10 bit word from the decoder  136  may be passed to a Gigabit Media Independent Interface (GMII). For data transmission, input data from the GMII is first encoded by the encoder  138  and then serialized by the serializer  140  to create a transmittable serial bit stream. The output from the serializer  140  controls the output of the laser  126 , said output being modulated by the modulator  128  and then amplified by the amplifier  130  prior to transmission on the fiber  116 . 
     While, the illustration of  FIG. 2  is an example, it will be understood by persons of ordinary skill in the art that additional control blocks and routines may be used or that some of the control blocks of  FIG. 2  may be eliminated or replaced. For example, the controller  132  may include an internal clock, a clock and data recovery device (CDR), phase control via phase locked loops (PLL), and/or error correction control circuitry. Furthermore, while not necessary, the transponder  112  may be compliant with any known network communications standards of which SONET formats OC-48 (2.5 Gbps), OC-192 (10 Gbps), and OC-768 (40 Gbps) are examples. 
       FIGS. 3A and 3B  illustrate an example tunable laser  150  that may be used as the laser  126  of FIG.  2 . The tunable laser  150  includes an ultrasonic transducer  152  formed of an acoustically-responsive substrate  154 , e.g., a piezoelectric substrate. Suitable materials for the substrate  154  include any known piezoelectric material, like crystalline quartz or lead zurcinate titinate, also known as PZT. PZT, for example, may be undoped or doped. In the latter case, many available dopant atoms are known, one being La which forms a material commonly termed PLZT. In any event, numerous suitable glasses and ceramics will be known to persons of ordinary skill in the art. The substrate  154  may spin coated on a support substrate  156 , which may be silicon. 
     First and second electrodes  158  and  160  (see  FIG. 3B ) are formed on the acoustically-responsive substrate  154  and may receive an AC drive signal for forming the ultrasonic wave in the transducer  152 . Different AC drive signals will result in different ultrasonic waves. The transducer  152  is adjacent a laser source  162 , such that the ultrasonic wave from the transducer  152  may be coupled into the laser source  162 . In this manner, the ultrasonic transducer  152  may be used to form a tunable ultrasonic wave that appears in the laser source  162 . Further, while ultrasonic waves are described, many different sound waves may be formed and coupled into the laser source  162 , with this example. 
     As illustrated in  FIG. 3B , the laser source  162  includes a first cladding layer  164  and a second cladding layer  166  with a gain region  168  extending between the two cladding layers  164  and  166 . The cladding layer  164  may be directly formed on or mounted to the top surface of the substrate  154 , thus providing direct coupling of the ultrasonic wave created in the transducer  152 . Suitable mountings include adhesive mounting, clamp mounting, chemical bonding, soldering, or welding. For example, an acoustically-transparent layer, like an acoustically-transparent glue, may be used to contact the cladding  164  adjacent the substrate  154 . In a further example, the laser source  162  may have a suspended mounting or a slidable engagement with the substrate  154 , to avoid any unwanted transfer of mechanical energy during transducer operation, e.g., to restrain the laser source  162  against movement. 
     In an example, the gain region  168  is a semiconductor optical amplifier (SOA), such as a III-V compound semiconductor structure. Suitable SOA materials will be known to those of ordinary skill in the art and include InP and InGaAS, though any suitable lasing material that may also function as an acousto-optic material may be used for the gain region  168 . 
     In operation of the tunable laser  150 , an RF drive signal is applied across the electrodes  158  and  160  to form an acoustic wave  170  within the substrate  154 . The acoustic wave  170 , with peaks and troughs  170 A and  170 B, respectively, may be a standing wave or a traveling wave. The acoustic wave  170  is coupled into the gain region  168  through the cladding layer  164 , creating an acoustic wave  172  therein. The acoustic wave  172  will have a similar effect to that of a DBR. The peaks and troughs of the acoustic wave  172  create a periodic, alternating index of refraction change within the gain region  168 . The result is a perturbation on the index of refraction seen by light propagating in the gain region  168 . As a result, the output energy from laser source  162  will have a peak frequency dependent upon the periodicity of acoustic wave  172 . As the spacing between peaks of the wave  172  changes, i.e., the frequency of the acoustic wave  172  changes, the laser frequency of the laser source output energy changes. Typical laser frequencies may be tuned around 1550 or 1310 nanometers (nm), the low-loss windows or bands commonly used in optical fiber communications, although any laser frequency range may be formed. 
