Patent Application
20050094696
The present disclosure relates to a laser device and, more particularly, to a laser transmitter using a photodiode for laser performance monitoring.
Laser devices like network transmitters and transponders often monitor output power levels as part of a control scheme to optimize device performance. The importance of regulating output power levels has been known for years. Yet, as the number of optical components within a communications network (e.g., transponders, optical fibers, switches, amplifiers, repeaters, etc.) increases, power regulation has become more of an issue for component designers. Across the network, power regulation improves data integrity and prolongs device lifetime.
The typical power regulation technique uses monitoring photodiodes to measure a portion of the laser device's output energy. A controller receives a current signal from the photodiode and determines if the laser device is operating within an acceptable output power range. If the device is not, then the controller may correspondingly adjust the laser's power supply, an external modulator, or an associated attenuator to achieve the desired output power level.
Generally, there are two techniques for measuring the output power of a laser.
One problem with this design is that it is unusable for certain lasers. For external cavity lasers, the front and back power may not be proportional. In some lasers, the laser cavity is formed by a semiconductor chip with one face acting as the first mirror and an external mirror acting as the second. For Vertical Surface Emitting Lasers (VCSEL), there may be no emission from the back face at all.
Another technique for monitoring output power is shown in
In some communication devices, optical isolators are used to prevent backward traveling or backward scattered waves from impinging on the device's laser source. Substantial amounts of energy can exist in a backward wave as a result of boundary reflections or back-scattering phenomena, such as stimulated Brillouin scattering. Proposed herein are techniques and apparatuses for using existing optical isolators to assist in the monitoring of output energy of a laser device. Although the examples may be described with reference to laser devices such as those that may be used in transponders and transmitters, persons of ordinary skill in the art will recognize that the attendant descriptions may be implemented in sensors, amplifiers, switches, routers, and other optical devices having optical isolators and which may benefit from output signal monitoring.
The device 200 includes a coupling stage 208 that includes a lens 210 with a principle axis, x1, aligned with an axis, x2, extending between the laser 202 and the output device, fiber 206. Alignment is not necessary; the axes x1 and x2 may be misaligned, instead. In the illustrated example, the lens 210 couples part of the energy 204 into the fiber 206 through the isolator 216. The laser source 202, the fiber 206 and the lens 210 are positioned to reach the adequate coupling for the application. This coupling typically ranges from 10% in short range transmitter to 90% for long-range transmitter.
The isolator 216 protects the laser source 202 from backward waves. Unlike the lens 210, the optical isolator 216 is not aligned about the axis, x2. Instead, the isolator 216 has a front face 218 and a parallel, back face 220 that are both angled an angle, φ, with respect to the axis, x2. The angle, φ, is adjustable across a range of angles, and may be between 0-15°, in one example.
A majority of the energy 204 is provided as the energy 204′. A portion of the energy 204, is reflected off of the front face 218 of the isolator 216. Laser energy reflected from the face 218 is reflected back into the lens 210 and focused onto a photodiode 222 that is laterally displaced from the laser 202 by a distance, D. The lateral distance, D, is dependent upon the angle, φ. The walk-off distance D increases with higher angles, φ. In the illustrated example, the photodiode 222 being laterally displaced from the laser 202 allows for greater compactness. Nevertheless, the photodiode may be positioned elsewhere. The amount of energy reflected by the isolator 216 is adjustable by applying different reflective coatings to the face 218.
The lens 210 is mounted to the substrate 250 via a flexure 254, in the illustrated example. A fiber support 256 is also mounted to the substrate 250 using techniques described herein. The support 256 may include a v-groove recess sized to accept the fiber 206 glued or clamped therein. Other fiber or pigtail mountings will be known to persons of ordinary skill in the art.
The optical isolator 216 is shown in more detail in
The offset angle, φ, for the isolator 216 may be predetermined prior to assembly. Alteratively, the offset may be set by operating the laser 202 with the isolator 216 temporarily in place and rotating the position of the isolator 216 relative to the axis, x2, until the desired amount of reflected energy is detected by the photodiode 222. From this calibrated position, the isolator 216 may be bonded in place on the substrate 250. Because only slight tilting angles, φ, are used, the isolator offset will have a limited affect on the position of the coupled light 204′ into the fiber 206.
The illustrated examples of
The transceiver 402 is connected to a controller 424, which may represent a microprocessor, for example. A bus 426 connects the receiver stage 408 to the controller 424, and a bus 428 connects the transmitter stage to the controller 424. For the receiver stage 408, the controller 424 may include a deserializer and decoder coupled with the bus 416. For the transmitter stage 416, the controller 424 may include an encoder and a serializer coupled with the bus 428.
In operation, a multi-channel or single channel data stream is received on the fiber 406. The multi-channel data-stream is coupled into the photodiode 410 for optical-to-electrical signal conversion. Data from the photodiode 410 is coupled to the trans-impedance amplifier 412 and sent on to the amplifier 414 prior to being sent to the deserializer within the controller 424 via the bus 426. The deserializer provides a 10 bit signal to the decoder, which decodes the input signal and creates a 10 bit word that may be passed to a Gigabit Media Independent Interface (GMII) bus 430. For data transmission, input data from the GMII is first encoded by the encoder and then serialized by the serializer to create a transmittable serial bit stream. The output from the serializer is provided on the bus 428 and controls the output of the laser device 418. The laser device 418 includes an optical isolator positioned to tap a portion of the output energy back into a photodiode positioned adjacent the laser source of the laser device 418. The monitored signal from this photodiode is used by the controller 424 or control circuitry within the transmitter stage 416 to adjust the output intensity of the laser device 418 or to adjust the amount of amplification from the amplifier 422. This feedback control may be a separate control having predetermined output power intensity levels. The control may measure output energy from the laser device 418 directly or, alternatively, it may measure output energy from the amplifier 422, for example, by positioning the optical isolator downstream of the amplifier 422 and positioning a photodiode to collect the partially reflected energy. In the illustrated example, the output signal from the laser device 418 is modulated by the modulator 420 and then amplified by the amplifier 422 prior to transmission on the fiber 404.
While the illustration of
The embodiments illustrated and described herein are provided by way of example only numerous modifications and changes may be made to the illustrated embodiments. For example, the laser device may couple the collimated beam to other downstream devices, such as active or passive optical devices (e.g., modulators, amplifiers, or filters) in place of a focusing lens. Furthermore, the distances between the laser and the fiber may be chosen to increase compactness. Further still, the focal lengths and tilt angles, φ, may be chosen to focus the reflected beam onto various spot sizes. As a result, a photodiode ranging from above 500 μm in diameter to below 100 μm in diameter may be used. These are provided by way of example only.
Also, other optical objective components may be used in place of or in addition to the lenses described. Multiple lens objectives, prisms, and mirrors are examples, as well as apertures that reduce beam spot size.
By way of further example, the laser sources described above may be any of a variety of laser sources including semiconducting edge emitting lasers, VCSELS, external cavity lasers, and laser amplifiers. The lasers may represent active or passive laser sources, as well. Persons of ordinary skill in the art will appreciate other alternatives from the foregoing description and in light of the following claims.
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.
