REVIEW OF SCIENTIFIC INSTRUMENTS 85, 046104 (2014)
Note: Efficient generation of optical sidebands at GHz with a high-power tapered amplifier J. C. Zappala,1,2 K. Bailey,1 Z.-T. Lu,1,2 T. P. O’Connor,1 and W. Jiang1,a) 1 2
Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Department of Physics and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
(Received 7 January 2014; accepted 22 March 2014; published online 4 April 2014) Two methods using a laser-diode tapered amplifier to produce high-power, high-efficiency optical frequency sidebands over a wide tunable frequency range are studied and compared. For a total output of 500 mW at 811 nm, 20% of the power can be placed in each of the first-order sidebands. Functionality and characterization are presented within the sideband frequency region of 0.8–2.3 GHz, and it is shown that both methods can be applied beyond this frequency range. These methods provide a versatile and effective tool for atomic physics experiments. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4870412] Optical sideband production has been a widely used technique in atomic physics for decades. For instance, it has been necessary for the trapping and cooling of atoms1 and for providing optical access to several fine or hyperfine transitions for laser spectroscopy using a single laser.2, 3 Furthermore, optical sidebands have also proven an integral part of laserfrequency lock in the Pound-Drever-Hall technique,4 and in generating optical frequency combs.5 Most optical sideband generation schemes involve using an electro-optic modulator (EOM).6 This form of modulation has long offered optical sidebands as high as tens of GHz away from the carrier frequency.7 Bulk crystal phase modulators can produce sidebands on a laser beam of several Watts, but have a high Vπ (the voltage required for π radian phase modulation is a few hundred volts). Although one can use the resonant type phase modulators to achieve good efficiency with a lower Vπ , sidebands from resonant devices are restricted to a narrow frequency range, limiting their tunability. Fiber-based phase modulators have both a low Vπ and a wide tunable frequency range thanks to waveguide technology. However, this same waveguide technology typically limits the maximum optical power the modulator can handle to tens of milliwatts as a result of the photorefractive effect.8 If the intensity of light inside the waveguide is too high, the index of refraction of the waveguide changes significantly due to the photorefractive effect and the light leaks out of the waveguide. Here we report on two methods of generating optical sidebands with laser-diode tapered amplifiers. We demonstrate that these methods produce highpower and high-efficiency sidebands with the sideband frequency tunable across a wide range of interest, and we have characterized their functionality across this range. The first method considered is the injection of the seed laser with sidebands from the fiber phase modulator into a tapered amplifier. Our investigation of this method has sought to prove that a faithful amplification of the sidebands occurs through the tapered amplifier. The setup is shown in Fig. 1(a). a) Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0034-6748/2014/85(4)/046104/3/$30.00
Twenty milliwatt of power from an 811 nm external-cavity diode laser (ECDL) with a linewidth of 500 kHz is coupled into a fiber phase modulator (EOSpace, PM-0K3-10-PFUPFU-811) for sideband generation. The output from the phase modulator is then used to seed a tapered amplifier. The phase modulator is driven by a RF signal generator (Marconi 2024). The tapered amplifier (Sacher, TEC400) is capable of generating 500 mW of optical power at 1.7 A of input current. The output from the tapered amplifier is analyzed by a Fabry-Perot cavity with a free spectral range (FSR) of 300 MHz. A typical output spectrum appears in Fig. 2(a). The second method uses direct current modulation of the tapered amplifier chip to produce the phase modulation necessary for the production of optical sidebands during the amplification process. This method has often been applied to traditional laser diodes, such as in ECDLs,9, 10 but has only recently been demonstrated on a tapered amplifier.11 In this prior demonstration, a sideband efficiency of 0.3% was achieved at a single frequency (6.6 GHz) on a 1 W beam. In principle, modulation on the current changes the carrier density inside the tapered amplifier chip, which in turn alters the index of refraction of the gain region and thus the phase of the laser beam. However, the operating current and internal capacitance of the chip are much larger than a conventional low power laser diode. The setup of this direct modulation method is shown in Fig. 1(b). The 20 mW seed is produced by an 811 nm ECDL. This light is injected into a tapered amplifier (Eagleyard, EYP-TPA-0808-01000-4006-CMT04-0000). A bias tee (Picosecond Pulse Labs, Model 5589) is used to combine the DC current and the RF modulation. The RF current modulation comes from a RF signal generator (Marconi 2024) and a RF amplifier (Amplifier Research 5S1G4). The RF signal is passed through a 50 1 dB attenuator for impedance matching. The output light from the amplifier, carrying sidebands from the current modulation, is fed into the Fabry-Perot cavity for analysis. A typical output spectrum appears in Fig. 2(b). The efficiency of each method has been tested between 800 MHz and 2.3 GHz, the results of which are plotted in Fig. 3. We observed efficiencies for the positive (or negative)
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FIG. 1. Schematic setup of the two investigated methods: (a) Amplification of the fiber phase modulator sidebands. The RF modulation signal is passed into the phase modulator. The resulting sidebands are then amplified by the tapered amplifier. (b) The direct modulation method. A RF signal is passed through an attenuator for impedance matching and then into a bias tee to modulate the current on the tapered amplifier’s diode chip. In both methods, sidebands are measured in a Fabry-Perot cavity.
first-order sideband up to 20%, where the efficiency is defined as the power in a given signed order divided by the total power in all orders. In the case of amplification of the output from the fiber phase modulator, the efficiency was checked both before (green curve) and after (blue curve) the light entered the tapered amplifier. Our results demonstrate that, not only are the sidebands faithfully amplified alongside the main carrier, but the amplification of the fiber phase modulator is relatively flat across the frequency range. Additionally, it is verified from the Fabry-Perot analysis that the entire output spectrum appears as a faithful copy of the input carrier and its sidebands, alleviating concerns of mode competition. The slight roll-off in efficiency after 2 GHz matches the expected frequency
FIG. 2. (a) Typical output spectra from the tapered amplifier with sidebands created by the fiber phase modulator and (b) those generated by current modulation on the tapered amplifier. The 300 MHz FSR of the Fabry-Perot cavity causes the first-order sidebands of 1.25 GHz to appear 50 MHz from the nearest carrier peak.
Rev. Sci. Instrum. 85, 046104 (2014)
FIG. 3. Efficiency of sideband production versus frequency for both methods. Amplification for the fiber phase modulator is presented prior to the amplifier (green circles and dashed line) and after amplification (blue squares and solid line) at a modulating voltage of 0.4 V (5 dBm). These curves demonstrate a fairly flat response undisturbed by the amplification process. The direct modulation method (red diamonds and solid line), presented at a modulation depth of 160 mA (28 dBm) on a DC input of 1 A, shows functionality across the full range with a characteristic shape due to the internal capacitance of the chip. The curves displayed are guides to the eye.
