Patent Application
20160373191
The present invention relates to optical systems and more particularly to optical devices having a laser cavity.
Lasers are commonly used as the source of light signals in optical communications systems. These lasers are often integrated onto optical chips and/or onto optoelectronic chips. The laser cavities in these lasers can be external cavity lasers configured to output a light signal with a single wavelength or a single longitudinal cavity mode. One of the challenges with these lasers is mode hopping. Mode hopping refers to shift in output light wavelength when laser switches from one longitudinal mode to another. The change between modes is associated with an undesirable discrete change in the wavelength (and sometimes power) of the light signal output by the laser. These changes are a source of bit error in optical links.
The mode hopping can be a result of influences that change the index of refraction of the media through which the light signals are guided in the laser cavity. Examples of influences that can cause these effects are temperature changes, changes in the level of electrical current applied to the laser cavity, or aging of the gain medium. In order to address these problems, many of these devices include temperature control devices such as heaters and/or other feedback control devices for stabilizing the indices of refraction of the media through which the light signals are guided. These temperature control devices and/or other feedback control devices increase the complexity, cost, and power consumption of the device.
An optical link transmits light between a transmitter and a receiver. The transmitter includes a laser cavity that outputs a laser light signal. The laser cavity is configured such that a longitudinal cavity mode hop may occur operation of the optical link. The transmitter outputs an output light signal that includes light from the laser light signal. The output light signal travels a data travel distance before being received at the receiver. The data travel distance is greater than 0 and less than 1 km and the optical link has a Bit Error Rate less than 10−12. In some instances, the laser cavity is an external cavity laser.
An optical system has a laser cavity on a substrate. The laser cavity outputs a laser light signal where one or more mode hops can occur. The laser cavity has one, two, or three conditions selected from a group consisting of a wavelength error that is greater than 0.15 nm and less than 0.25 nm for at least one of the mode hops, a power variation that is greater than −4 dBm and less than 0.2 dBm for at least one of the mode hops, and a SideMode Suppression Ratio (SMSR) that is less than 100 dB, and greater than 30 dB.
An optical link includes a transmitter and a receiver. The transmitter includes a laser cavity that experiences one or more mode hops during operation of the laser cavity. The inventors have been able to design the laser cavity such that the mode hops result in an acceptable rate of bit errors for optical links of certain distances. For instance, for optical lengths less than 1 km, the inventors have found that the laser cavity can be designed such that the mode hops produce an optical link with a Bit Error Rate (BER) less than 10−12 or even less than 10−15 even though the laser cavity still experiences mode hops during the operation of the optical link. The tolerance of mode hopping in these optical links permits the use of laser types that are more prone to mode hopping but that are more desirable due to reduced costs and/or complexity. For instance, external cavity lasers can be used in these optical links. Additionally or alternately, the tolerance for mode hopping means that devices that include these laser cavities can optionally exclude power hungry temperature control devices such as heaters and/or other devices for stabilizing the indices of refraction of the media through which the light signals are guided in the laser cavity.
A coupled waveguide 24 may optionally be optically coupled with the output waveguide 20 such that a portion of the output light signal is coupled into the coupled waveguide 24. The coupled waveguide 24 guides the tapped portion of the output light signal to a light sensor 26. The light sensor 26 is configured to convert the received light signal to an electrical signal. Electronics (not shown) can be in electrical communication with the light sensor 26 and can receive the electrical signal from the light sensor 26. In some instances, the electronics are also in electrical communication with the gain element 10. For instance, the electronics can apply electrical energy to the gain element 10.
During operation of the device, the cavity waveguide 16 carries a laser light signal from the gain medium 12 to the partial return device 18. The partial return device 18 returns a first portion of the laser light signal along its original path and permits a second portion of the laser light signal to enter the output waveguide 20. As a result, the second portion of the laser light signal serves as the light signal output by the laser.