     Generally, the desired frequency of the acoustic wave  170  and the RF drive signal is provided by the relation c=λv, where v is the frequency of the acoustic wave  172 , c is the speed of sound in the gain region  168 , and λ is the wavelength of the acoustic wave  172 , which would be the same as the wavelength of the lightwave from the laser  150 , e.g., 1550 nm. As a result, the acoustic wave  170  is an ultrasonic wave. By changing the RF drive signal applied across electrodes  158  and  160 , the frequency of the acoustic waves  170  and  172  will change, as will the frequency of the light wave from the laser  150 . Thus, by adjusting the spacing between the peaks of the wave  172 , the output (or laser) frequency of the laser  150  may be tuned across a range of values. 
     Numerous alternative examples based on the principles inherent in the above example may be implemented. For example, while the substrate  154  is described as a single piezoelectric material, the substrate  154  may include multiple sections each of different material. In fact, these materials need not be piezoelectric materials. Further, the substrate  154  may be modified to shape the acoustic wave  170  or block all or part of the acoustic wave  170  from coupling into certain portions of the laser source  162 . Further still, while a single electrode pair  158 - 160  is shown, dual electrode pairs may be used. 
       FIG. 4  illustrates another example tunable laser  200  that may be used as the laser  126 . For like components, reference numbers from  FIGS. 3A and 3B  have been retained. The laser source  162  is replaced with a laser source  202 , that includes a first cladding layer  204  and a second cladding layer  206  disposed on opposing sides of a gain region  208 , all similar to structures described above. The laser source  202  may be formed on or mounted on the ultrasonic transducer  194 . The tunable laser  200  differs from that of the example laser  150  by the existence of a first passive region  210  and a second passive region  212  disposed on opposing sides of the gain region  208 . The passive regions  210  and  212  may be transparent throughout the entire frequency range of operation of the tunable laser  200 . Alternatively, one or both of the regions  210  and  212  may be wholly or partially reflective to direct output energy to one side or the other of the laser source  202 . The passive regions  210  and  212  protect the gain region  208  from environmental conditions, as well as from edge damage. The passive regions  210  and  212  are optional. 
     On the cladding layer  206  is a second ultrasonic transducer  214  formed of a piezoelectric substrate  216  and two drive electrodes  218  and  220 . The substrate  216 , for example, may be any of the piezoelectric materials herein described and may be formed in a known manner. The two ultrasonic transducers  152  and  214  may have a cumulative effect in the gain region  208 , where the gain region acoustic wave becomes the superposition of the two transducer acoustic waves. If the two electrode pairs, ( 158  and  160 ) and ( 218  and  220 ), receive the same drive signal, a lower intensity drive signal may be used to induce the same index of refraction perturbation within gain region  208 . The two drive signals need not be identical, though. Furthermore, by altering the relative frequencies of the drive signal, complete cancellation or a beat frequency periodicity profile may be imposed on the index of refraction perturbation. 
       FIG. 5  illustrates another example tunable laser  300  that may be used as the laser  126 . Whereas the example tunable lasers  150  and  200  are formed such that an ultrasonic standing wave extends into a gain region of a laser, the example tunable laser  300  employs a gain region that is un-perturbed by an acoustic wave. The tunable laser  300  is formed of two parts, a laser source  302  and an external acousto-optic modulator (AOM)  304 . The laser source  302  includes a first cladding layer  306  and a second cladding layer  308  sandwiching a gain region  310 . A highly reflective mirror  312  is displayed at one end of the laser assembly  302 . Suitable mirrors will be known to persons of ordinary skill in the art; a DBR stack is one example. The AOM  304  is at an opposing end of the laser source  302  than the mirror  312  and is driven by two electrodes,  314 ,  316  on the AOM  304 . 