response of the fiber phase modulator provided by the manufacturer. These high efficiencies are achieved at the moderate modulation voltage of 0.4 V (5 dBm) courtesy of the fiber phase modulator’s low Vπ . Extrapolation of our data confirms the provided Vπ of the fiber phase modulator to be approximately 1.8 V. Higher modulation frequency is possible for this method as the fiber phase modulator can provide sidebands as high as 20 GHz. For the direct modulation method the response is not flat. The roll-off at higher frequencies is due to the internal capacitance of the tapered amplifier chip. For this frequency range (800 MHz to 2.3 GHz), a modulation of 160 mA (28 dBm) was applied on top of 1 A of DC current. The modulation depth is determined by considering the RF power applied to the 50 attenuator in series with the diode chip, which has an effective resistance of approximately 2 , and calculating the resulting current passing through the total load. While this RF modulation amplitude is over 10% of the DC current, the chip was not adversely affected after 50 h of usage during a two-week time period. It is worth noting that if the modulation pushes the chip beyond its current limit (in this case, somewhere in excess of 2 A) the chip may be placed in danger of damage. Because the chip produces sidebands via current modulation, we characterize it in terms of radians/A rather than a Vπ . Given the change in response with respect to the frequency, the direct modulation method on this particular chip yields 7.4 rad/A to 0.4 rad/A over the presented frequency range. In order to better understand the internal capacitance of the chip in the direct modulation method, we reduced the modulation depth to 50 mA (18 dBm) and probed a lower range of frequencies, from 100 to 800 MHz. The modulation depth was lowered in order to remain a comfortable margin from the maximum efficiency allowed in the first order sideband. Using an equivalent circuit model for the chip, we produced curves to compare with both the low and high frequency data sets. We achieved a qualitative match for an
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Rev. Sci. Instrum. 85, 046104 (2014)
produces a resonant structure in the frequency response that reduces the usable range of the method. In conclusion we have demonstrated two methods of generating optical sidebands with high efficiency on a high-power tapered amplifier. Both methods can access a wide tunable frequency range, generating high efficiency sidebands up to a few GHz and are now being successfully applied to hyperfine repumping in cold atom experiments, such as the trapping of argon and krypton. In the first method the tapered amplifier preserves the modulation depth of the incident beam and the efficiency. The second method has a simpler construction. However, the high frequency response is limited by the internal capacitance of the tapered amplifier. Both techniques should find many applications in atomic physics experiments. FIG. 4. Frequency responses for the direct modulation method with modulation depths of (a) 50 mA and (b) 160 mA on 1 A of DC current. Black curves are model predictions for an equivalent circuit with 1 nF internal capacitance. A qualitative match is achieved.
We thank P. Mueller for many helpful discussions. This work is supported by Department of Energy, Office of Nuclear Physics, under Contract No. DEAC02-06CH11357. 1 E.
internal capacitance of 1 nF at 1 A DC current. The curves are shown against the data sets in Fig. 4. It is also important to state that in the implementation of this direct modulation, impedance matching (provided by the 1 dB attenuator in our experiment) is key. Without matching, much of the modulation power does not reach the chip, but is instead reflected, as we found by the use of a RF network analyzer. Additionally, the output of the bias tee should be fed directly to the cathode of the chip. Any circuitry in between the bias tee output and chip input produces stray inductance that, in combination with the internal capacitance of the chip,
L. Raab, M. Prentiss, A. Cable, S. Chu, and D. E. Pritchard, Phys. Rev. Lett. 59, 2631 (1987). 2 D. Shiner, R. Dixson, and P. Zhao, Phys. Rev. Lett. 72, 1802 (1994). 3 D. Shiner, R. Dixson, and V. Vedantham, Phys. Rev. Lett. 74, 3553 (1995). 4 R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward, Appl. Phys. B 31, 97 (1983). 5 J. Ye, L.-S. Ma, T. Day, and J. L. Hall, Opt. Lett. 22, 301 (1997). 6 T. A. Maldonado, “Electro-optic modulators,” in Handbook of Optics, 2nd ed., edited by M. Bass (McGraw-Hill, 2010), Vol. 2. 7 R. Kallenbach, B. Scheumann, C. Zimmermann, D. Meschede, and T. W. Hansch, Appl. Phys. Lett. 54, 1622 (1989). 8 A. Yariv, Quantum Electronics, 3rd ed. (Wiley, 1989). 9 C. J. Myatt, N. R. Newbury, and C. E. Wieman, Opt. Lett. 18, 649 (1993). 10 P. Feng and T. Walker, Am. J. Phys. 63, 905 (1995). 11 J. Mahnke, S. Kulas, I. Geisel, S. Jollenbeck, W. Ertmer, and C. Klempt, Rev. Sci. Instrum. 84, 063110 (2013).
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
Note: Efficient generation of optical sidebands at GHz with a high-power tapered amplifier J. C. Zappala,1,2 K. Bailey,1 Z.-T. Lu,1,2 T. P. O’Connor,1 and W. Jiang1,a) 1 2
Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Department of Physics and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA
(Received 7 January 2014; accepted 22 March 2014; published online 4 April 2014) Two methods using a laser-diode tapered amplifier to produce high-power, high-efficiency optical frequency sidebands over a wide tunable frequency range are studied and compared. For a total output of 500 mW at 811 nm, 20% of the power can be placed in each of the first-order sidebands. Functionality and characterization are presented within the sideband frequency region of 0.8–2.3 GHz, and it is shown that both methods can be applied beyond this frequency range. These methods provide a versatile and effective tool for atomic physics experiments. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4870412] Optical sideband production has been a widely used technique in atomic physics for decades. For instance, it has been necessary for the trapping and cooling of atoms1 and for providing optical access to several fine or hyperfine transitions for laser spectroscopy using a single laser.2, 3 Furthermore, optical sidebands have also proven an integral part of laserfrequency lock in the Pound-Drever-Hall technique,4 and in generating optical frequency combs.5 Most optical sideband generation schemes involve using an electro-optic modulator (EOM).6 This form of modulation has long offered optical sidebands as high as tens of GHz away from the carrier frequency.7 Bulk crystal phase modulators can produce sidebands on a laser beam of several Watts, but have a high Vπ (the voltage required for π radian phase modulation is a few hundred volts). Although one can use the resonant type phase modulators to achieve good efficiency with a lower Vπ , sidebands from resonant devices are restricted to a narrow frequency range, limiting their tunability. Fiber-based phase modulators have both a low Vπ and a wide tunable frequency range thanks to waveguide technology. However, this same waveguide technology typically limits the maximum optical power the modulator can handle to tens of milliwatts as a result of the photorefractive effect.8 If the intensity of light inside the waveguide is too high, the index of refraction of the waveguide changes significantly due to the photorefractive effect and the light leaks out of the waveguide. Here we report on two methods of generating optical sidebands with laser-diode tapered amplifiers. We demonstrate that these methods produce highpower and high-efficiency sidebands with the sideband frequency tunable across a wide range of interest, and we have characterized their functionality across this range. The first method considered is the injection of the seed laser with sidebands from the fiber phase modulator into a tapered amplifier. Our investigation of this method has sought to prove that a faithful amplification of the sidebands occurs through the tapered amplifier. The setup is shown in Fig. 1(a). a) Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0034-6748/2014/85(4)/046104/3/$30.00
Twenty milliwatt of power from an 811 nm external-cavity diode laser (ECDL) with a linewidth of 500 kHz is coupled into a fiber phase modulator (EOSpace, PM-0K3-10-PFUPFU-811) for sideband generation. The output from the phase modulator is then used to seed a tapered amplifier. The phase modulator is driven by a RF signal generator (Marconi 2024). The tapered amplifier (Sacher, TEC400) is capable of generating 500 mW of optical power at 1.7 A of input current. The output from the tapered amplifier is analyzed by a Fabry-Perot cavity with a free spectral range (FSR) of 300 MHz. A typical output spectrum appears in Fig. 2(a). The second method uses direct current modulation of the tapered amplifier chip