Patent Application
20150214693
The present disclosure generally relates to pulse laser output stabilizing apparatuses and, more particularly, to a pulse laser output stabilizing apparatus using real-time feedback control and an acousto-optic tunable modulator.
Since 2000s, optical sources have been developed vigorously. Besides industry of lasers, various other components such as a light-emitting diode (LED) and a semiconductor optical amplifier (SOA) have also been grown rapidly and have been used in various industrial applications such as lightings, tests, and skin treatments. In laser fields of bandwidth, ultra-broadband light source components of 250 nm to 2500 nm (Optical Parametric Oscillator, Super-continuum source, etc.) are also well developed using nonlinear effect. These broadband light sources have extended their application range using new bands of wavelength in optical communication, optical measurement, and bio-imaging fields.
However, in case of a broadband laser, time-dependent output power variation is very large. Since conventional low power lasers (<1 W) used in industries are continuous-wave (CW) type lasers, they are very stable lasers whose time-dependent output variation is less than 5 percent. Meanwhile, since broadband lasers are pulse-type lasers and use nonlinear crystal, their time-dependent output variation is very large.
Embodiments of the present disclosure provide a pulse laser output stabilizing apparatus for stabilizing a broadband pulse laser output.
A pulse laser output stabilizing apparatus according to an embodiment of the present disclosure may include a directional coupler configured to receive an output of pulse laser such that the output branches into a first optical path and a second optical path; a photodetector configured to receive light branching into the first optical path and output current according to the intensity of the light; a current-voltage converter configured to convert output current of the photodetector into a voltage and output the converted voltage; a function generator configured to provide an output proportional to an output signal of the current-voltage converter with a predetermined frequency; a time delay unit disposed on the second optical path to provide a predetermined time delay for feedback control; and an acousto-optic tunable modulator configured to receive an output signal of the functional generator and an optical signal provided from the time delay unit as an input and modulate and output the optical signal provided from the time delay unit according to the amplitude of the output signal of the function generator.
In example embodiments, the pulse laser output stabilizing apparatus may further include an amplifier disposed between the current-voltage converter and the function generator to amplify an output signal of the current-voltage converter and provide the amplified output signal as an input signal of the function generator.
In example embodiments, the acousto-optic tunable modulator may include a disc-shaped piezoelectric transducer having a first through-hole formed in its center, the piezoelectric transducer being configured to generate an acoustic wave; a conic dielectric cone having a second through-hole formed in its center; an optical fiber inserted into the first through-hole and the second through-hole to be disposed there; and an acoustic damper spaced apart from the dielectric cone by a predetermined distance to be coupled with the optical fiber.
In example embodiments, a wavelength of the pulse laser may be variable.
In example embodiments, delay time of the time delay unit may be between 50 and 70 microseconds.
A pulse laser output stabilizing method according to an embodiment of the inventive concept may include receiving output light of a pulse laser to branch into a first optical path and a second optical path; receiving light branching into the first optical path to output first current depending on light intensity; converting the first current into a first voltage; providing an output of a second voltage proportional to the first voltage with a predetermined frequency; providing a predetermined time-delay on the second optical path; receiving the second voltage to acousto-optically modulate and output a time-delayed optical signal of the second optical path according to the magnitude of the second voltage.
In example embodiments, the pulse laser output stabilizing method may further include amplifying the first voltage.
In example embodiments, a wavelength of the pulse laser may be variable.
In example embodiments, the delay time may be between 50 and 70 microseconds.
The present disclosure will become more apparent in view of the attached drawings and accompanying detailed description. The embodiments depicted therein are provided by way of example, not by way of limitation, wherein like reference numerals refer to the same or similar elements. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating aspects of the present disclosure.
Lasers operating in the wavelength range from 250 nm to 2500 nm such as an optical parametric oscillator (OPO) and a super-continuum source (SC) have been developed to meet a need for broadband lasers. However, due to pulse-type and nonlinear characteristics, outputs of these lasers are much more unstable than an output of continuous wave (CW) laser. Accordingly, an apparatus for stabilizing an output of broadband laser in real time is required.
The present disclosure provides an apparatus for stabilizing an output of pulse laser in real time which may operate with a broad wavelength range of 250 nm to 2500 nm.
First, the description will be given to the operation principle of an acousto-optic wavelength tunable modulator that is an essential component of a pulse laser output stabilizing apparatus according to the present disclosure.
Referring to
The acoustic generation part 142 may include a disc-shaped piezoelectric transducer 142a and a dielectric cone 142b. The piezoelectric transducer 142a may be a shear mode lead zirconate titanate (PZT) piezoelectric transducer. The piezoelectric transducer 142a and the dielectric cone 142b are bonded to each other. A first through-hole is formed in the center of the piezoelectric transducer 142a, and a second through-hole is formed on a central axis of the dielectric cone 142b. The optical fiber 144 is inserted into the first through-hole and the second through-hole.
Light of an LP01 mode of a Gaussian shape, which is irradiated from laser, may travel along the optical fiber 144. In this case, when a function generator 180 applies a sine-type voltage signal of a predetermined oscillation frequency to the piezoelectric transducer 142, the piezoelectric transducer 142a may generate an acoustic wave in a vertical direction according to the oscillation frequency. Accordingly, the dielectric cone 142b bonded to the piezoelectric transducer 142a concentrate the acoustic energy on a vertex of the dielectric cone 142b. Thus, the acoustic energy produces periodical bending at the optical fiber 144. When a period of the periodical bending satisfies a phase matching condition, the LP01 mode may be maximally converted into an LP1m mode. The phase matching condition corresponds to a case where the acoustic wavelength (A) is equal to effective refractive indices of the LP01 mode and the LP1m mode.
That is, β01-β11=2π/Λ,
Where, β01 represents a propagation constant of the LP01 mode and β11 represents a propagation constant of the LP11 mode.
The acoustic damper 145 is disposed to be sufficiently spaced apart from the dielectric cone 142b along the optical fiber 144. The acoustic damper 145 may absorb the acoustic energy that travels along the optical fiber 144.
The converted LP1m mode may be removed through a mode stripper (not shown) that is a higher mode stripper. Thus, a transmission of the LP01 mode provided to the AOTM 140 from the laser may be adjusted. That is, the AOTM 140 may perform intensity modulation.
The AOTM 140 may modulate a transmission of an input terminal by a frequency (f) to adjust the periodical bending provided to the function generator 180 and a voltage (V) to adjust amplitude of the bending.
Referring to
Laser output stabilizing apparatuses capable of providing feedback in real time may be classified into an optics module and an electronics module. The electronics module performs a feedback function to generate a stabilized pulse.
The pulse laser 10 outputs pulse light, and the intensity of the pulse light becomes unstable depending on time. The pulse laser 10 may be an optical parametric oscillator (OPO) or a super-continuum source (SC). A wavelength of the pulse laser 10 is variable. The frequency of the pulse laser 10 may vary in the entire or partial area within the range from 250 nm to 2500 nm.
The pulse light is provided to the directional coupler 120. The directional coupler 120 divides an optical path into the first optical path and the second optical path. More specifically, the output light of the pulse laser may branch into two optical paths via an optical fiber directional coupler.
First light propagating along the first optical path is provided to the photodetector 150 to provide a control signal to the AOTM 140. Second light propagating along the second optical path is modulated by the AOTM 140. The time delay unit 130 is disposed between the AOTM 140 and the directional coupler 120 to compensate a time delay for modulating the second light provided to the AOTM 140. The time delay unit 130 may be an optical fiber. The time delay provided by the time delay unit 130 may be between several microseconds (μsec) and tens of μsec.
The first light is provided to the photodetector 150. The photodetector 150 may be a photodiode capable of adjusting temperature. The photodector 150 may convert the intensity of the first light into current. An output signal of the photodetector 150 may be converted into a voltage signal through the current-voltage converter 160.
Since the magnitude of an output signal of the current-voltage converter 160 is small, the output signal of the current-voltage converter 160 may be provided to an amplifier 170. The amplifier 170 may amplify and output an input voltage signal. A gain of the amplifier 170 is variable.
An output signal of the amplifier 170 is provided to the function generator 180. An output signal of the function generator 180 may have a predetermined frequency and be in proportion to an input signal. In case of an input terminal of a laser pulse with a wavelength range of 1500 nm, the frequency may be 2.23 MHz to 2.27 MHz.
An output signal of the function generator 180 is provided to the AOTM 140. The AOTM 140 vibrates an optical fiber with the frequency of the function generator 180 and an output voltage of the amplifier 170 to adjust a transmission.
For example, when the pulse laser 10 outputs a high-power pulse, the time delay unit 130 delays time and an amplified voltage detected by the photodetector 150 is combined with the function generator 180 to provide a control signal the AOTM 140 such that the transmission is reduced to allow only the fixed amount of light to be transmitted for the delayed time. On the other hand, when the pulse laser 10 outputs a low-output pulse, a specific frequency of a low voltage is controlled by the function generator 180 to allow the AOTM 140 to increase the transmission. Thus, the AOTM 140 outputs a constant output pulse light bundle.
The most important point to manufacture a laser output stabilizing apparatus capable of providing feedback in real time is to minimize feedback time. For achieving this, performance evaluation of the AOTM 140 is required.
