Patent Application
20160134080
Certain applications, such as laser spectroscopy and laser metrology, for example, employ lasers having extremely precise and stable wavelengths, as well as sufficiently long coherence lengths to support the application. Typically, gas lasers are used in such applications because gas lasers are known to have such very precise and stable wavelengths, and sufficiently long coherence lengths.
Gas lasers convert electrical energy to laser light by discharging an electric current through a gas. A common and inexpensive gas laser is the Helium-Neon (HeNe) gas laser. HeNe lasers are available in a variety of colors, such as red (632.8 nm), orange (612 nm), yellow (594 nm), and green (543.5 nm). Other gas lasers, such as argon, krypton, and xenon, as also well known.
In some applications, however, gas lasers may be undesirable. In many applications, semiconductor laser diodes have largely taken the place of traditional gas lasers. However, though laser diodes are often advertised as replacements for gas lasers, it is well known that traditional laser diodes have not been able to replicate the coherence length and wavelength precision and stability of gas lasers.
But laser diodes do have some practical advantages that make them desirable as gas laser replacements. For example, laser diodes are smaller, more efficient, and more versatile than gas lasers. It would be desirable therefore, if there were available a semiconductor laser diode that could replicate the coherence length and wavelength precision and stability of a gas laser.
As disclosed herein, a semiconductor laser diode system may include a single longitudinal mode laser diode and a feedback system that monitors and controls the emission characteristics of the laser diode. The feedback system may include a wavelength discriminator, an optical detector, a microprocessor, and a laser controller.
The laser diode may be stabilized according to known laser stabilization techniques. The laser diode may include a gain medium and an optical feedback device. The gain medium may be chosen to most nearly approximate the desired laser emission wavelength. The optical feedback device may feed a narrowband portion of the emitted radiation back into the gain medium to cause the laser diode to achieve single longitudinal mode at the desired wavelength. The optical feedback device may be a volume Bragg grating element, i.e., a three-dimensional optical element having a Bragg grating recorded therein. The volume Bragg grating element may be a bulk of photorefractive glass, having the Bragg grating holographically recorded therein. The Bragg grating may cause a portion of the light emitted from the gain medium to be reflected back into the gain medium as seed light at a very precisely known wavelength.
The wavelength discriminator may receive light emitted from the laser diode. The wavelength discriminator may be a partially reflective optical element that allows most of the light emitted from the laser diode to pass through transparently, and yet diverts a small portion of the light toward the optical detector. The wavelength discriminator may be a second volume Bragg grating element having a Bragg grating recorded therein. The Bragg grating may be formed such that the volume Bragg grating element is basically transparent to the light emitted from the laser diode, and yet diverts a small portion of the light toward the optical detector. The Bragg grating may be formed to be a wavelength-selective Bragg grating. That is, the Bragg grating may be formed such that the portion of the light that is diverted to the optical detector consists of only a certain subset of the wavelengths present in the light emitted from the laser diode. The wavelength discriminator could be an etalon or a diffraction grating.
The optical detector receives the light diverted by the wavelength discriminator. The optical detector detects the optical power distribution over several frequency channels, and produces an electrical signal representative of the optical power distribution. The optical detector may include an arrangement of one or more photodiodes, or it may include one or more charge-coupled devices (CCDs). The optical detector may pass the electrical signal to the microprocessor.
The microprocessor analyzes the electrical signal to assess the emission characteristics of the laser diode. If the microprocessor determines that the emission characteristics of the laser diode are not exactly as they should be, the microprocessor commands the controller to alter a characteristic of the laser diode.
The laser controller may control the temperature, drive current, or cavity length of the laser diode. Based on the commands received from the microprocessor, the laser controller determines whether to alter one, or more, or any, of the laser diode characteristics. The laser controller may issue commands to one or more of a temperature controller, drive current controller, or cavity length controller. The several controllers may all be executed in a single microprocessor, or in different processors.
The temperature controller may cause the temperature of the laser diode to be adjusted by causing a thermoelectric cooler to draw more or less thermal energy from the laser diode. The drive current controller may cause the drive current of the laser diode to be adjusted by causing a current driver to provide more or less drive current to the laser diode. The cavity length controller may cause the cavity length to be adjusted by causing an electromechanical element, such as a piezo element, for example, to increase or decrease the distance between the gain medium and the feedback grating element.
As disclosed herein, such a semiconductor laser diode system may be used to produce laser light having coherence length, wavelength precision, and wavelength stability that is equivalent to that of a gas laser. Accordingly, such a semiconductor laser diode system may be used in place of a traditional gas laser.
To be used in place of a traditional gas laser, the laser diode system 100 may be configured to achieve a coherence length of at least 30 meters, wavelength precision of ±5 pm, and wavelength stability of less than ±5 pm. It should be understood, of course, that the laser diode system disclosed herein may be used in applications other than in place of traditional gas lasers.
As shown in
The laser diode 110 may include a gain medium 112 and an optical feedback device 114. The gain medium 112 may be chosen to most nearly approximate the desired laser emission wavelength. The optical feedback device 114 may feed a narrowband portion of radiation emitted from the gain medium 112 back into the gain medium 112 to cause the laser diode 110 to achieve single longitudinal mode at a desired wavelength.
The optical feedback device 114 may be a volume Bragg grating element, i.e., a three-dimensional optical element having a Bragg grating recorded therein. The volume Bragg grating element may be a bulk of photorefractive glass, having the Bragg grating holographically recorded therein. The Bragg grating may cause a portion of the radiation emitted from the gain medium 112 to be reflected back into the gain medium 112 as seed light at a very precisely known wavelength.
It should be understood, of course, that the laser diode system disclosed herein may produce laser light having a wavelength that is not equivalent to that of any known gas laser. However, to be used in place of a traditional gas laser, the laser diode system 100 may be configured to produce laser light having a wavelength that is equivalent to that of a known gas laser. For example, the laser diode system 100 may be configured to produce laser light having a wavelength that is equivalent to that of a HeNe gas laser. Specifically, the laser diode system 100 may be configured to produce laser light having a wavelength of 632.8 nm for red, 612.0 nm for orange, 594.0 nm for yellow, or 543.5 nm for green. Argon lasers typically emit at 514.5 nm for green, 457.9 nm for blue, or 488.0 nm for blue-green, among others. Krypton lasers typically emit at 647.1 nm, 413.1 nm, or 530.9 nm, among others.
The optical feedback device 114 can be configured to cause the laser diode 110 to achieve single longitudinal mode. As shown in
The wavelength discriminator 122 may receive light emitted from the laser diode 110. The wavelength discriminator 122 may be a partially reflective optical element that allows most of the light emitted from the laser diode 110 to pass through transparently, and yet diverts a small portion of the light toward the optical detector 124. The wavelength discriminator 122 may be a second volume Bragg grating element having a Bragg grating recorded therein. The Bragg grating may be formed such that the volume Bragg grating element is basically transparent to the light emitted from the laser diode 110, and yet diverts a small portion of the light toward the optical detector 124. The Bragg grating may be formed to be a wavelength-selective Bragg grating. That is, the Bragg grating may be formed such that the portion of the light that is diverted to the optical detector 124 consists of only a certain subset of the wavelengths present in the light emitted from the laser diode 110. Other examples of wavelength discriminators include etalons, diffraction gratings, and gas cells, all of which are well known.
The optical detector 124 receives the light diverted by the wavelength discriminator 122. The optical detector 124 detects the optical power distribution over several frequency channels, and produces an electrical signal representative of the optical power distribution. The optical detector 124 may include an arrangement of one or more photodiodes, or it may include one or more charge-coupled devices (CCDs). The optical detector 124 may pass the electrical signal to the processor 126.
The processor 126 may be a microprocessor, for example, that is configured to analyze the electrical signal to assess one or more emission characteristics of the laser diode. If the processor 126 determines that an emission characteristic of the laser diode is undesirable (e.g., the laser diode 110 is emitting laser light having a wavelength or bandwidth that is outside the desired range for performance as an equivalent to a gas laser), the processor 126 may command the laser controller 128 to alter a characteristic of the laser diode 110 to thereby alter the undesired emission characteristic.
The laser controller 128 may control the temperature, drive current, cavity length, or other characteristic of the laser diode. Based on commands received from the processor 126, the laser controller 128 may determine whether to adjust one, or more, or any, of the laser diode characteristics (e.g., temperature, drive current, and cavity length). The laser controller 128 may issue commands to one or more laser diode characteristic controllers 129. Examples of characteristic controllers are a temperature controller, a drive current controller, and a cavity length controller (see
The processor 126 and the laser controller 128 may be implemented in single microprocessor, or in different microprocessors. Likewise, the laser diode characteristic controller(s) 129 may be implemented in a single microprocessor, which may be the same microprocessor as the processor 126 and/or laser controller 128, or they may be implemented in different microprocessors.
Sudden jumps in output power correspond to changes in the operating condition of the laser. That is, the sudden jumps in output power indicate when the laser switches between SLM and non-SLM operation, and also between different longitudinal modes within the SLM regime. Using the features of the output power vs operating current it is possible to identify regions of SLM operation, as noted in
The system may be configured such that, on startup, the system executes a search algorithm to perform a scan of monitor current vs drive current to determine what drive current regions produce SLM operation. Depending on the desired output power, a corresponding drive current may be determined from the monitor current that corresponds to the desired output power. For example, if the desired output power corresponds to a monitor current of 120 μA, then SLM operation of the laser diode may be achieved at a drive current of 142 mA. The laser is tuned to an SLM condition by monitoring its output power and adjusting the drive current and operating temperature.
Note that SLM operation may not be achievable for all output powers at any given temperature. For example, as shown in
The laser controller 528 may be instructed by commands received from the processor 526. In response to the commands received from the processor 526, the laser controller 528 may instruct one or more of the characteristic controller(s) 529 to alter a respective characteristic of the laser diode 510.
For example, the cavity length controller may instruct a cavity length adjuster to adjust the cavity length of the laser diode 510. The cavity length controller may cause the cavity length to be adjusted by causing the cavity length adjuster to increase or decrease the distance between the gain medium and the feedback grating element. The cavity length adjuster may be an electromechanical element, such as a piezo element, for example.
The temperature controller may instruct a heating/cooling element to adjust the temperature of the laser diode 510. The temperature controller may cause the temperature of the laser diode 510 to be adjusted by causing the heating/cooling element to draw more or less thermal energy from the laser diode 510. The heating/cooling element may be a thermoelectric cooler, for example.
The drive current controller may instruct a current driver to adjust the drive current of the laser diode 510. The drive current controller may cause the drive current of the laser diode 510 to be adjusted by causing the current driver to provide more or less drive current to the laser diode 510.
A typical HeNe multimode laser diode may have a wavelength precision of about +/−1 picometer, a wavelength stability of +/−1 picometer, and a coherence length of about 30 centimeters. A typical SLM HeNe laser diode without wavelength stabilization may have a wavelength precision of about +/−5 MHz, a wavelength stability of +/−5 MHz, and a coherence length of about 30 meters. A typical SLM HeNe laser diode with wavelength stabilization may have a wavelength precision of about +/−2 MHz, a wavelength stability of +/−2 MHz, and a coherence length of more than about 100 meters.
The wavelength selective element 806 directs at least a portion of the incident light toward a second wavelength selective element 808. The wavelength selective element 808 may be, for example, a VBG element or a diffractive grating. The wavelength selective element 808 directs light having a second wavelength, λ+, toward a second photodetector 809. The photodetector 809 produces a current, I+, that is proportional to the energy received at the photodetector 809. The wavelengths, λ+ and λ−, respectively, may be chosen to be plus and minus a small offset to the desired operating wavelength, λ0.
The currents I+ and I− are fed into an analog-to-digital converter 810. The digitized current streams are provided to the processor 804. The processor 804 determines from the digitized current streams whether the laser diode 801 is emitting at the desired operating wavelength, λ0. For example, the processor 804 may determine that the laser diode is operating at the desired operating wavelength, λ0, if the currents I+ and I− are balanced. If the processor determines that the currents I+and I− are not balanced, and therefore that the laser diode is not emitting at the desired operating wavelength, λ0, then the processor may instruct the laser controller 803 to adjust one or more characteristics of the laser diode in a manner that would be expected to move the actual operating wavelength closer to the desired operating wavelength, λ0.
Basically, it should be understood that the wavelength discriminator will have a certain bandwidth. The bandwidth of the wavelength discriminator may correspond to a desired stabilization range, that is, the range of wavelength in which the wavelength stabilization system is able to control the operating wavelength of the laser diode. The laser diode can be stabilized more precisely when it can detect in this range.
The wavelength selective element 906 directs a portion of the light having a wavelength, λ, toward a second photodetector 909. The photodetector 909 produces a current, I−, that is proportional to the energy received at the photodetector 809. The wavelengths, λ+ and λ−, respectively, may be chosen to be plus and minus a small offset to the desired operating wavelength, λ0.
The currents I+ and I− are fed into an analog-to-digital converter 910. The digitized current streams are provided to the processor 904. The processor determines from the digitized current streams whether the laser diode is emitting at the desired operating wavelength, λ0. If the processor determines that the laser diode is not emitting at the desired operating wavelength, λ0, then the processor instructs the laser controller 903 to adjust one or more characteristics of the laser diode in a manner that would be expected to move the actual operating wavelength closer to the desired operating wavelength, λ0.
The etalon 1006 transmits a portion of the light having a wavelength, λ, toward a second photodetector 1009. The photodetector 1009 produces a current, It, that is proportional to the energy received at the photodetector 1009. The wavelengths, λr and λt, respectively, may be chosen to be plus and minus a small offset to the desired operating wavelength, λ0. The etalon may be rotated to tune to the desired reflected and transmitted wavelengths, λr and λt.
The currents I+ and I− are fed into an analog-to-digital converter 1010. The digitized current streams are provided to the processor 1004. The processor 1004 determines from the digitized current streams whether the laser diode 1001 is emitting at the desired operating wavelength, λ0. If the processor 1004 determines that the laser diode 1001 is not emitting at the desired operating wavelength, λ0, then the processor 1004 instructs the laser controller 1003 to adjust one or more characteristics of the laser diode 1001 in a manner that would be expected to move the actual operating wavelength closer to the desired operating wavelength, λ0.
The etalon 1106 transmits a portion of the light having a wavelength, λt1, toward a second photodetector 1109. The photodetector 1109 produces a current, It1, that is proportional to the energy received at the photodetector 1109. The wavelengths, λr1 and λt1, respectively, may be chosen to be plus and minus a small offset to the desired operating wavelength, λ0. The etalon 1106 may be rotated to tune to the desired reflected and transmitted wavelengths, λr1 and πt1.
The beam sampler 1105 directs a portion of the incident light toward a second etalon 1116. The etalon 1116 reflects a portion of the light, having a first wavelength, λr2, toward a photodetector 1117. The photodetector 1117 produces a current, Ir2, that is proportional to the energy received at the photodetector 1117.
The etalon 1116 transmits a portion of the light having a wavelength, πt2, toward a photodetector 1119. The photodetector 1119 produces a current, It2, that is proportional to the energy received at the photodetector 1119. The wavelengths, λr2 and λr2, respectively, may be chosen to be plus and minus a small offset to the desired operating wavelength, λ0. The etalon 1116 may be rotated to tune to the desired reflected and transmitted wavelengths, λr2 and λt2.
The currents It1, It2, Ir1, and Ir2 are fed into an analog-to-digital converter 1110. The digitized current streams are provided to the processor 1104. The processor 1104 determines from the digitized current streams whether the laser diode 1101 is emitting at the desired operating wavelength, λ0. If the processor 1104 determines that the laser diode 1101 is not emitting at the desired operating wavelength, λ0, then the processor 1104 instructs the laser controller 1103 to adjust one or more characteristics of the laser diode 1101 in a manner that would be expected to move the actual operating wavelength closer to the desired operating wavelength, λ0.