The cavity waveguide 16 carries the first portion of the laser light signal back to the gain waveguide 14. The gain waveguide 14 guides the received first portion of the laser light signal through the gain medium 12 to a reflector 28. The reflector 28 reflects the laser light signal portion such that the first laser light signal portion returns to the gain waveguide 14 and eventually to the partial return device 18. Accordingly, the first laser light signal portion travels through the gain waveguide 14 twice before returning to the partial return device 18. The gain medium 12 in combination with the multiple passes of the laser light signal through the gain medium 12 are the source of optical gain in the laser. Energy can be applied to the gain medium 12 to provide the optical gain. In some instances, the energy is electrical energy provided by the electronics but other forms of energy can be used. The reflector 28 can be highly reflective so substantially all of the first laser light signal portion that is incident on the reflector 28 is returned to the gain waveguide 14.
During the generation of the output light signal, the electronics receive the electrical signal from the light sensor 26. The electronics can also adjust the level of electrical energy applied to the gain element 10 in response to the electrical signal received from the light sensor 26 in a feedback loop. For instance, in the event that the electrical signal from the light sensor 26 indicates that the intensity of the output light signal is above a threshold, the electronics can reduce the electrical energy applied to the gain medium 12 in order to reduce the intensity of the output light signal.
A suitable partial return device 18 is a reflective optical grating such as a Bragg grating.
The base 32 can include the optical insulator 34 positioned on a substrate 36. As will become evident below, the substrate 36 can be configured to transmit light signals. For instance, the substrate 36 can be constructed of a second light-transmitting medium 30 that is different from the light-transmitting medium 30 or the same as the light-transmitting medium 30. The illustrated device is constructed on a silicon-on-insulator wafer. A silicon-on-insulator wafer includes a silicon layer that serves as the light-transmitting medium 30. The silicon-on-insulator wafer also includes a layer of silica positioned on a silicon substrate 36. The layer of silica can serve as the optical insulator 34 and the silicon substrate 36 can serve as the substrate 36.
The illustrated portion of the device shows a Bragg grating at an interface between the cavity waveguide 16 and the output waveguide 20. A ridge of the light-transmitting medium 30 extends outward from slab regions 38 of the light-transmitting medium 30. The ridge partially defines each of the waveguides. For instance, the ridges and the base 32 together define a portion of a light signal-carrying region where light signals are constrained within each of the waveguides. When the device is constructed on a silicon-on-insulator wafer, the silica that serves as the insulator 34 has an index of refraction that is less than an index of refraction of the silicon light-transmitting medium 30. The reduced index of refraction prevents the light signals from entering the substrate 36 from the silicon. Different waveguides on the device can have different dimensions or the same dimensions.
Recesses 40 extend into the top of the ridge. The recesses 40 are filled with a medium having a lower index of refraction than the light-transmitting medium 30. The medium can be a solid or a gas such as air. Accordingly, the recesses 40 provide the variations in the index of refraction of the waveguide that allow the recesses 40 to act as a Bragg grating. The Bragg grating is illustrated with only four recesses 40 in order to simplify the illustration. However, the Bragg grating can include more than four recesses 40. In some instances, the recesses 40 are arranged so as to form a periodic pattern in the ridge. The period is labeled P in
The recesses 40 need not extend only into top of the ridge. For instance, the recesses 40 can also extend into the slab regions 38 of the light-transmitting medium 30 as shown in
Hopping between the modes of
A laser cavity is generally associated with an operational temperature range and/or an operational applied current range. For instance, a laser cavity constructed according to
The laser cavity construction disclosed above is an example of an external cavity laser. An external cavity laser includes a passive region. For instance, the laser cavity guides the light through a medium other than the gain medium where light amplification does not occur at all or does not substantially occur. The region of the laser cavity where light is not amplified can serve as the passive region. As an example, the cavity waveguide 16 and gain waveguides 14 disclosed in the context of
Several variables of the laser cavity can be altered to change the optical characteristics of the laser cavity. For instance, the passive length of the laser cavity can be altered. Additionally or alternately, when the partial return device 18 is a Bragg grating, variables such as the depth of the recesses (dr in
The portion of the device disclosed above can be included in a transmitter. For instance,
An optical fiber 44 can be positioned in the fiber recess such that the core 45 of the optical fiber is aligned with the facet 42. As a result, the device output light signal can be transmitted through the facet 42 and be received in the core 45 of the optical fiber 44. Although the transmitter of
The transmitter can be included in an optical link. For instance,
One measure of the performance of an optical link such as the optical link illustrated in
The inventors have found that the laser cavity can be designed so that for certain data travel distances, the BER is at an acceptable level despite the occurrence of mode hops. As an example, for certain data travel distances, the laser cavity can be designed so the BER can be less than 10−12 or 10−15. Further, when Forward Error Correction (FEC) is employed, the laser cavity can be designed so the BER before EFC is less than 10−3 or 10−5. Examples of data travel distances where there BERs can be achieved include distances greater than 0 m, 0.1 m, 1 m or 10 m and/or less than 500 m or less than 1 km. While optical links have traditional been used over longer distances, the adoption of optical links into smaller systems has made these data travel distances more desirable.