If the current-voltage converter 160, the amplifier 170 or the function generator 180 is a programmed device, time required to execute a command is about several milliseconds (ms). Accordingly, the time delay unit 130 needs an optical fiber having a length of hundreds of kilometers (km) for time delay. For example, if time delay of 1 ms occurs, a length of an optical fiber is about 200 km when a refractive index of the optical fiber is 1.45. Loss of a typical optical fiber is about 0.22 dB/km at a wavelength of 1550 nm. Therefore, when light travels a distance of 200 km, 44 dB is lost and thus the light reaching the AOTM 140 is substantially entirely lost.
Accordingly, all steps for real-time feedback may be preferably performed by a passive apparatus having no program command. In this case, the time taken by the current-voltage converter 160, the amplifier 170 or the function generator 180 is about 1 μsec. Meanwhile, it is necessary to measure operation time of the AOTM 140 depending on a frequency and a voltage.
First, frequency-dependent operation time of the AOTM 140 was investigated. The operation principle of the AOTM 140 serves to remove the LP01 mode, which is an output mode of an input terminal at laser, through modulation caused by periodical bending that is produced by applying an acoustic wave to an optical fiber. The periodical bending may vary depending on a frequency applied from the function generator 180.
Referring to
Referring to
Referring to
The AOTM 140 may perform modulation only by changing a frequency. However, a current-frequency converter or a voltage-frequency converter is required to provide feedback in real time. However, most of the above devices require a field programmable gate array (FPGA) that an active device where a program need to be executed. The number of FPGAs to change a frequency is limited according to a channel, and command execution time increases in units of several milliseconds (ms) when a program command is input to an FPGA. Therefore, an FPGA is not suitable for modulation.
Accordingly, voltage-dependent modulation of an AOTM is required to decrease command execution time or feedback time.
Although a test device has the same configuration as shown in
Referring to
Output light of the pulse laser 10 is converted into current in the photodetector 150, and the current of the photodetector 150 is converted into a voltage signal by the current-voltage converter 160. If a voltage applied to the AOTM 140 increases when the intensity of the output light of the pulse laser 10 is high, the AOTM 140 outputs an input signal after significantly reducing the input signal. If a voltage applied to the AOTM 140 decreases when the intensity of the output light of the pulse laser 10 is low, the AOTM 140 outputs an input signal after transmitting the most of the small input signal. Thus, an output of the AOTM 140 may be constantly maintained although the output intensity of the pulse laser 10 becomes unstable with time.
Operation delay time of the AOTM 140 was measured. The operation delay time of the AOTM 140 is preferably less than tens of microseconds (μsec) to implement an active output stabilizing apparatus that is capable of providing feedback in real time. If the time taken for providing feedback to operate the AOTM 140 is too long, energy loss of an optical fiber serving as time delay is seriously great. Therefore, it is necessary to measure the operation delay time of the AOTM 140.
Referring to
The function generator 180 applies a voltage to the AOTM 140, and rising time in which the AOTM 140 operates normally is about 60 μsec. In addition, when the AOTM 140 stops operating, falling time is about 60 μsec.
If the time taken before the AOTM 140 during a feedback procedure is expected to be 1 μsec, the total feedback time is about 61 μsec. The delay time of 61 μsec corresponds to optical fiber length of 12 kilometers (km). The optical fiber has a loss of 0.22 dB/km at 1550 nm. Accordingly, an output of the optical fiber is 25 percent lost. As a result, the delay time of 61 μsec may be applied to an output stabilizing apparatus which is capable of providing feedback in real time.
The AOTM 140 is an active system modulated by an applied voltage.
However, it was confirmed that delay time did not vary depending the magnitude of an applied voltage.
The operation delay time or rising time of the AOTM 140 depending on an applied voltage is shown. When voltages of 1 to 5.6 Vpp are applied to the AOTM 140, modulation values are different from each other but the AOTM 140 has the same operation delay time of 60 μsec.
As described above, a pulse laser output stabilizing apparatus according to an embodiment of the present disclosure may provide stabilized output characteristics using an acousto-optic tunable modulator and real-time feedback control.
Although the present disclosure has been described in connection with the embodiment of the present disclosure illustrated in the accompanying drawings, it is not limited thereto. It will be apparent to those skilled in the art that various substitutions, modifications and changes may be made without departing from the scope and spirit of the present disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2012-0110099 | Oct 2012 | KR | national |
This application is a continuation of and claims priority to PCT/KR2013/008661 filed on Sep. 27, 2013, which claims priority to Korea Patent Application No. 10-2012-0110099 filed on Oct. 4, 2010, the entireties of both of which are hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/KR2013/008661 | Sep 2013 | US |
| Child | 14674306 | US |