to produce the phase modulation necessary for the production of optical sidebands during the amplification process. This method has often been applied to traditional laser diodes, such as in ECDLs,9, 10 but has only recently been demonstrated on a tapered amplifier.11 In this prior demonstration, a sideband efficiency of 0.3% was achieved at a single frequency (6.6 GHz) on a 1 W beam. In principle, modulation on the current changes the carrier density inside the tapered amplifier chip, which in turn alters the index of refraction of the gain region and thus the phase of the laser beam. However, the operating current and internal capacitance of the chip are much larger than a conventional low power laser diode. The setup of this direct modulation method is shown in Fig. 1(b). The 20 mW seed is produced by an 811 nm ECDL. This light is injected into a tapered amplifier (Eagleyard, EYP-TPA-0808-01000-4006-CMT04-0000). A bias tee (Picosecond Pulse Labs, Model 5589) is used to combine the DC current and the RF modulation. The RF current modulation comes from a RF signal generator (Marconi 2024) and a RF amplifier (Amplifier Research 5S1G4). The RF signal is passed through a 50 1 dB attenuator for impedance matching. The output light from the amplifier, carrying sidebands from the current modulation, is fed into the Fabry-Perot cavity for analysis. A typical output spectrum appears in Fig. 2(b). The efficiency of each method has been tested between 800 MHz and 2.3 GHz, the results of which are plotted in Fig. 3. We observed efficiencies for the positive (or negative)
85, 046104-1
© 2014 AIP Publishing LLC
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FIG. 1. Schematic setup of the two investigated methods: (a) Amplification of the fiber phase modulator sidebands. The RF modulation signal is passed into the phase modulator. The resulting sidebands are then amplified by the tapered amplifier. (b) The direct modulation method. A RF signal is passed through an attenuator for impedance matching and then into a bias tee to modulate the current on the tapered amplifier’s diode chip. In both methods, sidebands are measured in a Fabry-Perot cavity.
first-order sideband up to 20%, where the efficiency is defined as the power in a given signed order divided by the total power in all orders. In the case of amplification of the output from the fiber phase modulator, the efficiency was checked both before (green curve) and after (blue curve) the light entered the tapered amplifier. Our results demonstrate that, not only are the sidebands faithfully amplified alongside the main carrier, but the amplification of the fiber phase modulator is relatively flat across the frequency range. Additionally, it is verified from the Fabry-Perot analysis that the entire output spectrum appears as a faithful copy of the input carrier and its sidebands, alleviating concerns of mode competition. The slight roll-off in efficiency after 2 GHz matches the expected frequency
FIG. 2. (a) Typical output spectra from the tapered amplifier with sidebands created by the fiber phase modulator and (b) those generated by current modulation on the tapered amplifier. The 300 MHz FSR of the Fabry-Perot cavity causes the first-order sidebands of 1.25 GHz to appear 50 MHz from the nearest carrier peak.
Rev. Sci. Instrum. 85, 046104 (2014)
FIG. 3. Efficiency of sideband production versus frequency for both methods. Amplification for the fiber phase modulator is presented prior to the amplifier (green circles and dashed line) and after amplification (blue squares and solid line) at a modulating voltage of 0.4 V (5 dBm). These curves demonstrate a fairly flat response undisturbed by the amplification process. The direct modulation method (red diamonds and solid line), presented at a modulation depth of 160 mA (28 dBm) on a DC input of 1 A, shows functionality across the full range with a characteristic shape due to the internal capacitance of the chip. The curves displayed are guides to the eye.
response of the fiber phase modulator provided by the manufacturer. These high efficiencies are achieved at the moderate modulation voltage of 0.4 V (5 dBm) courtesy of the fiber phase modulator’s low Vπ . Extrapolation of our data confirms the provided Vπ of the fiber phase modulator to be approximately 1.8 V. Higher modulation frequency is possible for this method as the fiber phase modulator can provide sidebands as high as 20 GHz. For the direct modulation method the response is not flat. The roll-off at higher frequencies is due to the internal capacitance of the tapered amplifier chip. For this frequency range (800 MHz to 2.3 GHz), a modulation of 160 mA (28 dBm) was applied on top of 1 A of DC current. The modulation depth is determined by considering the RF power applied to the 50 attenuator in series with the diode chip, which has an effective resistance of approximately 2 , and calculating the resulting current passing through the total load. While this RF modulation amplitude is over 10% of the DC current, the chip was not adversely affected after 50 h of usage during a two-week time period. It is worth noting that if the modulation pushes the chip beyond its current limit (in this case, somewhere in excess of 2 A) the chip may be placed in danger of damage. Because the chip produces sidebands via current modulation, we characterize it in terms of radians/A rather than a Vπ . Given the change in response with respect to the frequency, the direct modulation method on this particular chip yields 7.4 rad/A to 0.4 rad/A over the presented frequency range. In order to better understand the internal capacitance of the chip in the direct modulation method, we reduced the modulation depth to 50 mA (18 dBm) and probed a lower range of frequencies, from 100 to 800 MHz. The modulation depth was lowered in order to remain a comfortable margin from the maximum efficiency allowed in the first order sideband. Using an equivalent circuit model for the chip, we produced curves to compare with both the low and high frequency data sets. We achieved a qualitative match for an
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Rev. Sci. Instrum. 85, 046104 (2014)
produces a resonant structure in the frequency response that reduces the usable range of the method. In conclusion we have demonstrated two methods of generating optical sidebands with high efficiency on a high-power tapered amplifier. Both methods can access a wide tunable frequency range, generating high efficiency sidebands up to a few GHz and are now being successfully applied to hyperfine repumping in cold atom experiments, such as the trapping of argon and krypton. In the first method the tapered amplifier preserves the modulation depth of the incident beam and the efficiency. The second method has a simpler construction. However, the high frequency response is limited by the internal capacitance of the tapered amplifier. Both techniques should find many applications in atomic physics experiments. FIG. 4. Frequency responses for the direct modulation method with modulation depths of (a) 50 mA and (b) 160 mA on 1 A of DC current. Black curves are model predictions for an equivalent circuit with 1 nF internal capacitance. A qualitative match is achieved.
We thank P. Mueller for many helpful discussions. This work is supported by Department of Energy, Office of Nuclear Physics, under Contract No. DEAC02-06CH11357. 1 E.
internal capacitance of 1 nF at 1 A DC current. The curves are shown against the data sets in Fig. 4. It is also important to state that in the implementation of this direct modulation, impedance matching (provided by the 1 dB attenuator in our experiment) is key. Without matching, much of the modulation power does not reach the chip, but is instead reflected, as we found by the use of a RF network analyzer. Additionally, the output of the bias tee should be fed directly to the cathode of the chip. Any circuitry in between the bias tee output and chip input produces stray inductance that, in combination with the internal capacitance of the chip,
L. Raab, M. Prentiss, A. Cable, S. Chu, and D. E. Pritchard, Phys. Rev. Lett. 59, 2631 (1987). 2 D. Shiner, R. Dixson, and P. Zhao, Phys. Rev. Lett. 72, 1802 (1994). 3 D. Shiner, R. Dixson, and V. Vedantham, Phys. Rev. Lett. 74, 3553 (1995). 4 R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, and H. Ward, Appl. Phys. B 31, 97 (1983). 5 J. Ye, L.-S. Ma, T. Day, and J. L. Hall, Opt. Lett. 22, 301 (1997). 6 T. A. Maldonado, “Electro-optic modulators,” in Handbook of Optics, 2nd ed., edited by M. Bass (McGraw-Hill, 2010), Vol. 2. 7 R. Kallenbach, B. Scheumann, C. Zimmermann, D. Meschede, and T. W. Hansch, Appl. Phys. Lett. 54, 1622 (1989). 8 A. Yariv, Quantum Electronics, 3rd ed. (Wiley, 1989). 9 C. J. Myatt, N. R. Newbury, and C. E. Wieman, Opt. Lett. 18, 649 (1993). 10 P. Feng and T. Walker, Am. J. Phys. 63, 905 (1995). 11 J. Mahnke, S. Kulas, I. Geisel, S. Jollenbeck, W. Ertmer, and C. Klempt, Rev. Sci. Instrum. 84, 063110 (2013).
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