As noted above, the above Bit Error Rates (BER) can be achieved through design of the laser cavity. In some instances, the laser cavity is designed such that the wavelength error is greater than greater than 0.15 nm and/or less than 0.30 nm for at least one, for at least a portion, or for all of the mode hops in the functional operational range of the laser cavity or for all mode hops experienced by the laser cavity. In one example, the laser cavity is designed such that the wavelength error is greater than 0.178 nm, or 0.189 nm and/or less than 0.239 nm, 0.245 nm, or 0.257 nm for at least one, for at least a portion, or for all of the mode hops in the functional operational range of the laser cavity or for all mode hops experienced by the laser cavity. As noted above, the desired level of wavelength error for all or a portion of the mode hops can be generally be achieved by altering the length of the passive section of the laser cavity, grating length, and/or length of the gain medium. In some instances, the laser cavity is designed such that the power variation is greater than −10 dBm and/or less than 0.6 dBm for at least one, for at least a portion, or for all of the mode hops in the functional operational range of the laser cavity or for all mode hops experienced by the laser cavity. In one example, the laser cavity is designed such that the power variation is greater than −5 dBm, or −3 dBm and/or less than 0.1 dBm, 0.2 dBm, or 0.3 dBm for at least one, for at least a portion, or for all of the mode hops in the functional operational range of the laser cavity or for all mode hops experienced by the laser cavity. As noted above, the desired value of the power variation for all or a portion of the mode hops can generally be achieved by tuning the length of the grating, the recess depth (dr in
In some instances, the laser cavity is configured to have one, two, or three conditions selected from a group consisting of a wavelength error that is greater than 0.15 nm and/or less than 0.30 nm for at least one, for at least a portion, or for all of the mode hops in the functional operational range of the laser cavity or for all mode hops experienced by the laser cavity; a power variation that is greater than −10 dBm and/or less than 0.6 dBm for at least one, for a portion, or for all of the mode hops in the functional operational range of the laser cavity or for all mode hops experienced by the laser cavity; and a SideMode Suppression Ratio (SMSR) that is less than 100 dB and/or greater than 30 dB for at least the functional operational range of the laser cavity. In some instances, a device having the laser cavity with these one, two, or three conditions is included in a optical link having a data travel distance less than 1 km, 500 m, or 100 m. In some instances, the laser cavity is configured to have two of the conditions satisfied for the same mode hop.
In some instances the desired conditions can be achieved using a device where one, two, three, four, or five parameters are selected from the group consisting of a grating greater than 300 um and/or less than 2000 um; a passive section length greater than 150 um and/or less than 800 um; recess depth (dr in
A first recess 71 extends through the silicon light-transmitting medium 30 and the silica insulator 34. A second recess 72 extends into the bottom of the first recess 71 such that the silicon substrate 36 forms shelves 73 in the bottom of the second recess 72. A first conducting layer 75 is positioned in the bottom of the second recess 72. A first conductor 76 on the silicon slab is in electrical communication with the first conducting layer 75. A second conductor 77 on the silicon slab is positioned adjacent to the first recess 71.
A gain element 10 is positioned in the first recess 71 and rests on the shelves 73. The gain element 10 includes a gain medium 12. A second conducting layer 78 is positioned on the gain medium 12. A third conductor 79 provides electrical communication between the second conducting layer 78 and the second conductor 77.
Three ridges extend into the second recess 72. The outer-most ridges have a passivation layer. The central ridge defines a portion of the gain waveguide 14 and is in electrical communication with the first conducting layer 75. The electrical communication between the central ridge and the first conducting layer 75 can be achieved through a conducting medium 80 such as solder. Since the first conductor 76 is in electrical communication with the first conducting layer 75, the first conductor 76 is in electrical communication with the central ridge.
The beam of light can be generated from the gain medium 12 by causing an electrical current to flow through the gain medium 12. The electrical current can be generated by applying a potential difference between the first conductor 76 and the second conductor 77. The potential difference can be provided by the electronics. The electronics can be included on the device or can be separate from the device but electrically coupled with the device.
The gain element 10 includes a reflecting surface on the gain medium 12. The reflecting surface can serve as the reflector 28 of
As is evident from
Suitable gain elements 10 include, but are not limited to, InP chips. The electrical communication between the second conducting layer 78 and the second conductor 77 can be achieved using traditional techniques such as wire bonding. The electrical communication between the central ridge and the first conductor 76 can be achieved through traditional techniques such as solder bonding.
Although
The receiver of
The seed portion 105 of the light-transmitting medium 30 can be continuous with the light-transmitting medium 30 included in the input waveguide 90 or spaced apart from the input waveguide 90. When the light signal enters the light sensor 92, a portion of the light signal can enter the seed portion 105 of the light-transmitting medium 30 and another portion of the light signal can enter the light-absorbing medium 112. Accordingly, the light-absorbing medium 112 can receive only a portion of the light signal. In some instances, the light sensor can be configured such that the light-absorbing material receives the entire light signal.
During the fabrication of the device, the seed portion 105 of the light-transmitting medium 30 can be used to grow the light-absorbing medium 112. For instance, when the light-transmitting medium 30 is silicon and the light-absorbing medium 112 is germanium, the germanium can be grown on the silicon using a process such as epitaxial growth. As a result, the use of the light-transmitting medium 30 in both the input waveguide 90 and as a seed layer for growth of the light-absorbing medium 112 can simplify the process for fabricating the device.
As is evident from
The input waveguide 90 optionally includes a taper 107. The taper 107 can be a horizontal taper and need not include a vertical taper although a vertical taper is optional. The taper 107 is positioned before the light sensor 92. For instance, the horizontal taper occurs in the light-transmitting medium 30 rather than in the light-absorbing medium 112. The taper 107 allows the light-absorbing medium 112 to have a narrower width than the input waveguide 90. The reduced width of the light-absorbing medium 112 increases the speed of the light sensor 92. The optical component preferably excludes additional components between the taper and light sensor although other components may be present.
During operation of the light sensor 92, a reverse bias electrical field is applied across the light-absorbing medium 112. When the light-absorbing medium 112 absorbs a light signal, an electrical current flows through the light-absorbing medium 112. As a result, the level of electrical current through the light-absorbing medium 112 indicates receipt of a light signal. Additionally, the magnitude of the current can indicate the power and/or intensity of the light signal. Different light-absorbing media 112 can absorb different wavelengths and are accordingly suitable for use in a light sensor 92 depending on the function of the light sensor 92. A light-absorbing medium 112 that is suitable for detection of light signals used in communications applications includes, but are not limited to, germanium, silicon germanium, silicon germanium quantum well, GaAs, and InP. Germanium is suitable for detection of light signals having wavelengths in a range of 1300 nm to 1650 nm.
Doped regions 116 of the light-absorbing medium 112 are positioned on the lateral sides of the ridge 110 of the light-absorbing medium 112. The doped regions 116 extend from the ridge 110 into the slab region of the light-absorbing medium 112. The transition of a doped region 116 from the ridge 110 of the light-absorbing medium 112 into the slab region of the light-absorbing medium 112 can be continuous and unbroken as is evident from
Each of the doped regions 116 can be an N-type doped regions or a P-type doped region. For instance, each of the N-type doped regions can include an N-type dopant and each of the P-type doped regions can include a P-type dopant. In some instances, the light-absorbing medium 112 includes a doped region 116 that is an N-type doped region and a doped region 116 that is a P-type doped region. The separation between the doped regions 116 in the light-absorbing medium 112 results in the formation of PIN junction in the light sensor 92.