     The electrodes  314 ,  316  receive an RF drive signal that is varied to tune the laser frequency of the tunable laser  300 . In operation, the AOM  304  acts as a tunable mirror opposite the mirror  312 . The reflection characteristics of the AOM  304  will change with the changes to the frequency of the drive signal to the electrodes  314  and  316 . A higher frequency drive signal may result in a reduction in the wavelength of the lightwave from the laser  300 , while a lower frequency drive signal may result in a longer wavelength on that lightwave. The AOM  304 , although highly reflective, is also partially transmitting to allow laser energy to escape. Alternatively, or additionally, the mirror  312  may be partially transmitting. 
     The AOM  304  and the laser assembly  302  of the illustrated example are mounted on a substrate  318 . The substrate  318  may be a microbench with etched recesses for the assemblies  302  and  304 , although other mounting, clamping, or bonding techniques will be known to persons of ordinary skill in the art. 
     The laser  302  and AOM  304  may be in direct contact with one another at their adjacent inner faces or they may be spaced apart as shown in FIG.  5 . Further, the spacing between the two may or may not include an anti-reflection region, such as an antireflection gel, to further reduce insertion or coupling losses. 
     Another example tunable laser  350  that may be used as the laser  126  is illustrated in FIG.  6 . The tunable laser  350  is similar to the tunable laser  300  and, therefore, the same reference numerals are used for like components. The tunable laser  350  includes a laser source  302 ′, the first tunable AOM  304 , and a second tunable AOM  352  in place of the reflector  312 . The AOM  352  includes electrodes  354 ,  356  that are controlled in a similar manner to the electrodes  304 ,  316  of the AOM  304 . 
     The same or different controllers may supply an identical drive signal to the AOM  304  and the AOM  352 . As with the tunable laser  300 , the laser source  302 ′ and each AOMs  304  and  352  may be coupled together through an index matching antireflection coating gel or layer in the spacings therebetween. The laser source  302 ′ may be in direct contact with the AOMs  304  and  352 . With the tunable laser  350 , smaller bandwidths may be achieved and the tuning of the laser assembly  302  may be achieved more quickly in comparison to the state of the art. Any of the known mounting, clamping, or bonding techniques may be used to assemble the tunable laser  350 . 
       FIG. 7  shows an example tunable laser  400  having a laser source  402  and a tunable AOM  404  similar to the structures  302  and  304  of FIG.  5 . The example laser assembly  402  includes a first cladding layer  406 , a second cladding layer  408  and a gain region  410  therebetween. A mirror  412 , which may be highly reflective at all tunable wavelengths, is disposed on one end of the assembly  402 . Whereas components  302  and  304  were adjacent to one another, the laser source  402  and the AOM  404  are spaced apart and are coupled to one another via a waveguide, e.g., an optical fiber  414 . The optical fiber  414  couples laser energy from the laser source  402  into the tunable AOM  404 . Laser energy may exit from the partially transmitting AOM  404 . 
     In the example of  FIG. 7 , a lens  416  mounted to a support  420  collects laser energy and couples it into the fiber  414 . Supports  422  and  424 , affixedly mounted to a substrate  426 , retain the fiber  414  in position for optimum coupling and negligible bending losses. The supports  422  and  424  may be augmented, replaced, or removed, as desired. The fiber  414  may be coupled to the assembly  404  via an exit face, a pigtail, a buffer, or the like. Alignment between the modules  402  and  404  need not be controlled to high tolerances due to the use of the waveguide coupling. 
       FIG. 8  illustrates an example tunable laser  500  that also may be used as the laser  126 . The laser  500  is formed of a laser source  502  positioned above an ultrasonic transducer  504  that may be formed of a piezoelectric material. In the illustrated example, the laser assembly  502  includes a first cladding layer  506  and a second cladding layer  508  with a gain region  510  therebetween. A mirror  512  is disposed at one end of the laser assembly  502  and may be like those previously described, though it could also be an AOM. 
     In operation, an ultrasonic wave is formed in the ultrasonic transducer  504  via an RF drive signal provided to electrodes (not shown). The ultrasonic wave produced by the drive signal couples into the gain region  510  in a similar manner to that shown in FIG.  1 . In the tunable laser  500 , however, the location of the ultrasonic wave within the gain region  510  is controlled by patterning various couplers between the laser source  502  and the ultrasonic transducer  504 . In the illustration, for example, two couplers  514  and  516  are disposed on a top surface of the transducer  504  between the transducer  504  and the laser source  502 . 