In the light-absorbing medium 112, suitable dopants for N-type regions include, but are not limited to, phosphorus and/or arsenic. Suitable dopants for P-type regions include, but are not limited to, boron. The doped regions 116 are doped so as to be electrically conducting. A suitable concentration for the P-type dopant in a P-type doped region includes, but is not limited to, concentrations greater than 1×1015 cm−3, 1×1017 cm−3, or 1×1019 cm−3, and/or less than 1×1017 cm−3, 1×1019 cm−3, or 1×1021 cm−3. A suitable concentration for the N-type dopant in an N-type doped region includes, but is not limited to, concentrations greater than 1×1015 cm−3, 1×1017 cm−3, or 1×1019 cm−3, and/or less than 1×1017 cm−3, 1×1019 cm3, or 1×1021 cm−3.
Each doped region 116 is in contact with an electrical conductor 109 such as a metal. Accordingly, each of the doped regions 116 provides electrical communication between an electrical conductor 109 and a lateral side of the ridge of light-absorbing medium 112. As a result, electrical energy can be applied to the electrical conductors 109 in order to apply electrical energy to the lateral side of the ridge of light-absorbing medium 112. As a result, the resulting electrical field can be substantially parallel to the base 32.
A variety of other light sensor 92 constructions are suitable for use with waveguides on a silicon-on-insulator platform. For instance, the light sensor 92 can be constructed and/or operated as disclosed in U.S. patent application Ser. No. 12/380,016, filed Feb. 19, 2009, and entitled “Optical Device Having Light Sensor Employing Horizontal Electrical Field;” U.S. patent application Ser. No. 12/804,769, filed Jul. 28, 2010, and entitled “Light Monitor Configured to Tap Portion of Light Signal from Mid-Waveguide;” and/or in U.S. patent application Ser. No. 12/803,136, filed Jun. 18, 2010, and entitled “System Having Light Sensor with Enhanced Sensitivity;” and/or in U.S. patent application Ser. No. 12/799,633, filed Apr. 28, 2010, and entitled “Optical Device Having Partially Butt-Coupled Light Sensor;” and/or in U.S. patent application Ser. No. 12/589,501, filed Oct. 23, 2009, and entitled “System Having Light Sensor with Enhanced Sensitivity;” and/or in U.S. patent application Ser. No. 12/584,476, filed Sep. 4, 2009, and entitled “Optical Device Having Light Sensor Employing Horizontal Electrical Field;” each of which is incorporated herein in its entirety.
Although the transmitter disclosed above is disclosed in the context of a transmitter having a single laser cavity, the transmitter can include multiple laser cavities. Additionally or alternately, the receiver can include multiple light sensors rather than the single light sensor disclosed above. Examples of suitable transmitters and receivers are disclosed in U.S. patent application Ser. No. 14/280,067, filed on May 16, 2014 and entitled “Reducing Power Requirements for Optical Links” and also in U.S. patent application Ser. No. 14/048,685, filed on Oct. 8, 2013 and entitled “Use of Common Active Materials in Optical Components,” each of which is incorporated herein in its entirety.
A transmitter having a first laser cavity according to
A mode hop occurs when a current of about 80 mA is applied to the gain medium in the transmitter of Example 1 and the laser cavity is at a temperature of 60° C. The laser cavity is configured such that the wavelength error for the mode hop is 0.27 nm, and the power variation for this mode hop is 0.172 mW. Since the most intensely output mode in FIG. 3A is the fundamental mode, the SideMode Suppression Ratio (SMSR) is 35.13 dB.
The transmitter was included in optical links with different data travel distances and eye diagrams were generated using a data rate of 25 Gb/s. The applied current was swept from 40 to 100 mA. At a data travel distance of 500 m, a Bit Error Rate (BER) of 10−12 was achieved and did not substantially change in response to the mode hop at the current level of about 80 mA. At a data travel distance of 1 km, a BER of 10−11 was achieved and did not substantially change in response to the mode hop. At a data travel distance of 2 km, a BER of 10−9 was achieved and did not substantially change in response to the mode hop.
Other embodiments, combinations and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.