     The first coupler  514  is a poor acousto-optic coupler that shields the portion of the gain region above it (region  518 ) from the ultrasonic wave within the ultrasonic transducer  504 . The poor coupler  514  may be a spongy material, such as a silicon gel that absorbs any sound waves before they impinge upon the region  518 . Other suitable materials will be known to those of ordinary skill in the art. The thickness will also affect the coupling properties of the coupler  514 . In contrast, the coupler  516  is formed of a material that is a good acousto-optic coupler to ensure that ultrasonic waves from the transducer  504  are coupled into the gain region  510 , in particular, into a region thereof labeled  520 . A resultant index of refraction perturbation  522  is shown by a sinusoidal line in FIG.  8 . Typically the coupler  516  is a hard material such as solder. 
     As illustrated, a laser source having a single gain region and only one highly reflective mirror at one end may be formed into an ultrasonic tunable laser by forming a tunable mirror at the opposing end of the gain region. In the example of  FIG. 8 , therefore, region  520  forms a tunable reflector opposite the reflector  512 . The laser assembly  502  has been formed into a distributed feedback laser system tunable by changing the drive signal that controls the ultrasonic transducer  504 . The layers  514  and  516  are examples of intermediate couplers that define an index of refraction pattern within the gain region. Of course, any number of couplers may be used to define any desired patterns. 
     The tunable lasers described herein may be used in a feedback control system, an example of which is shown in FIG.  9 . In the illustrated control system of  FIG. 9 , a tunable laser  600  includes a laser source  602  and an acousto-optic modulator or ultrasonic transducer (AOM/UT)  604 . The AOM/UT  604  may be any of the AOMs or ultrasonic transducers described above. For example, the AOM/UT  604  may supply an ultrasonic wave to the laser source  602  or may serve as an external mirror. If desired, the AOM/UT  604  may include multiple AOMs or multiple ultrasonic transducers. 
     A controller  606  controls the AOM/UT  604  by controlling or changing the RF drive signal applied to the AOM/UT  604  by an FR signal generator  608 . The controller  606  may be a microprocessor, analog controller, chipset, ASIC, or the like. Examples available from Intel Corporation have been provided above. 
     Tuning of the laser  602  is performed through a feedback loop, whereby the controller  606  receives a signal from a detector  610  coupled to receive an output  612  from the laser source  602 . The detector  610  may by a photo-detector, for example. If the detected laser light frequency does not match the desired frequency, then the controller  606  determines what adjustments need to be made to the RF drive signal sent to the AOM/UT  604  to produce the desired frequency. 
     In the example of  FIG. 9 , the detector  610  is partially transmitting so that only a small portion of the output  612  is detected and the remaining portion  612 ′ is transmitted. In this way, the detector  610  need not disrupt operation of the tunable laser  600 . The detector  610  may also be used in an intensity feedback loop control and specifically to prevent intensity dependent losses like stimulated Brillouin scattering (SBS). Numerous SBS suppression techniques are known. The illustration shows the controller  606  connected to a modulator  614 . The controller  606  may signal the modulator  614  to apply a slow Hz ripple on the laser output wavelength. Such modulation is a known SBS suppression technique. This kind of feedback control may be particularly beneficial in long-haul and ultra-long haul applications where higher output intensity signals are generally employed. 
     Numerous alternatives will be apparent to persons of ordinary skill in the art. While certain blocks are shown, these may be replaced or augmented as desired. Further, while blocks are illustrated as forming part of the laser  600 , it will be understood that any number of blocks may be separate structures coupled to the laser  600 . 
     Examples described herein may exhibit greater stability over known devices, because there is no float in the acoustic wave wavelength over time. Therefore, recalibration is reduced or eliminated. Any of the above described tunable lasers may be formed into a transponder, as might be used in an optical network. Yet, these tunable lasers may be stand-alone or incorporated into other optical devices that may utilize a tunable laser source. Furthermore, while the teachings herein were described in illustrated examples, any of the techniques may be combined with other techniques as desired. 
     Although certain apparatus constructed in accordance with the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all embodiments of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalence.